Merge pull request #85 from UnityTech/yczhang

Yczhang

Runtime/material/checkbox.cs

             TickerProvider vsync = null,
             BoxConstraints additionalConstraints = null
         ) : base(key: key) {
-            D.assert(tristate != null);
             D.assert(tristate || value != null);
             D.assert(activeColor != null);
             D.assert(inactiveColor != null);

Runtime/widgets/safe_area.cs

     }
 
 
-    class SliverSafeArea : StatelessWidget {
+    public class SliverSafeArea : StatelessWidget {
         public SliverSafeArea(
             Key key = null,
             bool left = true,

Runtime/widgets/sliver.cs

 using System;
 using System.Collections.Generic;
 using System.Linq;
+using com.unity.uiwidgets.Runtime.rendering;
 using Unity.UIWidgets.foundation;
 using Unity.UIWidgets.painting;
 using Unity.UIWidgets.rendering;
             return new SliverMultiBoxAdaptorElement(this);
         }
 
-        public float? estimateMaxScrollOffset(
+        public virtual float? estimateMaxScrollOffset(
             SliverConstraints constraints,
             int firstIndex,
             int lastIndex,
         public override void updateRenderObject(BuildContext context, RenderObject renderObjectRaw) {
             var renderObject = (RenderSliverFixedExtentList) renderObjectRaw;
             renderObject.itemExtent = this.itemExtent;
+        }
+    }
+
+    public class SliverGrid : SliverMultiBoxAdaptorWidget {
+        public SliverGrid(
+            Key key = null,
+            SliverChildDelegate layoutDelegate = null,
+            SliverGridDelegate gridDelegate = null
+        ) : base(key: key, del: layoutDelegate) {
+            D.assert(layoutDelegate != null);
+            D.assert(gridDelegate != null);
+            this.gridDelegate = gridDelegate;
+        }
+
+        public static SliverGrid count(
+            Key key = null,
+            int? crossAxisCount = null,
+            float mainAxisSpacing = 0.0f,
+            float crossAxisSpacing = 0.0f,
+            float childAspectRatio = 1.0f,
+            List<Widget> children = null
+        ) {
+            return new SliverGrid(
+                key: key,
+                layoutDelegate: new SliverChildListDelegate(children ?? new List<Widget> { }),
+                gridDelegate: new SliverGridDelegateWithFixedCrossAxisCount(
+                    crossAxisCount: crossAxisCount ?? 0,
+                    mainAxisSpacing: mainAxisSpacing,
+                    crossAxisSpacing: crossAxisSpacing,
+                    childAspectRatio: childAspectRatio
+                )
+            );
+        }
+
+        public static SliverGrid extent(
+            Key key = null,
+            float? maxCrossAxisExtent = null,
+            float mainAxisSpacing = 0.0f,
+            float crossAxisSpacing = 0.0f,
+            float childAspectRatio = 1.0f,
+            List<Widget> children = null
+        ) {
+            return new SliverGrid(key: key,
+                layoutDelegate: new SliverChildListDelegate(new List<Widget> { }),
+                gridDelegate: new SliverGridDelegateWithMaxCrossAxisExtent(
+                    maxCrossAxisExtent: maxCrossAxisExtent ?? 0,
+                    mainAxisSpacing: mainAxisSpacing,
+                    crossAxisSpacing: crossAxisSpacing,
+                    childAspectRatio: childAspectRatio
+                ));
+        }
+
+        public readonly SliverGridDelegate gridDelegate;
+
+        public override RenderObject createRenderObject(BuildContext context) {
+            SliverMultiBoxAdaptorElement element = context as SliverMultiBoxAdaptorElement;
+            return new RenderSliverGrid(childManager: element, gridDelegate: this.gridDelegate);
+        }
+
+        public override void updateRenderObject(BuildContext context, RenderObject renderObject) {
+            (renderObject as RenderSliverGrid).gridDelegate = this.gridDelegate;
+        }
+
+        public override float? estimateMaxScrollOffset(
+            SliverConstraints constraints,
+            int firstIndex,
+            int lastIndex,
+            float leadingScrollOffset,
+            float trailingScrollOffset
+        ) {
+            return base.estimateMaxScrollOffset(
+                       constraints,
+                       firstIndex,
+                       lastIndex,
+                       leadingScrollOffset,
+                       trailingScrollOffset
+                   ) ?? this.gridDelegate.getLayout(constraints)
+                       .computeMaxScrollOffset(this.del.estimatedChildCount ?? 0);
         }
     }
 

Runtime/widgets/sliver_persistent_header.cs

             bool floating = false
         ) : base(key: key) {
             D.assert(del != null);
-            this.del = del;
+            this.layoutDelegate = del;
-        public readonly SliverPersistentHeaderDelegate del;
+        public readonly SliverPersistentHeaderDelegate layoutDelegate;
 
         public readonly bool pinned;
 
             if (this.floating && this.pinned) {
-                return new _SliverFloatingPinnedPersistentHeader(del: this.del);
+                return new _SliverFloatingPinnedPersistentHeader(layoutDelegate: this.layoutDelegate);
-                return new _SliverPinnedPersistentHeader(del: this.del);
+                return new _SliverPinnedPersistentHeader(layoutDelegate: this.layoutDelegate);
-                return new _SliverFloatingPersistentHeader(del: this.del);
+                return new _SliverFloatingPersistentHeader(layoutDelegate: this.layoutDelegate);
-            return new _SliverScrollingPersistentHeader(del: this.del);
+            return new _SliverScrollingPersistentHeader(layoutDelegate: this.layoutDelegate);
-            properties.add(new DiagnosticsProperty<SliverPersistentHeaderDelegate>("del", this.del));
+            properties.add(new DiagnosticsProperty<SliverPersistentHeaderDelegate>("layoutDelegate", this.layoutDelegate));
             List<string> flags = new List<string> { };
             if (this.pinned) {
                 flags.Add("pinned");
             _SliverPersistentHeaderRenderObjectWidget newWidget =
                 _newWidget as _SliverPersistentHeaderRenderObjectWidget;
             _SliverPersistentHeaderRenderObjectWidget oldWidget = this.widget;
-            SliverPersistentHeaderDelegate newDelegate = newWidget.del;
-            SliverPersistentHeaderDelegate oldDelegate = oldWidget.del;
+            SliverPersistentHeaderDelegate newDelegate = newWidget.layoutDelegate;
+            SliverPersistentHeaderDelegate oldDelegate = oldWidget.layoutDelegate;
             if (newDelegate != oldDelegate &&
                 (newDelegate.GetType() != oldDelegate.GetType() || newDelegate.shouldRebuild(oldDelegate))) {
                 (this.renderObject as _RenderSliverPersistentHeaderForWidgetsMixin).triggerRebuild();
             this.owner.buildScope(this,
                 () => {
                     this.child = this.updateChild(this.child,
-                        this.widget.del.build(this, shrinkOffset, overlapsContent), null);
+                        this.widget.layoutDelegate.build(this, shrinkOffset, overlapsContent), null);
                 });
         }
 
     abstract class _SliverPersistentHeaderRenderObjectWidget : RenderObjectWidget {
         public _SliverPersistentHeaderRenderObjectWidget(
             Key key = null,
-            SliverPersistentHeaderDelegate del = null
+            SliverPersistentHeaderDelegate layoutDelegate = null
-            D.assert(del != null);
-            this.del = del;
+            D.assert(layoutDelegate != null);
+            this.layoutDelegate = layoutDelegate;
-        public readonly SliverPersistentHeaderDelegate del;
+        public readonly SliverPersistentHeaderDelegate layoutDelegate;
 
         public override Element createElement() {
             return new _SliverPersistentHeaderElement(this);
 
         public override void debugFillProperties(DiagnosticPropertiesBuilder description) {
             base.debugFillProperties(description);
-            description.add(new DiagnosticsProperty<SliverPersistentHeaderDelegate>("del", this.del));
+            description.add(new DiagnosticsProperty<SliverPersistentHeaderDelegate>("layoutDelegate", this.layoutDelegate));
         }
     }
 
     class _SliverScrollingPersistentHeader : _SliverPersistentHeaderRenderObjectWidget {
         public _SliverScrollingPersistentHeader(
             Key key = null,
-            SliverPersistentHeaderDelegate del = null
-        ) : base(key: key, del: del) {
+            SliverPersistentHeaderDelegate layoutDelegate = null
+        ) : base(key: key, layoutDelegate: layoutDelegate) {
         }
 
         public override RenderObject createRenderObject(BuildContext context) {
         _SliverPersistentHeaderElement _ele;
 
         public override float? minExtent {
-            get { return this._element.widget.del.minExtent; }
+            get { return this._element.widget.layoutDelegate.minExtent; }
-            get { return this._element.widget.del.maxExtent; }
+            get { return this._element.widget.layoutDelegate.maxExtent; }
         }
 
         protected override void updateChild(float shrinkOffset, bool overlapsContent) {
     class _SliverPinnedPersistentHeader : _SliverPersistentHeaderRenderObjectWidget {
         public _SliverPinnedPersistentHeader(
             Key key = null,
-            SliverPersistentHeaderDelegate del = null
-        ) : base(key: key, del: del) {
+            SliverPersistentHeaderDelegate layoutDelegate = null
+        ) : base(key: key, layoutDelegate: layoutDelegate) {
         }
 
         public override RenderObject createRenderObject(BuildContext context) {
         _SliverPersistentHeaderElement _ele;
 
         public override float? minExtent {
-            get { return this._element.widget.del.minExtent; }
+            get { return this._element.widget.layoutDelegate.minExtent; }
-            get { return this._element.widget.del.maxExtent; }
+            get { return this._element.widget.layoutDelegate.maxExtent; }
         }
 
         protected override void updateChild(float shrinkOffset, bool overlapsContent) {
     class _SliverFloatingPersistentHeader : _SliverPersistentHeaderRenderObjectWidget {
         public _SliverFloatingPersistentHeader(
             Key key = null,
-            SliverPersistentHeaderDelegate del = null
-        ) : base(key: key, del: del) {
+            SliverPersistentHeaderDelegate layoutDelegate = null
+        ) : base(key: key, layoutDelegate: layoutDelegate) {
-            ret.snapConfiguration = this.del.snapConfiguration;
+            ret.snapConfiguration = this.layoutDelegate.snapConfiguration;
             return ret;
         }
 
-            renderObject.snapConfiguration = this.del.snapConfiguration;
+            renderObject.snapConfiguration = this.layoutDelegate.snapConfiguration;
         }
     }
 
         _SliverPersistentHeaderElement _ele;
 
         public override float? minExtent {
-            get { return this._element.widget.del.minExtent; }
+            get { return this._element.widget.layoutDelegate.minExtent; }
-            get { return this._element.widget.del.maxExtent; }
+            get { return this._element.widget.layoutDelegate.maxExtent; }
         }
 
         protected override void updateChild(float shrinkOffset, bool overlapsContent) {
     class _SliverFloatingPinnedPersistentHeader : _SliverPersistentHeaderRenderObjectWidget {
         public _SliverFloatingPinnedPersistentHeader(
             Key key = null,
-            SliverPersistentHeaderDelegate del = null
-        ) : base(key: key, del: del) {
+            SliverPersistentHeaderDelegate layoutDelegate = null
+        ) : base(key: key, layoutDelegate: layoutDelegate) {
-            ret.snapConfiguration = this.del.snapConfiguration;
+            ret.snapConfiguration = this.layoutDelegate.snapConfiguration;
             return ret;
         }
 
-            renderObject.snapConfiguration = this.del.snapConfiguration;
+            renderObject.snapConfiguration = this.layoutDelegate.snapConfiguration;
         }
     }
 
         _SliverPersistentHeaderElement _ele;
 
         public override float? minExtent {
-            get { return this._element.widget.del.minExtent; }
+            get { return this._element.widget.layoutDelegate.minExtent; }
-            get { return this._element.widget.del.maxExtent; }
+            get { return this._element.widget.layoutDelegate.maxExtent; }
         }
 
         protected override void updateChild(float shrinkOffset, bool overlapsContent) {

Samples/UIWidgetsGallery/gallery/demo.cs

         void _showApiDocumentation(BuildContext context) {
             string url = this.demos[DefaultTabController.of(context).index].documentationUrl;
             if (url != null) {
-                // TODO: find Unity equivalent
-                // Open the URL in browser
-                // launch(url, forceWebView: true);
+                Application.OpenURL(url);
             }
         }
 

Samples/UIWidgetsGallery/gallery/demos.cs

     public static partial class DemoUtils {
         static List<GalleryDemo> _buildGalleryDemos() {
             List<GalleryDemo> galleryDemos = new List<GalleryDemo> {
-//                // Demos
-//                new GalleryDemo(
-//                    title: "Shrine",
-//                    subtitle: "Basic shopping app",
-//                    icon: GalleryIcons.shrine,
-//                    category: GalleryDemoCategory._kDemos,
-//                    routeName: ShrineDemo.routeName,
-//                    buildRoute: (BuildContext context) => new ShrineDemo()
-//                ),
+                // Demos
+                new GalleryDemo(
+                    title: "Shrine",
+                    subtitle: "Basic shopping app",
+                    icon: GalleryIcons.shrine,
+                    category: DemoUtils._kDemos,
+                    routeName: ShrineDemo.routeName,
+                    buildRoute: (BuildContext context) => new ShrineDemo()
+                ),
                 new GalleryDemo(
                     title: "Contact profile",
                     subtitle: "Address book entry with a flexible appbar",

Runtime/rendering/sliver_grid.cs

+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.painting;
+using Unity.UIWidgets.rendering;
+using Unity.UIWidgets.ui;
+using UnityEngine;
+
+namespace com.unity.uiwidgets.Runtime.rendering {
+    public class SliverGridGeometry {
+        public SliverGridGeometry(
+            float? scrollOffset = null,
+            float? crossAxisOffset = null,
+            float? mainAxisExtent = null,
+            float? crossAxisExtent = null
+        ) {
+            this.scrollOffset = scrollOffset;
+            this.crossAxisOffset = crossAxisOffset;
+            this.mainAxisExtent = mainAxisExtent;
+            this.crossAxisExtent = crossAxisExtent;
+        }
+
+        public readonly float? scrollOffset;
+
+        public readonly float? crossAxisOffset;
+
+        public readonly float? mainAxisExtent;
+
+        public readonly float? crossAxisExtent;
+
+        public float? trailingScrollOffset {
+            get { return this.scrollOffset + this.mainAxisExtent; }
+        }
+
+        public BoxConstraints getBoxConstraints(SliverConstraints constraints) {
+            return constraints.asBoxConstraints(
+                minExtent: this.mainAxisExtent ?? 0.0f,
+                maxExtent: this.mainAxisExtent ?? 0.0f,
+                crossAxisExtent: this.crossAxisExtent ?? 0.0f
+            );
+        }
+
+        public override string ToString() {
+            return "SliverGridGeometry(" +
+                   "scrollOffset: $scrollOffset, " +
+                   "crossAxisOffset: $crossAxisOffset, " +
+                   "mainAxisExtent: $mainAxisExtent, " +
+                   "crossAxisExtent: $crossAxisExtent" +
+                   ")";
+        }
+    }
+
+    public abstract class SliverGridLayout {
+        public SliverGridLayout() {
+        }
+
+        public abstract int getMinChildIndexForScrollOffset(float scrollOffset);
+
+        public abstract int getMaxChildIndexForScrollOffset(float scrollOffset);
+
+        public abstract SliverGridGeometry getGeometryForChildIndex(int index);
+
+        public abstract float computeMaxScrollOffset(int childCount);
+    }
+
+    public class SliverGridRegularTileLayout : SliverGridLayout {
+        public SliverGridRegularTileLayout(
+            int? crossAxisCount = null,
+            float? mainAxisStride = null,
+            float? crossAxisStride = null,
+            float? childMainAxisExtent = null,
+            float? childCrossAxisExtent = null,
+            bool? reverseCrossAxis = null
+        ) {
+            D.assert(crossAxisCount != null && crossAxisCount > 0);
+            D.assert(mainAxisStride != null && mainAxisStride >= 0);
+            D.assert(crossAxisStride != null && crossAxisStride >= 0);
+            D.assert(childMainAxisExtent != null && childMainAxisExtent >= 0);
+            D.assert(childCrossAxisExtent != null && childCrossAxisExtent >= 0);
+            D.assert(reverseCrossAxis != null);
+            this.crossAxisCount = crossAxisCount;
+            this.mainAxisStride = mainAxisStride;
+            this.crossAxisStride = crossAxisStride;
+            this.childMainAxisExtent = childMainAxisExtent;
+            this.childCrossAxisExtent = childCrossAxisExtent;
+            this.reverseCrossAxis = reverseCrossAxis;
+        }
+
+        public readonly int? crossAxisCount;
+
+        public readonly float? mainAxisStride;
+
+        public readonly float? crossAxisStride;
+
+        public readonly float? childMainAxisExtent;
+
+        public readonly float? childCrossAxisExtent;
+
+        public readonly bool? reverseCrossAxis;
+
+        public override int getMinChildIndexForScrollOffset(float scrollOffset) {
+            return (this.mainAxisStride > 0.0f
+                       ? this.crossAxisCount * ((int) (scrollOffset / this.mainAxisStride))
+                       : 0) ?? 0;
+        }
+
+        public override int getMaxChildIndexForScrollOffset(float scrollOffset) {
+            if (this.mainAxisStride > 0.0f) {
+                int? mainAxisCount = (scrollOffset / this.mainAxisStride)?.ceil();
+                return Mathf.Max(0, (this.crossAxisCount * mainAxisCount - 1) ?? 0);
+            }
+
+            return 0;
+        }
+
+        float _getOffsetFromStartInCrossAxis(float crossAxisStart) {
+            if (this.reverseCrossAxis == true) {
+                return (this.crossAxisCount * this.crossAxisStride - crossAxisStart - this.childCrossAxisExtent) ??
+                       0.0f;
+            }
+
+            return crossAxisStart;
+        }
+
+        public override SliverGridGeometry getGeometryForChildIndex(int index) {
+            float? crossAxisStart = (index % this.crossAxisCount) * this.crossAxisStride;
+            return new SliverGridGeometry(
+                scrollOffset: (index / this.crossAxisCount) * this.mainAxisStride,
+                crossAxisOffset:
+                this._getOffsetFromStartInCrossAxis(crossAxisStart ?? 0.0f),
+                mainAxisExtent:
+                this.childMainAxisExtent,
+                crossAxisExtent:
+                this.childCrossAxisExtent
+            );
+        }
+
+        public override float computeMaxScrollOffset(int childCount) {
+            int? mainAxisCount = ((childCount - 1) / this.crossAxisCount) + 1;
+            float? mainAxisSpacing = this.mainAxisStride - this.childMainAxisExtent;
+            return (this.mainAxisStride * mainAxisCount - mainAxisSpacing) ?? 0.0f;
+        }
+    }
+
+    public abstract class SliverGridDelegate {
+        public abstract SliverGridLayout getLayout(SliverConstraints constraints);
+
+        public abstract bool shouldRelayout(SliverGridDelegate oldDelegate);
+    }
+
+    public class SliverGridDelegateWithFixedCrossAxisCount : SliverGridDelegate {
+        public SliverGridDelegateWithFixedCrossAxisCount(
+            int crossAxisCount,
+            float mainAxisSpacing = 0.0f,
+            float crossAxisSpacing = 0.0f,
+            float childAspectRatio = 1.0f
+        ) {
+            D.assert(crossAxisCount > 0);
+            D.assert(mainAxisSpacing >= 0);
+            D.assert(crossAxisSpacing >= 0);
+            D.assert(childAspectRatio > 0);
+            this.crossAxisCount = crossAxisCount;
+            this.mainAxisSpacing = mainAxisSpacing;
+            this.crossAxisSpacing = crossAxisSpacing;
+            this.childAspectRatio = childAspectRatio;
+        }
+
+        public readonly int crossAxisCount;
+
+        public readonly float mainAxisSpacing;
+
+        public readonly float crossAxisSpacing;
+
+        public readonly float childAspectRatio;
+
+        bool _debugAssertIsValid() {
+            D.assert(this.crossAxisCount > 0);
+            D.assert(this.mainAxisSpacing >= 0.0f);
+            D.assert(this.crossAxisSpacing >= 0.0f);
+            D.assert(this.childAspectRatio > 0.0f);
+            return true;
+        }
+
+        public override SliverGridLayout getLayout(SliverConstraints constraints) {
+            D.assert(this._debugAssertIsValid());
+            float usableCrossAxisExtent =
+                constraints.crossAxisExtent - this.crossAxisSpacing * (this.crossAxisCount - 1);
+            float childCrossAxisExtent = usableCrossAxisExtent / this.crossAxisCount;
+            float childMainAxisExtent = childCrossAxisExtent / this.childAspectRatio;
+            return new SliverGridRegularTileLayout(
+                crossAxisCount: this.crossAxisCount,
+                mainAxisStride: childMainAxisExtent + this.mainAxisSpacing,
+                crossAxisStride: childCrossAxisExtent + this.crossAxisSpacing,
+                childMainAxisExtent: childMainAxisExtent,
+                childCrossAxisExtent: childCrossAxisExtent,
+                reverseCrossAxis: AxisUtils.axisDirectionIsReversed(constraints.crossAxisDirection)
+            );
+        }
+
+        public override bool shouldRelayout(SliverGridDelegate _oldDelegate) {
+            SliverGridDelegateWithFixedCrossAxisCount oldDelegate =
+                _oldDelegate as SliverGridDelegateWithFixedCrossAxisCount;
+            return oldDelegate.crossAxisCount != this.crossAxisCount
+                   || oldDelegate.mainAxisSpacing != this.mainAxisSpacing
+                   || oldDelegate.crossAxisSpacing != this.crossAxisSpacing
+                   || oldDelegate.childAspectRatio != this.childAspectRatio;
+        }
+    }
+
+    public class SliverGridDelegateWithMaxCrossAxisExtent : SliverGridDelegate {
+        public SliverGridDelegateWithMaxCrossAxisExtent(
+            float maxCrossAxisExtent,
+            float mainAxisSpacing = 0.0f,
+            float crossAxisSpacing = 0.0f,
+            float childAspectRatio = 1.0f
+        ) {
+            D.assert(maxCrossAxisExtent >= 0);
+            D.assert(mainAxisSpacing >= 0);
+            D.assert(crossAxisSpacing >= 0);
+            D.assert(childAspectRatio > 0);
+            this.maxCrossAxisExtent = maxCrossAxisExtent;
+            this.mainAxisSpacing = mainAxisSpacing;
+            this.crossAxisSpacing = crossAxisSpacing;
+            this.childAspectRatio = childAspectRatio;
+        }
+
+        public readonly float maxCrossAxisExtent;
+
+        public readonly float mainAxisSpacing;
+
+        public readonly float crossAxisSpacing;
+
+        public readonly float childAspectRatio;
+
+        bool _debugAssertIsValid() {
+            D.assert(this.maxCrossAxisExtent > 0.0f);
+            D.assert(this.mainAxisSpacing >= 0.0f);
+            D.assert(this.crossAxisSpacing >= 0.0f);
+            D.assert(this.childAspectRatio > 0.0f);
+            return true;
+        }
+
+        public override SliverGridLayout getLayout(SliverConstraints constraints) {
+            D.assert(this._debugAssertIsValid());
+            int crossAxisCount =
+                (constraints.crossAxisExtent / (this.maxCrossAxisExtent + this.crossAxisSpacing)).ceil();
+            float usableCrossAxisExtent = constraints.crossAxisExtent - this.crossAxisSpacing * (crossAxisCount - 1);
+            float childCrossAxisExtent = usableCrossAxisExtent / crossAxisCount;
+            float childMainAxisExtent = childCrossAxisExtent / this.childAspectRatio;
+            return new SliverGridRegularTileLayout(
+                crossAxisCount: crossAxisCount,
+                mainAxisStride: childMainAxisExtent + this.mainAxisSpacing,
+                crossAxisStride: childCrossAxisExtent + this.crossAxisSpacing,
+                childMainAxisExtent: childMainAxisExtent,
+                childCrossAxisExtent: childCrossAxisExtent,
+                reverseCrossAxis: AxisUtils.axisDirectionIsReversed(constraints.crossAxisDirection)
+            );
+        }
+
+        public override bool shouldRelayout(SliverGridDelegate _oldDelegate) {
+            SliverGridDelegateWithMaxCrossAxisExtent oldDelegate =
+                _oldDelegate as SliverGridDelegateWithMaxCrossAxisExtent;
+            return oldDelegate.maxCrossAxisExtent != this.maxCrossAxisExtent
+                   || oldDelegate.mainAxisSpacing != this.mainAxisSpacing
+                   || oldDelegate.crossAxisSpacing != this.crossAxisSpacing
+                   || oldDelegate.childAspectRatio != this.childAspectRatio;
+        }
+    }
+
+    public class SliverGridParentData : SliverMultiBoxAdaptorParentData {
+        public float crossAxisOffset;
+
+        public override string ToString() {
+            return "crossAxisOffset=$crossAxisOffset; ${base.ToString()}";
+        }
+    }
+
+    public class RenderSliverGrid : RenderSliverMultiBoxAdaptor {
+        public RenderSliverGrid(
+            RenderSliverBoxChildManager childManager,
+            SliverGridDelegate gridDelegate
+        ) : base(childManager: childManager) {
+            D.assert(gridDelegate != null);
+            this._gridDelegate = gridDelegate;
+        }
+
+
+        public override void setupParentData(RenderObject child) {
+            if (!(child.parentData is SliverGridParentData)) {
+                child.parentData = new SliverGridParentData();
+            }
+        }
+
+        public SliverGridDelegate gridDelegate {
+            get { return this._gridDelegate; }
+            set {
+                D.assert(value != null);
+                if (this._gridDelegate == value) {
+                    return;
+                }
+
+                if (value.GetType() != this._gridDelegate.GetType() ||
+                    value.shouldRelayout(this._gridDelegate)) {
+                    this.markNeedsLayout();
+                }
+
+                this._gridDelegate = value;
+            }
+        }
+
+        SliverGridDelegate _gridDelegate;
+
+        public override float childCrossAxisPosition(RenderObject child) {
+            SliverGridParentData childParentData = (SliverGridParentData) child.parentData;
+            return childParentData.crossAxisOffset;
+        }
+
+        protected override void performLayout() {
+            this.childManager.didStartLayout();
+            this.childManager.setDidUnderflow(false);
+
+            float scrollOffset = this.constraints.scrollOffset + this.constraints.cacheOrigin;
+            D.assert(scrollOffset >= 0.0f);
+            float remainingExtent = this.constraints.remainingCacheExtent;
+            D.assert(remainingExtent >= 0.0f);
+            float targetEndScrollOffset = scrollOffset + remainingExtent;
+
+            SliverGridLayout layout = this._gridDelegate.getLayout(this.constraints);
+
+            int firstIndex = layout.getMinChildIndexForScrollOffset(scrollOffset);
+            int? targetLastIndex = targetEndScrollOffset.isFinite()
+                ? (int?) layout.getMaxChildIndexForScrollOffset(targetEndScrollOffset)
+                : null;
+
+            if (this.firstChild != null) {
+                int oldFirstIndex = this.indexOf(this.firstChild);
+                int oldLastIndex = this.indexOf(this.lastChild);
+                int leadingGarbage = (firstIndex - oldFirstIndex).clamp(0, this.childCount);
+                int trailingGarbage = targetLastIndex == null
+                    ? 0
+                    : ((oldLastIndex - targetLastIndex) ?? 0).clamp(0, this.childCount);
+                this.collectGarbage(leadingGarbage, trailingGarbage);
+            }
+            else {
+                this.collectGarbage(0, 0);
+            }
+
+            SliverGridGeometry firstChildGridGeometry = layout.getGeometryForChildIndex(firstIndex);
+            float? leadingScrollOffset = firstChildGridGeometry.scrollOffset;
+            float? trailingScrollOffset = firstChildGridGeometry.trailingScrollOffset;
+
+            if (this.firstChild == null) {
+                if (!this.addInitialChild(index: firstIndex,
+                    layoutOffset: firstChildGridGeometry.scrollOffset ?? 0.0f)) {
+                    float max = layout.computeMaxScrollOffset(this.childManager.childCount ?? 0);
+                    this.geometry = new SliverGeometry(
+                        scrollExtent: max,
+                        maxPaintExtent: max
+                    );
+                    this.childManager.didFinishLayout();
+                    return;
+                }
+            }
+
+            RenderBox trailingChildWithLayout = null;
+
+            for (int index = this.indexOf(this.firstChild) - 1; index >= firstIndex; --index) {
+                SliverGridGeometry gridGeometry = layout.getGeometryForChildIndex(index);
+                RenderBox child = this.insertAndLayoutLeadingChild(
+                    gridGeometry.getBoxConstraints(this.constraints)
+                );
+                SliverGridParentData childParentData = child.parentData as SliverGridParentData;
+                childParentData.layoutOffset = gridGeometry.scrollOffset ?? 0.0f;
+                childParentData.crossAxisOffset = gridGeometry.crossAxisOffset ?? 0.0f;
+                D.assert(childParentData.index == index);
+                trailingChildWithLayout = trailingChildWithLayout ?? child;
+                trailingScrollOffset =
+                    Mathf.Max(trailingScrollOffset ?? 0.0f, gridGeometry.trailingScrollOffset ?? 0.0f);
+            }
+
+            if (trailingChildWithLayout == null) {
+                this.firstChild.layout(firstChildGridGeometry.getBoxConstraints(this.constraints));
+                SliverGridParentData childParentData = this.firstChild.parentData as SliverGridParentData;
+                childParentData.layoutOffset = firstChildGridGeometry.scrollOffset ?? 0.0f;
+                childParentData.crossAxisOffset = firstChildGridGeometry.crossAxisOffset ?? 0.0f;
+                trailingChildWithLayout = this.firstChild;
+            }
+
+            for (int index = this.indexOf(trailingChildWithLayout) + 1;
+                targetLastIndex == null || index <= targetLastIndex;
+                ++index) {
+                SliverGridGeometry gridGeometry = layout.getGeometryForChildIndex(index);
+                BoxConstraints childConstraints = gridGeometry.getBoxConstraints(this.constraints);
+                RenderBox child = this.childAfter(trailingChildWithLayout);
+                if (child == null) {
+                    child = this.insertAndLayoutChild(childConstraints, after: trailingChildWithLayout);
+                    if (child == null) {
+                        break;
+                    }
+                }
+                else {
+                    child.layout(childConstraints);
+                }
+
+                trailingChildWithLayout = child;
+                D.assert(child != null);
+                SliverGridParentData childParentData = child.parentData as SliverGridParentData;
+                childParentData.layoutOffset = gridGeometry.scrollOffset ?? 0.0f;
+                childParentData.crossAxisOffset = gridGeometry.crossAxisOffset ?? 0.0f;
+                D.assert(childParentData.index == index);
+                trailingScrollOffset =
+                    Mathf.Max(trailingScrollOffset ?? 0.0f, gridGeometry.trailingScrollOffset ?? 0.0f);
+            }
+
+            int lastIndex = this.indexOf(this.lastChild);
+
+            D.assert(this.childScrollOffset(this.firstChild) <= scrollOffset);
+            D.assert(this.debugAssertChildListIsNonEmptyAndContiguous());
+            D.assert(this.indexOf(this.firstChild) == firstIndex);
+            D.assert(targetLastIndex == null || lastIndex <= targetLastIndex);
+
+            float estimatedTotalExtent = this.childManager.estimateMaxScrollOffset(this.constraints,
+                firstIndex: firstIndex,
+                lastIndex: lastIndex,
+                leadingScrollOffset: leadingScrollOffset ?? 0.0f,
+                trailingScrollOffset: trailingScrollOffset ?? 0.0f
+            );
+
+            float paintExtent = this.calculatePaintOffset(this.constraints,
+                from: leadingScrollOffset ?? 0.0f,
+                to: trailingScrollOffset ?? 0.0f
+            );
+            float cacheExtent = this.calculateCacheOffset(this.constraints,
+                from: leadingScrollOffset ?? 0.0f,
+                to: trailingScrollOffset ?? 0.0f
+            );
+
+            this.geometry = new SliverGeometry(
+                scrollExtent: estimatedTotalExtent,
+                paintExtent: paintExtent,
+                maxPaintExtent: estimatedTotalExtent,
+                cacheExtent: cacheExtent,
+                hasVisualOverflow: true
+            );
+
+            if (estimatedTotalExtent == trailingScrollOffset) {
+                this.childManager.setDidUnderflow(true);
+            }
+
+            this.childManager.didFinishLayout();
+        }
+    }
+}

Runtime/rendering/sliver_grid.cs.meta

+fileFormatVersion: 2
+guid: 5ba43ca1896804e9b8ca548384ee2abc
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

Runtime/widgets/heroes.cs

+using System;
+using System.Collections.Generic;
+using Unity.UIWidgets.animation;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.rendering;
+using Unity.UIWidgets.ui;
+
+namespace Unity.UIWidgets.widgets {
+    public delegate Tween<Rect> CreateRectTween(Rect begin, Rect end);
+
+    public delegate Widget HeroFlightShuttleBuilder(
+        BuildContext flightContext,
+        Animation<float> animation,
+        HeroFlightDirection flightDirection,
+        BuildContext fromHeroContext,
+        BuildContext toHeroContext
+    );
+
+    delegate void _OnFlightEnded(_HeroFlight flight);
+
+    public enum HeroFlightDirection {
+        push,
+        pop
+    }
+
+    class HeroUtils {
+        public static Rect _globalBoundingBoxFor(BuildContext context) {
+            RenderBox box = (RenderBox) context.findRenderObject();
+            D.assert(box != null && box.hasSize);
+            return box.getTransformTo(null).mapRect(Offset.zero & box.size);
+        }
+    }
+
+
+    public class Hero : StatefulWidget {
+        public Hero(
+            Key key = null,
+            object tag = null,
+            CreateRectTween createRectTween = null,
+            HeroFlightShuttleBuilder flightShuttleBuilder = null,
+            TransitionBuilder placeholderBuilder = null,
+            bool transitionOnUserGestures = false,
+            Widget child = null
+        ) : base(key: key) {
+            D.assert(tag != null);
+            D.assert(child != null);
+            this.tag = tag;
+            this.createRectTween = createRectTween;
+            this.child = child;
+            this.flightShuttleBuilder = flightShuttleBuilder;
+            this.placeholderBuilder = placeholderBuilder;
+            this.transitionOnUserGestures = transitionOnUserGestures;
+        }
+
+
+        public readonly object tag;
+
+        public readonly CreateRectTween createRectTween;
+
+        public readonly Widget child;
+
+        public readonly HeroFlightShuttleBuilder flightShuttleBuilder;
+
+        public readonly TransitionBuilder placeholderBuilder;
+
+        public readonly bool transitionOnUserGestures;
+
+        internal static Dictionary<object, _HeroState>
+            _allHeroesFor(BuildContext context, bool isUserGestureTransition) {
+            D.assert(context != null);
+            Dictionary<object, _HeroState> result = new Dictionary<object, _HeroState> { };
+
+            void visitor(Element element) {
+                if (element.widget is Hero) {
+                    StatefulElement hero = (StatefulElement) element;
+                    Hero heroWidget = (Hero) element.widget;
+                    if (!isUserGestureTransition || heroWidget.transitionOnUserGestures) {
+                        object tag = heroWidget.tag;
+                        D.assert(tag != null);
+                        D.assert(() => {
+                            if (result.ContainsKey(tag)) {
+                                throw new UIWidgetsError(
+                                    "There are multiple heroes that share the same tag within a subtree.\n" +
+                                    "Within each subtree for which heroes are to be animated (typically a PageRoute subtree), " +
+                                    "each Hero must have a unique non-null tag.\n" +
+                                    $"In this case, multiple heroes had the following tag: {tag}\n" +
+                                    "Here is the subtree for one of the offending heroes:\n" +
+                                    $"{element.toStringDeep(prefixLineOne: "# ")}"
+                                );
+                            }
+
+                            return true;
+                        });
+                        _HeroState heroState = (_HeroState) hero.state;
+                        result[tag] = heroState;
+                    }
+                }
+
+                element.visitChildren(visitor);
+            }
+
+            context.visitChildElements(visitor);
+            return result;
+        }
+
+        public override State createState() {
+            return new _HeroState();
+        }
+
+        public override void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+            base.debugFillProperties(properties);
+            properties.add(new DiagnosticsProperty<object>("tag", this.tag));
+        }
+    }
+
+    class _HeroState : State<Hero> {
+        GlobalKey _key = GlobalKey.key();
+        Size _placeholderSize;
+
+        public void startFlight() {
+            D.assert(this.mounted);
+            RenderBox box = (RenderBox) this.context.findRenderObject();
+            D.assert(box != null && box.hasSize);
+            this.setState(() => { this._placeholderSize = box.size; });
+        }
+
+        public void endFlight() {
+            if (this.mounted) {
+                this.setState(() => { this._placeholderSize = null; });
+            }
+        }
+
+        public override Widget build(BuildContext context) {
+            if (this._placeholderSize != null) {
+                if (this.widget.placeholderBuilder == null) {
+                    return new SizedBox(
+                        width: this._placeholderSize.width,
+                        height: this._placeholderSize.height
+                    );
+                }
+                else {
+                    return this.widget.placeholderBuilder(context, this.widget.child);
+                }
+            }
+
+            return new KeyedSubtree(
+                key: this._key,
+                child: this.widget.child
+            );
+        }
+    }
+
+    class _HeroFlightManifest {
+        public _HeroFlightManifest(
+            HeroFlightDirection type,
+            OverlayState overlay,
+            Rect navigatorRect,
+            PageRoute fromRoute,
+            PageRoute toRoute,
+            _HeroState fromHero,
+            _HeroState toHero,
+            CreateRectTween createRectTween,
+            HeroFlightShuttleBuilder shuttleBuilder,
+            bool isUserGestureTransition
+        ) {
+            D.assert(this.fromHero.widget.tag == this.toHero.widget.tag);
+            this.type = type;
+            this.overlay = overlay;
+            this.navigatorRect = navigatorRect;
+            this.fromRoute = fromRoute;
+            this.toRoute = toRoute;
+            this.fromHero = fromHero;
+            this.toHero = toHero;
+            this.createRectTween = createRectTween;
+            this.shuttleBuilder = shuttleBuilder;
+            this.isUserGestureTransition = isUserGestureTransition;
+        }
+
+        public readonly HeroFlightDirection type;
+        public readonly OverlayState overlay;
+        public readonly Rect navigatorRect;
+        public readonly PageRoute fromRoute;
+        public readonly PageRoute toRoute;
+        public readonly _HeroState fromHero;
+        public readonly _HeroState toHero;
+        public readonly CreateRectTween createRectTween;
+        public readonly HeroFlightShuttleBuilder shuttleBuilder;
+        public readonly bool isUserGestureTransition;
+
+        public object tag {
+            get { return this.fromHero.widget.tag; }
+        }
+
+        public Animation<float> animation {
+            get {
+                return new CurvedAnimation(
+                    parent: (this.type == HeroFlightDirection.push) ? this.toRoute.animation : this.fromRoute.animation,
+                    curve: Curves.fastOutSlowIn
+                );
+            }
+        }
+
+        public override string ToString() {
+            return $"_HeroFlightManifest($type tag: $tag from route: {this.fromRoute.settings} " +
+                   $"to route: {this.toRoute.settings} with hero: {this.fromHero} to {this.toHero})";
+        }
+    }
+
+    class _HeroFlight {
+        public _HeroFlight(_OnFlightEnded onFlightEnded) {
+            this.onFlightEnded = onFlightEnded;
+            this._proxyAnimation = new ProxyAnimation();
+            this._proxyAnimation.addStatusListener(this._handleAnimationUpdate);
+        }
+
+        public readonly _OnFlightEnded onFlightEnded;
+
+        Tween<Rect> heroRectTween;
+        Widget shuttle;
+
+        Animation<float> _heroOpacity = Animations.kAlwaysCompleteAnimation;
+        ProxyAnimation _proxyAnimation;
+        public _HeroFlightManifest manifest;
+        public OverlayEntry overlayEntry;
+        bool _aborted = false;
+
+        Tween<Rect> _doCreateRectTween(Rect begin, Rect end) {
+            CreateRectTween createRectTween =
+                this.manifest.toHero.widget.createRectTween ?? this.manifest.createRectTween;
+            if (createRectTween != null) {
+                return createRectTween(begin, end);
+            }
+
+            return new RectTween(begin: begin, end: end);
+        }
+
+        static readonly Animatable<float> _reverseTween = new FloatTween(begin: 1.0f, end: 0.0f);
+
+        Widget _buildOverlay(BuildContext context) {
+            D.assert(this.manifest != null);
+            this.shuttle = this.shuttle ?? this.manifest.shuttleBuilder(
+                               context, this.manifest.animation, this.manifest.type, this.manifest.fromHero.context,
+                               this.manifest.toHero.context
+                           );
+            D.assert(this.shuttle != null);
+
+            return new AnimatedBuilder(
+                animation: this._proxyAnimation,
+                child: this.shuttle,
+                builder: (BuildContext _, Widget child) => {
+                    RenderBox toHeroBox = (RenderBox) this.manifest.toHero.context?.findRenderObject();
+                    if (this._aborted || toHeroBox == null || !toHeroBox.attached) {
+                        if (this._heroOpacity.isCompleted) {
+                            this._heroOpacity = this._proxyAnimation.drive(
+                                _reverseTween.chain(
+                                    new CurveTween(curve: new Interval(this._proxyAnimation.value, 1.0f)))
+                            );
+                        }
+                    }
+                    else if (toHeroBox.hasSize) {
+                        RenderBox finalRouteBox = (RenderBox) this.manifest.toRoute.subtreeContext?.findRenderObject();
+                        Offset toHeroOrigin = toHeroBox.localToGlobal(Offset.zero, ancestor: finalRouteBox);
+                        if (toHeroOrigin != this.heroRectTween.end.topLeft) {
+                            Rect heroRectEnd = toHeroOrigin & this.heroRectTween.end.size;
+                            this.heroRectTween = this._doCreateRectTween(this.heroRectTween.begin, heroRectEnd);
+                        }
+                    }
+
+                    Rect rect = this.heroRectTween.evaluate(this._proxyAnimation);
+                    Size size = this.manifest.navigatorRect.size;
+                    RelativeRect offsets = RelativeRect.fromSize(rect, size);
+
+                    return new Positioned(
+                        top: offsets.top,
+                        right: offsets.right,
+                        bottom: offsets.bottom,
+                        left: offsets.left,
+                        child: new IgnorePointer(
+                            child: new RepaintBoundary(
+                                child: new Opacity(
+                                    opacity: this._heroOpacity.value,
+                                    child: child
+                                )
+                            )
+                        )
+                    );
+                }
+            );
+        }
+
+        void _handleAnimationUpdate(AnimationStatus status) {
+            if (status == AnimationStatus.completed || status == AnimationStatus.dismissed) {
+                this._proxyAnimation.parent = null;
+
+                D.assert(this.overlayEntry != null);
+                this.overlayEntry.remove();
+                this.overlayEntry = null;
+
+                this.manifest.fromHero.endFlight();
+                this.manifest.toHero.endFlight();
+                this.onFlightEnded(this);
+            }
+        }
+
+        public void start(_HeroFlightManifest initialManifest) {
+            D.assert(!this._aborted);
+            D.assert(() => {
+                Animation<float> initial = initialManifest.animation;
+                D.assert(initial != null);
+                HeroFlightDirection type = initialManifest.type;
+                switch (type) {
+                    case HeroFlightDirection.pop:
+                        return initial.value == 1.0f && initialManifest.isUserGestureTransition
+                            ? initial.status == AnimationStatus.completed
+                            : initial.status == AnimationStatus.reverse;
+                    case HeroFlightDirection.push:
+                        return initial.value == 0.0f && initial.status == AnimationStatus.forward;
+                }
+
+                throw new Exception("Unknown type: " + type);
+            });
+
+            this.manifest = initialManifest;
+
+            if (this.manifest.type == HeroFlightDirection.pop) {
+                this._proxyAnimation.parent = new ReverseAnimation(this.manifest.animation);
+            }
+            else {
+                this._proxyAnimation.parent = this.manifest.animation;
+            }
+
+            this.manifest.fromHero.startFlight();
+            this.manifest.toHero.startFlight();
+
+            this.heroRectTween = this._doCreateRectTween(
+                HeroUtils._globalBoundingBoxFor(this.manifest.fromHero.context),
+                HeroUtils._globalBoundingBoxFor(this.manifest.toHero.context)
+            );
+
+            this.overlayEntry = new OverlayEntry(builder: this._buildOverlay);
+            this.manifest.overlay.insert(this.overlayEntry);
+        }
+
+        public void divert(_HeroFlightManifest newManifest) {
+            D.assert(this.manifest.tag == newManifest.tag);
+
+            if (this.manifest.type == HeroFlightDirection.push && newManifest.type == HeroFlightDirection.pop) {
+                D.assert(newManifest.animation.status == AnimationStatus.reverse);
+                D.assert(this.manifest.fromHero == newManifest.toHero);
+                D.assert(this.manifest.toHero == newManifest.fromHero);
+                D.assert(this.manifest.fromRoute == newManifest.toRoute);
+                D.assert(this.manifest.toRoute == newManifest.fromRoute);
+
+                this._proxyAnimation.parent = new ReverseAnimation(newManifest.animation);
+                this.heroRectTween = new ReverseTween<Rect>(this.heroRectTween);
+            }
+            else if (this.manifest.type == HeroFlightDirection.pop && newManifest.type == HeroFlightDirection.push) {
+                D.assert(newManifest.animation.status == AnimationStatus.forward);
+                D.assert(this.manifest.toHero == newManifest.fromHero);
+                D.assert(this.manifest.toRoute == newManifest.fromRoute);
+
+                this._proxyAnimation.parent = newManifest.animation.drive(
+                    new FloatTween(
+                        begin: this.manifest.animation.value,
+                        end: 1.0f
+                    )
+                );
+
+                if (this.manifest.fromHero != newManifest.toHero) {
+                    this.manifest.fromHero.endFlight();
+                    newManifest.toHero.startFlight();
+                    this.heroRectTween = this._doCreateRectTween(this.heroRectTween.end,
+                        HeroUtils._globalBoundingBoxFor(newManifest.toHero.context));
+                }
+                else {
+                    this.heroRectTween = this._doCreateRectTween(this.heroRectTween.end, this.heroRectTween.begin);
+                }
+            }
+            else {
+                D.assert(this.manifest.fromHero != newManifest.fromHero);
+                D.assert(this.manifest.toHero != newManifest.toHero);
+
+                this.heroRectTween = this._doCreateRectTween(this.heroRectTween.evaluate(this._proxyAnimation),
+                    HeroUtils._globalBoundingBoxFor(newManifest.toHero.context));
+                this.shuttle = null;
+
+                if (newManifest.type == HeroFlightDirection.pop) {
+                    this._proxyAnimation.parent = new ReverseAnimation(newManifest.animation);
+                }
+                else {
+                    this._proxyAnimation.parent = newManifest.animation;
+                }
+
+                this.manifest.fromHero.endFlight();
+                this.manifest.toHero.endFlight();
+
+                newManifest.fromHero.startFlight();
+                newManifest.toHero.startFlight();
+
+                this.overlayEntry.markNeedsBuild();
+            }
+
+            this._aborted = false;
+            this.manifest = newManifest;
+        }
+
+        public void abort() {
+            this._aborted = true;
+        }
+
+        public override string ToString() {
+            RouteSettings from = this.manifest.fromRoute.settings;
+            RouteSettings to = this.manifest.toRoute.settings;
+            object tag = this.manifest.tag;
+            return "HeroFlight(for: $tag, from: $from, to: $to ${_proxyAnimation.parent})";
+        }
+    }
+
+    public class HeroController : NavigatorObserver {
+        HeroController(CreateRectTween createRectTween) {
+            this.createRectTween = createRectTween;
+        }
+
+        public readonly CreateRectTween createRectTween;
+
+        Dictionary<object, _HeroFlight> _flights = new Dictionary<object, _HeroFlight>();
+
+        public override void didPush(Route route, Route previousRoute) {
+            D.assert(this.navigator != null);
+            D.assert(route != null);
+            this._maybeStartHeroTransition(previousRoute, route, HeroFlightDirection.push, false);
+        }
+
+        public override void didPop(Route route, Route previousRoute) {
+            D.assert(this.navigator != null);
+            D.assert(route != null);
+            this._maybeStartHeroTransition(route, previousRoute, HeroFlightDirection.pop, false);
+        }
+
+        public override void didStartUserGesture(Route route, Route previousRoute) {
+            D.assert(this.navigator != null);
+            D.assert(route != null);
+            this._maybeStartHeroTransition(route, previousRoute, HeroFlightDirection.pop, true);
+        }
+
+        void _maybeStartHeroTransition(
+            Route fromRoute,
+            Route toRoute,
+            HeroFlightDirection flightType,
+            bool isUserGestureTransition
+        ) {
+            if (toRoute != fromRoute && toRoute is PageRoute && fromRoute is PageRoute) {
+                PageRoute from = (PageRoute) fromRoute;
+                PageRoute to = (PageRoute) toRoute;
+                Animation<float> animation = (flightType == HeroFlightDirection.push) ? to.animation : from.animation;
+
+                switch (flightType) {
+                    case HeroFlightDirection.pop:
+                        if (animation.value == 0.0f) {
+                            return;
+                        }
+
+                        break;
+                    case HeroFlightDirection.push:
+                        if (animation.value == 1.0f) {
+                            return;
+                        }
+
+                        break;
+                }
+
+                if (isUserGestureTransition && flightType == HeroFlightDirection.pop && to.maintainState) {
+                    this._startHeroTransition(from, to, animation, flightType, isUserGestureTransition);
+                }
+                else {
+                    to.offstage = to.animation.value == 0.0f;
+
+                    WidgetsBinding.instance.addPostFrameCallback((TimeSpan value) => {
+                        this._startHeroTransition(from, to, animation, flightType, isUserGestureTransition);
+                    });
+                }
+            }
+        }
+
+        void _startHeroTransition(
+            PageRoute from,
+            PageRoute to,
+            Animation<float> animation,
+            HeroFlightDirection flightType,
+            bool isUserGestureTransition
+        ) {
+            if (this.navigator == null || from.subtreeContext == null || to.subtreeContext == null) {
+                to.offstage = false; // in case we set this in _maybeStartHeroTransition
+                return;
+            }
+
+            Rect navigatorRect = HeroUtils._globalBoundingBoxFor(this.navigator.context);
+
+            Dictionary<object, _HeroState>
+                fromHeroes = Hero._allHeroesFor(from.subtreeContext, isUserGestureTransition);
+            Dictionary<object, _HeroState> toHeroes = Hero._allHeroesFor(to.subtreeContext, isUserGestureTransition);
+
+            to.offstage = false;
+
+            foreach (object tag in fromHeroes.Keys) {
+                if (toHeroes[tag] != null) {
+                    HeroFlightShuttleBuilder fromShuttleBuilder = fromHeroes[tag].widget.flightShuttleBuilder;
+                    HeroFlightShuttleBuilder toShuttleBuilder = toHeroes[tag].widget.flightShuttleBuilder;
+
+                    _HeroFlightManifest manifest = new _HeroFlightManifest(
+                        type: flightType,
+                        overlay: this.navigator.overlay,
+                        navigatorRect: navigatorRect,
+                        fromRoute: from,
+                        toRoute: to,
+                        fromHero: fromHeroes[tag],
+                        toHero: toHeroes[tag],
+                        createRectTween: this.createRectTween,
+                        shuttleBuilder:
+                        toShuttleBuilder ?? fromShuttleBuilder ?? _defaultHeroFlightShuttleBuilder,
+                        isUserGestureTransition: isUserGestureTransition
+                    );
+
+                    if (this._flights[tag] != null) {
+                        this._flights[tag].divert(manifest);
+                    }
+                    else {
+                        this._flights[tag] = new _HeroFlight(this._handleFlightEnded);
+                        this._flights[tag].start(manifest);
+                    }
+                }
+                else if (this._flights[tag] != null) {
+                    this._flights[tag].abort();
+                }
+            }
+        }
+
+        void _handleFlightEnded(_HeroFlight flight) {
+            this._flights.Remove(flight.manifest.tag);
+        }
+
+        static readonly HeroFlightShuttleBuilder _defaultHeroFlightShuttleBuilder = (
+            BuildContext flightContext,
+            Animation<float> animation,
+            HeroFlightDirection flightDirection,
+            BuildContext fromHeroContext,
+            BuildContext toHeroContext
+        ) => {
+            Hero toHero = (Hero) toHeroContext.widget;
+            return toHero.child;
+        };
+    }
+}

Runtime/widgets/heroes.cs.meta

+fileFormatVersion: 2
+guid: ae21d125110c4c989fd2ce9e5a0bc1c0
+timeCreated: 1554263028

Samples/UIWidgetsGallery/demo/shrine.meta

+fileFormatVersion: 2
+guid: 2621a6f94ad8a4b5ab7199025967dc90
+folderAsset: yes
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

Samples/UIWidgetsGallery/demo/shrine_demo.cs

+using Unity.UIWidgets.animation;
+using Unity.UIWidgets.material;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+
+namespace UIWidgetsGallery.gallery {
+    class ShrineDemoUtils {
+        public static Widget buildShrine(BuildContext context, Widget child) {
+            return new Theme(
+                data: new ThemeData(
+                    primarySwatch: Colors.grey,
+                    iconTheme: new IconThemeData(color: new Color(0xFF707070)),
+                    platform: Theme.of(context).platform
+                ),
+                child: new ShrineTheme(child: child)
+            );
+        }
+    }
+
+    public class ShrinePageRoute<T> : MaterialPageRoute {
+        public ShrinePageRoute(
+            WidgetBuilder builder,
+            RouteSettings settings
+        ) : base(builder: builder, settings: settings) {
+        }
+
+        public override Widget buildPage(BuildContext context, Animation<float> animation,
+            Animation<float> secondaryAnimation) {
+            return ShrineDemoUtils.buildShrine(context, base.buildPage(context, animation, secondaryAnimation));
+        }
+    }
+
+    public class ShrineDemo : StatelessWidget {
+        public const string routeName = "/shrine";
+
+        public override Widget build(BuildContext context) {
+            return ShrineDemoUtils.buildShrine(context, new ShrineHome());
+        }
+    }
+}

Samples/UIWidgetsGallery/demo/shrine_demo.cs.meta

+fileFormatVersion: 2
+guid: 5902905cca7447d2aa1fe5ae7f6c5cbc
+timeCreated: 1553239620

Tests/Resources/people.meta

+fileFormatVersion: 2
+guid: 8988da6435b0e414da67d5bb45802754
+folderAsset: yes
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

Samples/UIWidgetsGallery/demo/shrine/shrine_data.cs

+using System.Collections.Generic;
+using System.Linq;
+using Unity.UIWidgets.foundation;
+
+namespace UIWidgetsGallery.gallery {
+    class ShrineData {
+        const string _kGalleryAssetsPackage = "flutter_gallery_assets";
+
+        static Vendor _ali = new Vendor(
+            name: "Ali’s shop",
+            avatarAsset: "people/square/ali",
+            avatarAssetPackage: _kGalleryAssetsPackage,
+            description:
+            "Ali Connor’s makes custom goods for folks of all shapes and sizes " +
+            "made by hand and sometimes by machine, but always with love and care. " +
+            "Custom orders are available upon request if you need something extra special."
+        );
+
+        static Vendor _peter = new Vendor(
+            name: "Peter’s shop",
+            avatarAsset: "people/square/peter",
+            avatarAssetPackage: _kGalleryAssetsPackage,
+            description:
+            "Peter makes great stuff for awesome people like you. Super cool and extra " +
+            "awesome all of his shop’s goods are handmade with love. Custom orders are " +
+            "available upon request if you need something extra special."
+        );
+
+        static Vendor _sandra = new Vendor(
+            name: "Sandra’s shop",
+            avatarAsset: "people/square/sandra",
+            avatarAssetPackage: _kGalleryAssetsPackage,
+            description:
+            "Sandra specializes in furniture, beauty and travel products with a classic vibe. " +
+            "Custom orders are available if you’re looking for a certain color or material."
+        );
+
+        static Vendor _stella = new Vendor(
+            name: "Stella’s shop",
+            avatarAsset: "people/square/stella",
+            avatarAssetPackage: _kGalleryAssetsPackage,
+            description:
+            "Stella sells awesome stuff at lovely prices. made by hand and sometimes by " +
+            "machine, but always with love and care. Custom orders are available upon request " +
+            "if you need something extra special."
+        );
+
+        static Vendor _trevor = new Vendor(
+            name: "Trevor’s shop",
+            avatarAsset: "people/square/trevor",
+            avatarAssetPackage: _kGalleryAssetsPackage,
+            description:
+            "Trevor makes great stuff for awesome people like you. Super cool and extra " +
+            "awesome all of his shop’s goods are handmade with love. Custom orders are " +
+            "available upon request if you need something extra special."
+        );
+
+        static readonly List<Product> _allProducts = new List<Product> {
+            new Product(
+                name: "Vintage Brown Belt",
+                imageAsset: "products/belt",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"fashion", "latest"},
+                price: 300.00f,
+                vendor: _sandra,
+                description:
+                "Isn’t it cool when things look old, but they're not. Looks Old But Not makes " +
+                "awesome vintage goods that are base smart. This ol’ belt just got an upgrade. "
+            ),
+            new Product(
+                name: "Sunglasses",
+                imageAsset: "products/sunnies",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"travel", "fashion", "beauty"},
+                price: 20.00f,
+                vendor: _trevor,
+                description:
+                "Be an optimist. Carry Sunglasses with you at all times. All Tints and " +
+                "Shades products come with polarized lenses and base duper UV protection " +
+                "so you can look at the sun for however long you want. Sunglasses make you " +
+                "look cool, wear them."
+            ),
+            new Product(
+                name: "Flatwear",
+                imageAsset: "products/flatwear",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"furniture"},
+                price: 30.00f,
+                vendor: _trevor,
+                description:
+                "Leave the tunnel and the rain is fallin amazing things happen when you wait"
+            ),
+            new Product(
+                name: "Salmon Sweater",
+                imageAsset: "products/sweater",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"fashion"},
+                price: 300.00f,
+                vendor: _stella,
+                description:
+                "Looks can be deceiving. This sweater comes in a wide variety of " +
+                "flavors, including salmon, that pop as soon as they hit your eyes. " +
+                "Sweaters heat quickly, so savor the warmth."
+            ),
+            new Product(
+                name: "Pine Table",
+                imageAsset: "products/table",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"furniture"},
+                price: 63.00f,
+                vendor: _stella,
+                description:
+                "Leave the tunnel and the rain is fallin amazing things happen when you wait"
+            ),
+            new Product(
+                name: "Green Comfort Jacket",
+                imageAsset: "products/jacket",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"fashion"},
+                price: 36.00f,
+                vendor: _ali,
+                description:
+                "Leave the tunnel and the rain is fallin amazing things happen when you wait"
+            ),
+            new Product(
+                name: "Chambray Top",
+                imageAsset: "products/top",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"fashion"},
+                price: 125.00f,
+                vendor: _peter,
+                description:
+                "Leave the tunnel and the rain is fallin amazing things happen when you wait"
+            ),
+            new Product(
+                name: "Blue Cup",
+                imageAsset: "products/cup",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"travel", "furniture"},
+                price: 75.00f,
+                vendor: _sandra,
+                description:
+                "Drinksy has been making extraordinary mugs for decades. With each " +
+                "cup purchased Drinksy donates a cup to those in need. Buy yourself a mug, " +
+                "buy someone else a mug."
+            ),
+            new Product(
+                name: "Tea Set",
+                imageAsset: "products/teaset",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"furniture", "fashion"},
+                price: 70.00f,
+                vendor: _trevor,
+                featureTitle: "Beautiful glass teapot",
+                featureDescription:
+                "Teapot holds extremely hot liquids and pours them from the spout.",
+                description:
+                "Impress your guests with Tea Set by Kitchen Stuff. Teapot holds extremely " +
+                "hot liquids and pours them from the spout. Use the handle, shown on the right, " +
+                "so your fingers don’t get burnt while pouring."
+            ),
+            new Product(
+                name: "Blue linen napkins",
+                imageAsset: "products/napkins",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"furniture", "fashion"},
+                price: 89.00f,
+                vendor: _trevor,
+                description:
+                "Blue linen napkins were meant to go with friends, so you may want to pick " +
+                "up a bunch of these. These things are absorbant."
+            ),
+            new Product(
+                name: "Dipped Earrings",
+                imageAsset: "products/earrings",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"fashion", "beauty"},
+                price: 25.00f,
+                vendor: _stella,
+                description:
+                "WeDipIt does it again. These hand-dipped 4 inch earrings are perfect for " +
+                "the office or the beach. Just be sure you don’t drop it in a bucket of " +
+                "red paint, then they won’t look dipped anymore."
+            ),
+            new Product(
+                name: "Perfect Planters",
+                imageAsset: "products/planters",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"latest", "furniture"},
+                price: 30.00f,
+                vendor: _ali,
+                description:
+                "The Perfect Planter Co makes the best vessels for just about anything you " +
+                "can pot. This set of Perfect Planters holds succulents and cuttings perfectly. " +
+                "Looks great in any room. Keep out of reach from cats."
+            ),
+            new Product(
+                name: "Cloud-White Dress",
+                imageAsset: "products/dress",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"fashion"},
+                price: 54.00f,
+                vendor: _sandra,
+                description:
+                "Trying to find the perfect outift to match your mood? Try no longer. " +
+                "This Cloud-White Dress has you covered for those nights when you need " +
+                "to get out, or even if you’re just headed to work."
+            ),
+            new Product(
+                name: "Backpack",
+                imageAsset: "products/backpack",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"travel", "fashion"},
+                price: 25.00f,
+                vendor: _peter,
+                description:
+                "This backpack by Bags ‘n’ stuff can hold just about anything: a laptop, " +
+                "a pen, a protractor, notebooks, small animals, plugs for your devices, " +
+                "sunglasses, gym clothes, shoes, gloves, two kittens, and even lunch!"
+            ),
+            new Product(
+                name: "Charcoal Straw Hat",
+                imageAsset: "products/hat",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"travel", "fashion", "latest"},
+                price: 25.00f,
+                vendor: _ali,
+                description:
+                "This is the  helmet for those warm summer days on the road. " +
+                "Jetset approved, these hats have been rigorously tested. Keep that face " +
+                "protected from the sun."
+            ),
+            new Product(
+                name: "Ginger Scarf",
+                imageAsset: "products/scarf",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"latest", "fashion"},
+                price: 17.00f,
+                vendor: _peter,
+                description:
+                "Leave the tunnel and the rain is fallin amazing things happen when you wait"
+            ),
+            new Product(
+                name: "Blush Sweats",
+                imageAsset: "products/sweats",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"travel", "fashion", "latest"},
+                price: 25.00f,
+                vendor: _stella,
+                description:
+                "Leave the tunnel and the rain is fallin amazing things happen when you wait"
+            ),
+            new Product(
+                name: "Mint Jumper",
+                imageAsset: "products/jumper",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"travel", "fashion", "beauty"},
+                price: 25.00f,
+                vendor: _peter,
+                description:
+                "Leave the tunnel and the rain is fallin amazing things happen when you wait"
+            ),
+            new Product(
+                name: "Ochre Shirt",
+                imageAsset: "products/shirt",
+                imageAssetPackage: _kGalleryAssetsPackage,
+                categories: new List<string> {"fashion", "latest"},
+                price: 120.00f,
+                vendor: _stella,
+                description:
+                "Leave the tunnel and the rain is fallin amazing things happen when you wait"
+            )
+        };
+
+        public static List<Product> allProducts() {
+            D.assert(_allProducts.All((Product product) => product.isValid()));
+            return new List<Product>(_allProducts);
+        }
+    }
+}

Samples/UIWidgetsGallery/demo/shrine/shrine_data.cs.meta

+fileFormatVersion: 2
+guid: f29bb63cff3148cbb3ef3be8196d5709
+timeCreated: 1553240837

Samples/UIWidgetsGallery/demo/shrine/shrine_home.cs

+using System.Collections.Generic;
+using System.Linq;
+using com.unity.uiwidgets.Runtime.rendering;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.gestures;
+using Unity.UIWidgets.material;
+using Unity.UIWidgets.painting;
+using Unity.UIWidgets.rendering;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+using Constants = Unity.UIWidgets.material.Constants;
+using Image = Unity.UIWidgets.widgets.Image;
+
+namespace UIWidgetsGallery.gallery {
+    class ShrineHomeUtils {
+        public const float unitSize = Constants.kToolbarHeight;
+
+        public static readonly List<Product> _products = new List<Product>(ShrineData.allProducts());
+        public static readonly Dictionary<Product, Order> _shoppingCart = new Dictionary<Product, Order>();
+
+        public const int _childrenPerBlock = 8;
+        public const int _rowsPerBlock = 5;
+
+        public static int _minIndexInRow(int rowIndex) {
+            int blockIndex = rowIndex / _rowsPerBlock;
+            return new List<int> {0, 2, 4, 6, 7}[rowIndex % _rowsPerBlock] + blockIndex * _childrenPerBlock;
+        }
+
+        public static int _maxIndexInRow(int rowIndex) {
+            int blockIndex = rowIndex / _rowsPerBlock;
+            return new List<int> {1, 3, 5, 6, 7}[rowIndex % _rowsPerBlock] + blockIndex * _childrenPerBlock;
+        }
+
+        public static int _rowAtIndex(int index) {
+            int blockCount = index / _childrenPerBlock;
+            return new List<int> {0, 0, 1, 1, 2, 2, 3, 4}[index - blockCount * _childrenPerBlock] +
+                   blockCount * _rowsPerBlock;
+        }
+
+        public static int _columnAtIndex(int index) {
+            return new List<int> {0, 1, 0, 1, 0, 1, 0, 0}[index % _childrenPerBlock];
+        }
+
+        public static int _columnSpanAtIndex(int index) {
+            return new List<int> {1, 1, 1, 1, 1, 1, 2, 2}[index % _childrenPerBlock];
+        }
+    }
+
+    class _ShrineGridLayout : SliverGridLayout {
+        public _ShrineGridLayout(
+            float? rowStride = null,
+            float? columnStride = null,
+            float? tileHeight = null,
+            float? tileWidth = null
+        ) {
+            this.rowStride = rowStride;
+            this.columnStride = columnStride;
+            this.tileHeight = tileHeight;
+            this.tileWidth = tileWidth;
+        }
+
+        public readonly float? rowStride;
+        public readonly float? columnStride;
+        public readonly float? tileHeight;
+        public readonly float? tileWidth;
+
+        public override int getMinChildIndexForScrollOffset(float scrollOffset) {
+            return ShrineHomeUtils._minIndexInRow((int) (scrollOffset / this.rowStride));
+        }
+
+        public override int getMaxChildIndexForScrollOffset(float scrollOffset) {
+            return ShrineHomeUtils._maxIndexInRow((int) (scrollOffset / this.rowStride));
+        }
+
+        public override SliverGridGeometry getGeometryForChildIndex(int index) {
+            int row = ShrineHomeUtils._rowAtIndex(index);
+            int column = ShrineHomeUtils._columnAtIndex(index);
+            int columnSpan = ShrineHomeUtils._columnSpanAtIndex(index);
+            return new SliverGridGeometry(
+                scrollOffset: row * this.rowStride,
+                crossAxisOffset: column * this.columnStride,
+                mainAxisExtent: this.tileHeight,
+                crossAxisExtent: this.tileWidth + (columnSpan - 1) * this.columnStride
+            );
+        }
+
+        public override float computeMaxScrollOffset(int childCount) {
+            if (childCount == 0) {
+                return 0.0f;
+            }
+
+            int rowCount = ShrineHomeUtils._rowAtIndex(childCount - 1) + 1;
+            float? rowSpacing = this.rowStride - this.tileHeight;
+            return (this.rowStride * rowCount - rowSpacing) ?? 0.0f;
+        }
+    }
+
+    class _ShrineGridDelegate : SliverGridDelegate {
+        const float _spacing = 8.0f;
+
+        public override SliverGridLayout getLayout(SliverConstraints constraints) {
+            float tileWidth = (constraints.crossAxisExtent - _spacing) / 2.0f;
+            const float tileHeight = 40.0f + 144.0f + 40.0f;
+            return new _ShrineGridLayout(
+                tileWidth: tileWidth,
+                tileHeight: tileHeight,
+                rowStride: tileHeight + _spacing,
+                columnStride: tileWidth + _spacing
+            );
+        }
+
+        public override bool shouldRelayout(SliverGridDelegate oldDelegate) {
+            return false;
+        }
+    }
+
+    class _VendorItem : StatelessWidget {
+        public _VendorItem(Key key = null, Vendor vendor = null) : base(key: key) {
+            D.assert(vendor != null);
+            this.vendor = vendor;
+        }
+
+        public readonly Vendor vendor;
+
+        public override Widget build(BuildContext context) {
+            return new SizedBox(
+                height: 24.0f,
+                child: new Row(
+                    children: new List<Widget> {
+                        new SizedBox(
+                            width: 24.0f,
+                            child: new ClipRRect(
+                                borderRadius: BorderRadius.circular(12.0f),
+                                child: Image.asset(
+                                    this.vendor.avatarAsset,
+                                    fit: BoxFit.cover
+                                )
+                            )
+                        ),
+                        new SizedBox(width: 8.0f),
+                        new Expanded(
+                            child: new Text(this.vendor.name, style: ShrineTheme.of(context).vendorItemStyle)
+                        )
+                    }
+                )
+            );
+        }
+    }
+
+    abstract class _PriceItem : StatelessWidget {
+        public _PriceItem(Key key = null, Product product = null)
+            : base(key: key) {
+            D.assert(product != null);
+            this.product = product;
+        }
+
+        public readonly Product product;
+
+        public Widget buildItem(BuildContext context, TextStyle style, EdgeInsets padding) {
+            BoxDecoration decoration = null;
+            if (ShrineHomeUtils._shoppingCart.getOrDefault(this.product) != null) {
+                decoration = new BoxDecoration(color: ShrineTheme.of(context).priceHighlightColor);
+            }
+
+            return new Container(
+                padding: padding,
+                decoration: decoration,
+                child: new Text(this.product.priceString, style: style)
+            );
+        }
+    }
+
+    class _ProductPriceItem : _PriceItem {
+        public _ProductPriceItem(Key key = null, Product product = null) : base(key: key, product: product) {
+        }
+
+        public override Widget build(BuildContext context) {
+            return this.buildItem(
+                context,
+                ShrineTheme.of(context).priceStyle,
+                EdgeInsets.symmetric(horizontal: 16.0f, vertical: 8.0f)
+            );
+        }
+    }
+
+    class _FeaturePriceItem : _PriceItem {
+        public _FeaturePriceItem(Key key = null, Product product = null) : base(key: key, product: product) {
+        }
+
+        public override Widget build(BuildContext context) {
+            return this.buildItem(
+                context,
+                ShrineTheme.of(context).featurePriceStyle,
+                EdgeInsets.symmetric(horizontal: 24.0f, vertical: 16.0f)
+            );
+        }
+    }
+
+    class _HeadingLayout : MultiChildLayoutDelegate {
+        public _HeadingLayout() {
+        }
+
+        public const string price = "price";
+        public const string image = "image";
+        public const string title = "title";
+        public const string description = "description";
+        public const string vendor = "vendor";
+
+        public override void performLayout(Size size) {
+            Size priceSize = this.layoutChild(price, BoxConstraints.loose(size));
+            this.positionChild(price, new Offset(size.width - priceSize.width, 0.0f));
+
+            float halfWidth = size.width / 2.0f;
+            float halfHeight = size.height / 2.0f;
+            const float halfUnit = ShrineHomeUtils.unitSize / 2.0f;
+            const float margin = 16.0f;
+
+            Size imageSize = this.layoutChild(image, BoxConstraints.loose(size));
+            float imageX = imageSize.width < halfWidth - halfUnit
+                ? halfWidth / 2.0f - imageSize.width / 2.0f - halfUnit
+                : halfWidth - imageSize.width;
+            this.positionChild(image, new Offset(imageX, halfHeight - imageSize.height / 2.0f));
+
+            float maxTitleWidth = halfWidth + ShrineHomeUtils.unitSize - margin;
+            BoxConstraints titleBoxConstraints = new BoxConstraints(maxWidth: maxTitleWidth);
+            Size titleSize = this.layoutChild(title, titleBoxConstraints);
+            float titleX = halfWidth - ShrineHomeUtils.unitSize;
+            float titleY = halfHeight - titleSize.height;
+            this.positionChild(title, new Offset(titleX, titleY));
+
+            Size descriptionSize = this.layoutChild(description, titleBoxConstraints);
+            float descriptionY = titleY + titleSize.height + margin;
+            this.positionChild(description, new Offset(titleX, descriptionY));
+
+            this.layoutChild(vendor, titleBoxConstraints);
+            float vendorY = descriptionY + descriptionSize.height + margin;
+            this.positionChild(vendor, new Offset(titleX, vendorY));
+        }
+
+        public override bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) {
+            return false;
+        }
+    }
+
+    class _HeadingShrineHome : StatelessWidget {
+        public _HeadingShrineHome(Key key = null, Product product = null) : base(key: key) {
+            D.assert(product != null);
+            D.assert(product.featureTitle != null);
+            D.assert(product.featureDescription != null);
+            this.product = product;
+        }
+
+        public readonly Product product;
+
+        public override Widget build(BuildContext context) {
+            Size screenSize = MediaQuery.of(context).size;
+            ShrineTheme theme = ShrineTheme.of(context);
+            return new SizedBox(
+                height: screenSize.width > screenSize.height
+                    ? (screenSize.height - Constants.kToolbarHeight) * 0.85f
+                    : (screenSize.height - Constants.kToolbarHeight) * 0.70f,
+                child: new Container(
+                    decoration: new BoxDecoration(
+                        color: theme.cardBackgroundColor,
+                        border: new Border(bottom: new BorderSide(color: theme.dividerColor))
+                    ),
+                    child: new CustomMultiChildLayout(
+                        layoutDelegate: new _HeadingLayout(),
+                        children: new List<Widget> {
+                            new LayoutId(
+                                id: _HeadingLayout.price,
+                                child: new _FeaturePriceItem(product: this.product)
+                            ),
+                            new LayoutId(
+                                id: _HeadingLayout.image,
+                                child: Image.asset(
+                                    this.product.imageAsset,
+                                    fit: BoxFit.cover
+                                )
+                            ),
+                            new LayoutId(
+                                id: _HeadingLayout.title,
+                                child: new Text(this.product.featureTitle, style: theme.featureTitleStyle)
+                            ),
+                            new LayoutId(
+                                id: _HeadingLayout.description,
+                                child: new Text(this.product.featureDescription, style: theme.featureStyle)
+                            ),
+                            new LayoutId(
+                                id: _HeadingLayout.vendor,
+                                child: new _VendorItem(vendor: this.product.vendor)
+                            )
+                        }
+                    )
+                )
+            );
+        }
+    }
+
+    class _ProductItem : StatelessWidget {
+        public _ProductItem(Key key = null, Product product = null, VoidCallback onPressed = null) : base(key: key) {
+            D.assert(product != null);
+            this.product = product;
+            this.onPressed = onPressed;
+        }
+
+        public readonly Product product;
+        public readonly VoidCallback onPressed;
+
+        public override Widget build(BuildContext context) {
+            return new Card(
+                child: new Stack(
+                    children: new List<Widget> {
+                        new Column(
+                            children: new List<Widget> {
+                                new Align(
+                                    alignment: Alignment.centerRight,
+                                    child: new _ProductPriceItem(product: this.product)
+                                ),
+                                new Container(
+                                    width: 144.0f,
+                                    height: 144.0f,
+                                    padding: EdgeInsets.symmetric(horizontal: 8.0f),
+                                    child:new Hero(
+                                        tag: this.product.tag,
+                                        child: Image.asset(this.product.imageAsset,
+                                            fit: BoxFit.contain
+                                        )
+                                    )
+                                ),
+                                new Padding(
+                                    padding: EdgeInsets.symmetric(horizontal: 8.0f),
+                                    child: new _VendorItem(vendor: this.product.vendor)
+                                )
+                            }
+                        ),
+                        new Material(
+                            type: MaterialType.transparency,
+                            child: new InkWell(onTap: this.onPressed == null
+                                ? (GestureTapCallback) null
+                                : () => { this.onPressed(); })
+                        )
+                    }
+                )
+            );
+        }
+    }
+
+    public class ShrineHome : StatefulWidget {
+        public override State createState() {
+            return new _ShrineHomeState();
+        }
+    }
+
+    class _ShrineHomeState : State<ShrineHome> {
+        public _ShrineHomeState() {
+        }
+
+        readonly GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>.key(debugLabel: "Shrine Home");
+        static readonly _ShrineGridDelegate gridDelegate = new _ShrineGridDelegate();
+
+        void _showOrderPage(Product product) {
+            Order order = ShrineHomeUtils._shoppingCart.getOrDefault(product) ?? new Order(product: product);
+            Navigator.push(this.context, new ShrineOrderRoute(
+                order: order,
+                builder: (BuildContext context) => {
+                    return new OrderPage(
+                        order: order,
+                        products: ShrineHomeUtils._products,
+                        shoppingCart: ShrineHomeUtils._shoppingCart
+                    );
+                }
+            )).Then(completedOrder => {
+                D.assert((completedOrder as Order).product != null);
+                if ((completedOrder as Order).quantity == 0) {
+                    ShrineHomeUtils._shoppingCart.Remove((completedOrder as Order).product);
+                }
+            });
+        }
+
+        public override Widget build(BuildContext context) {
+            Product featured = ShrineHomeUtils._products.First((Product product) => product.featureDescription != null);
+            return new ShrinePage(
+                scaffoldKey: this._scaffoldKey,
+                products: ShrineHomeUtils._products,
+                shoppingCart: ShrineHomeUtils._shoppingCart,
+                body: new CustomScrollView(
+                    slivers: new List<Widget> {
+                        new SliverToBoxAdapter(child: new _HeadingShrineHome(product: featured)),
+                        new SliverSafeArea(
+                            top: false,
+                            minimum: EdgeInsets.all(16.0f),
+                            sliver: new SliverGrid(
+                                gridDelegate: gridDelegate,
+                                layoutDelegate: new SliverChildListDelegate(
+                                    ShrineHomeUtils._products.Select<Product, Widget>((Product product) => {
+                                        return new _ProductItem(
+                                            product: product,
+                                            onPressed: () => { this._showOrderPage(product); }
+                                        );
+                                    }).ToList()
+                                )
+                            )
+                        )
+                    }
+                )
+            );
+        }
+    }
+}

Samples/UIWidgetsGallery/demo/shrine/shrine_home.cs.meta

+fileFormatVersion: 2
+guid: ec9f3366e13014909b0287e1305641e1
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

Samples/UIWidgetsGallery/demo/shrine/shrine_order.cs.meta

+fileFormatVersion: 2
+guid: b0720924e140457dbc8626755c82f3ba
+timeCreated: 1553244438

Samples/UIWidgetsGallery/demo/shrine/shrine_page.cs

+using System.Collections.Generic;
+using System.Linq;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.material;
+using Unity.UIWidgets.painting;
+using Unity.UIWidgets.service;
+using Unity.UIWidgets.widgets;
+
+namespace UIWidgetsGallery.gallery {
+    public enum ShrineAction {
+        sortByPrice,
+        sortByProduct,
+        emptyCart
+    }
+
+    public class ShrinePage : StatefulWidget {
+        public ShrinePage(
+            Key key = null,
+            GlobalKey<ScaffoldState> scaffoldKey = null,
+            Widget body = null,
+            Widget floatingActionButton = null,
+            List<Product> products = null,
+            Dictionary<Product, Order> shoppingCart = null
+        ) : base(key: key) {
+            D.assert(body != null);
+            D.assert(scaffoldKey != null);
+            this.scaffoldKey = scaffoldKey;
+            this.body = body;
+            this.floatingActionButton = floatingActionButton;
+            this.products = products;
+            this.shoppingCart = shoppingCart;
+        }
+
+        public readonly GlobalKey<ScaffoldState> scaffoldKey;
+        public readonly Widget body;
+        public readonly Widget floatingActionButton;
+        public readonly List<Product> products;
+        public readonly Dictionary<Product, Order> shoppingCart;
+
+        public override State createState() {
+            return new ShrinePageState();
+        }
+    }
+
+    public class ShrinePageState : State<ShrinePage> {
+        float _appBarElevation = 0.0f;
+
+        bool _handleScrollNotification(ScrollNotification notification) {
+            float elevation = notification.metrics.extentBefore() <= 0.0f ? 0.0f : 1.0f;
+            if (elevation != this._appBarElevation) {
+                this.setState(() => { this._appBarElevation = elevation; });
+            }
+
+            return false;
+        }
+
+        void _showShoppingCart() {
+            BottomSheetUtils.showModalBottomSheet<object>(context: this.context, builder: (BuildContext context) => {
+                if (this.widget.shoppingCart.isEmpty()) {
+                    return new Padding(
+                        padding: EdgeInsets.all(24.0f),
+                        child: new Text("The shopping cart is empty")
+                    );
+                }
+
+                return new ListView(
+                    padding: Constants.kMaterialListPadding,
+                    children: this.widget.shoppingCart.Values.Select<Order, Widget>((Order order) => {
+                        return new ListTile(
+                            title: new Text(order.product.name),
+                            leading: new Text($"{order.quantity}"),
+                            subtitle: new Text(order.product.vendor.name)
+                        );
+                    }).ToList()
+                );
+            });
+        }
+
+        void _sortByPrice() {
+            this.widget.products.Sort((Product a, Product b) => a.price?.CompareTo(b.price) ?? (b == null ? 0 : -1));
+        }
+
+        void _sortByProduct() {
+            this.widget.products.Sort((Product a, Product b) => a.name.CompareTo(b.name));
+        }
+
+        void _emptyCart() {
+            this.widget.shoppingCart.Clear();
+            this.widget.scaffoldKey.currentState.showSnackBar(
+                new SnackBar(content: new Text("Shopping cart is empty")));
+        }
+
+        public override Widget build(BuildContext context) {
+            ShrineTheme theme = ShrineTheme.of(context);
+            return new Scaffold(
+                key: this.widget.scaffoldKey,
+                appBar: new AppBar(
+                    elevation: this._appBarElevation,
+                    backgroundColor: theme.appBarBackgroundColor,
+                    iconTheme: Theme.of(context).iconTheme,
+                    brightness: Brightness.light,
+                    flexibleSpace: new Container(
+                        decoration: new BoxDecoration(
+                            border: new Border(
+                                bottom: new BorderSide(color: theme.dividerColor)
+                            )
+                        )
+                    ),
+                    title: new Text("SHRINE", style: ShrineTheme.of(context).appBarTitleStyle),
+                    centerTitle: true,
+                    actions: new List<Widget> {
+                        new IconButton(
+                            icon: new Icon(Icons.shopping_cart),
+                            tooltip: "Shopping cart",
+                            onPressed: this._showShoppingCart
+                        ),
+                        new PopupMenuButton<ShrineAction>(
+                            itemBuilder: (BuildContext _) => new List<PopupMenuEntry<ShrineAction>> {
+                                new PopupMenuItem<ShrineAction>(
+                                    value: ShrineAction.sortByPrice,
+                                    child: new Text("Sort by price")
+                                ),
+                                new PopupMenuItem<ShrineAction>(
+                                    value: ShrineAction.sortByProduct,
+                                    child: new Text("Sort by product")
+                                ),
+                                new PopupMenuItem<ShrineAction>(
+                                    value: ShrineAction.emptyCart,
+                                    child: new Text("Empty shopping cart")
+                                )
+                            },
+                            onSelected: (ShrineAction action) => {
+                                switch (action) {
+                                    case ShrineAction.sortByPrice:
+                                        this.setState(this._sortByPrice);
+                                        break;
+                                    case ShrineAction.sortByProduct:
+                                        this.setState(this._sortByProduct);
+                                        break;
+                                    case ShrineAction.emptyCart:
+                                        this.setState(this._emptyCart);
+                                        break;
+                                }
+                            }
+                        )
+                    }
+                ),
+                floatingActionButton: this.widget.floatingActionButton,
+                body: new NotificationListener<ScrollNotification>(
+                    onNotification: this._handleScrollNotification,
+                    child: this.widget.body
+                )
+            );
+        }
+    }
+}

Samples/UIWidgetsGallery/demo/shrine/shrine_page.cs.meta

+fileFormatVersion: 2
+guid: 81f9ca5ad3e848f4889059abd3037d17
+timeCreated: 1553246179

Samples/UIWidgetsGallery/demo/shrine/shrine_theme.cs

+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.material;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+using TextStyle = Unity.UIWidgets.painting.TextStyle;
+
+namespace UIWidgetsGallery.gallery {
+    public class ShrineStyle : TextStyle {
+        public ShrineStyle(bool inherit, Color color, float fontSize, FontWeight fontWeight, TextBaseline textBaseline,
+            string fontFamily = null
+        ) : base(inherit: inherit, color: color, fontSize: fontSize, fontWeight: fontWeight, fontFamily: fontFamily,
+            textBaseline: textBaseline) {
+        }
+
+        public static ShrineStyle roboto(float size, FontWeight weight, Color color) {
+            return new ShrineStyle(inherit: false, color: color, fontSize: size, fontWeight: weight,
+                textBaseline: TextBaseline.alphabetic);
+        }
+
+        public static ShrineStyle abrilFatface(float size, FontWeight weight, Color color) {
+            return new ShrineStyle(inherit: false, color: color, fontFamily: "AbrilFatface", fontSize: size,
+                fontWeight: weight, textBaseline: TextBaseline.alphabetic);
+        }
+    }
+
+    public class ShrineThemeUtils {
+        public static TextStyle robotoRegular12(Color color) {
+            return ShrineStyle.roboto(12.0f, FontWeight.w400, color);
+        }
+
+        public static TextStyle robotoLight12(Color color) {
+            return ShrineStyle.roboto(12.0f, FontWeight.w400, color);
+        }
+
+        public static TextStyle robotoRegular14(Color color) {
+            return ShrineStyle.roboto(14.0f, FontWeight.w400, color);
+        }
+
+        public static TextStyle robotoMedium14(Color color) {
+            return ShrineStyle.roboto(14.0f, FontWeight.w700, color);
+        }
+
+        public static TextStyle robotoLight14(Color color) {
+            return ShrineStyle.roboto(14.0f, FontWeight.w400, color);
+        }
+
+        public static TextStyle robotoRegular16(Color color) {
+            return ShrineStyle.roboto(16.0f, FontWeight.w400, color);
+        }
+
+        public static TextStyle robotoRegular20(Color color) {
+            return ShrineStyle.roboto(20.0f, FontWeight.w400, color);
+        }
+
+        public static TextStyle abrilFatfaceRegular24(Color color) {
+            return ShrineStyle.abrilFatface(24.0f, FontWeight.w400, color);
+        }
+
+        public static TextStyle abrilFatfaceRegular34(Color color) {
+            return ShrineStyle.abrilFatface(34.0f, FontWeight.w400, color);
+        }
+    }
+
+    public class ShrineTheme : InheritedWidget {
+        public ShrineTheme(Key key = null, Widget child = null)
+            : base(key: key, child: child) {
+            D.assert(child != null);
+        }
+
+        public readonly Color cardBackgroundColor = Colors.white;
+        public readonly Color appBarBackgroundColor = Colors.white;
+        public readonly Color dividerColor = new Color(0xFFD9D9D9);
+        public readonly Color priceHighlightColor = new Color(0xFFFFE0E0);
+
+        public readonly TextStyle appBarTitleStyle = ShrineThemeUtils.robotoRegular20(Colors.black87);
+        public readonly TextStyle vendorItemStyle = ShrineThemeUtils.robotoRegular12(new Color(0xFF81959D));
+        public readonly TextStyle priceStyle = ShrineThemeUtils.robotoRegular14(Colors.black87);
+
+        public readonly TextStyle featureTitleStyle =
+            ShrineThemeUtils.abrilFatfaceRegular34(new Color(0xFF0A3142));
+
+        public readonly TextStyle featurePriceStyle = ShrineThemeUtils.robotoRegular16(Colors.black87);
+        public readonly TextStyle featureStyle = ShrineThemeUtils.robotoLight14(Colors.black54);
+        public readonly TextStyle orderTitleStyle = ShrineThemeUtils.abrilFatfaceRegular24(Colors.black87);
+        public readonly TextStyle orderStyle = ShrineThemeUtils.robotoLight14(Colors.black54);
+        public readonly TextStyle vendorTitleStyle = ShrineThemeUtils.robotoMedium14(Colors.black87);
+        public readonly TextStyle vendorStyle = ShrineThemeUtils.robotoLight14(Colors.black54);
+        public readonly TextStyle quantityMenuStyle = ShrineThemeUtils.robotoLight14(Colors.black54);
+
+        public static ShrineTheme of(BuildContext context) {
+            return (ShrineTheme) context.inheritFromWidgetOfExactType(typeof(ShrineTheme));
+        }
+
+        public override bool updateShouldNotify(InheritedWidget oldWidget) {
+            return false;
+        }
+    }
+}

Samples/UIWidgetsGallery/demo/shrine/shrine_theme.cs.meta

+fileFormatVersion: 2
+guid: 468619e1fd4f4437ba3f0929e269aab4
+timeCreated: 1553241961

Samples/UIWidgetsGallery/demo/shrine/shrine_types.cs

+using System.Collections.Generic;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.ui;
+
+namespace UIWidgetsGallery.gallery {
+    public class Vendor {
+        public Vendor(
+            string name,
+            string description,
+            string avatarAsset,
+            string avatarAssetPackage
+        ) {
+            this.name = name;
+            this.description = description;
+            this.avatarAsset = avatarAsset;
+            this.avatarAssetPackage = avatarAssetPackage;
+        }
+
+        public readonly string name;
+        public readonly string description;
+        public readonly string avatarAsset;
+        public readonly string avatarAssetPackage;
+
+        public bool isValid() {
+            return this.name != null && this.description != null && this.avatarAsset != null;
+        }
+
+        public override string ToString() {
+            return "Vendor($name)";
+        }
+    }
+
+    public class Product {
+        public Product(
+            string name = null,
+            string description = null,
+            string featureTitle = null,
+            string featureDescription = null,
+            string imageAsset = null,
+            string imageAssetPackage = null,
+            List<string> categories = null,
+            float? price = null,
+            Vendor vendor = null
+        ) {
+            this.name = name;
+            this.description = description;
+            this.featureTitle = featureTitle;
+            this.featureDescription = featureDescription;
+            this.imageAsset = imageAsset;
+            this.imageAssetPackage = imageAssetPackage;
+            this.categories = categories;
+            this.price = price;
+            this.vendor = vendor;
+        }
+
+        public readonly string name;
+        public readonly string description;
+        public readonly string featureTitle;
+        public readonly string featureDescription;
+        public readonly string imageAsset;
+        public readonly string imageAssetPackage;
+        public readonly List<string> categories;
+        public readonly float? price;
+        public readonly Vendor vendor;
+
+        public string tag {
+            get { return this.name; }
+        }
+
+        public string priceString {
+            get { return $"${(this.price ?? 0.0f).floor()}"; }
+        }
+
+        public bool isValid() {
+            return this.name != null && this.description != null && this.imageAsset != null &&
+                   this.categories != null && this.categories.isNotEmpty() && this.price != null &&
+                   this.vendor.isValid();
+        }
+
+        public override string ToString() {
+            return $"Product({this.name})";
+        }
+    }
+
+    public class Order {
+        public Order(Product product, int quantity = 1, bool inCart = false) {
+            D.assert(product != null);
+            D.assert(quantity >= 0);
+            this.product = product;
+            this.quantity = quantity;
+            this.inCart = inCart;
+        }
+
+        public readonly Product product;
+        public readonly int quantity;
+        public readonly bool inCart;
+
+        public Order copyWith(Product product = null, int? quantity = null, bool? inCart = null) {
+            return new Order(
+                product: product ?? this.product,
+                quantity: quantity ?? this.quantity,
+                inCart: inCart ?? this.inCart
+            );
+        }
+
+        public static bool operator ==(Order left, Order right) {
+            if (left is null && right is null) return true;
+            if (left is null || right is null) return false;
+            return left.Equals(right);
+        }
+
+        public static bool operator !=(Order left, Order right) {
+            if (left is null && right is null) return false;
+            if (left is null || right is null) return true;
+            return !left.Equals(right);
+        }
+
+        public bool Equals(Order other) {
+            return this.product == other.product &&
+                   this.quantity == other.quantity &&
+                   this.inCart == other.inCart;
+        }
+
+        public override bool Equals(object obj) {
+            if (ReferenceEquals(null, obj)) {
+                return false;
+            }
+
+            if (ReferenceEquals(this, obj)) {
+                return true;
+            }
+
+            if (obj.GetType() != this.GetType()) {
+                return false;
+            }
+
+            return this.Equals((Order) obj);
+        }
+
+        public override int GetHashCode() {
+            unchecked {
+                var hashCode = (this.product != null ? this.product.GetHashCode() : 0);
+                hashCode = (hashCode * 397) ^ this.quantity.GetHashCode();
+                hashCode = (hashCode * 397) ^ this.inCart.GetHashCode();
+                return hashCode;
+            }
+        }
+
+        public override string ToString() {
+            return $"Order({this.product}, quantity={this.quantity}, inCart={this.inCart})";
+        }
+    }
+}

Samples/UIWidgetsGallery/demo/shrine/shrine_types.cs.meta

+fileFormatVersion: 2
+guid: 924c1662fb504687af28a4f05901a20a
+timeCreated: 1553240139

Samples/UIWidgetsGallery/demo/shrine/shrine_order.cs

+using System.Collections.Generic;
+using System.Linq;
+using com.unity.uiwidgets.Runtime.rendering;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.material;
+using Unity.UIWidgets.painting;
+using Unity.UIWidgets.rendering;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+using Image = Unity.UIWidgets.widgets.Image;
+
+namespace UIWidgetsGallery.gallery {
+    class _ProductItemShrineOrder : StatelessWidget {
+        public _ProductItemShrineOrder(
+            Key key = null,
+            Product product = null,
+            int? quantity = null,
+            ValueChanged<int> onChanged = null
+        ) : base(key: key) {
+            D.assert(product != null);
+            D.assert(quantity != null);
+            D.assert(onChanged != null);
+            this.product = product;
+            this.quantity = quantity;
+            this.onChanged = onChanged;
+        }
+
+        public readonly Product product;
+        public readonly int? quantity;
+        public readonly ValueChanged<int> onChanged;
+
+        public override Widget build(BuildContext context) {
+            ShrineTheme theme = ShrineTheme.of(context);
+            return new Column(
+                mainAxisSize: MainAxisSize.min,
+                crossAxisAlignment: CrossAxisAlignment.stretch,
+                children: new List<Widget> {
+                    new Text(this.product.name, style: theme.featureTitleStyle),
+                    new SizedBox(height: 24.0f),
+                    new Text(this.product.description, style: theme.featureStyle),
+                    new SizedBox(height: 16.0f),
+                    new Padding(
+                        padding: EdgeInsets.only(top: 8.0f, bottom: 8.0f, right: 88.0f),
+                        child: new DropdownButtonHideUnderline(
+                            child: new Container(
+                                decoration: new BoxDecoration(
+                                    border: Border.all(
+                                        color: new Color(0xFFD9D9D9)
+                                    )
+                                ),
+                                child: new DropdownButton<string>(
+                                    items: new List<int> {0, 1, 2, 3, 4, 5}.Select<int, DropdownMenuItem<string>>(
+                                        (int value) => {
+                                            return new DropdownMenuItem<string>(
+                                                value: $"{value}",
+                                                child: new Padding(
+                                                    padding: EdgeInsets.only(left: 8.0f),
+                                                    child: new Text($"Quantity {value}", style: theme.quantityMenuStyle)
+                                                )
+                                            );
+                                        }).ToList(),
+                                    value: $"{this.quantity}",
+                                    onChanged: (value) => { this.onChanged(int.Parse(value)); })
+                            )
+                        )
+                    )
+                }
+            );
+        }
+    }
+
+    class _VendorItemShrineOrder : StatelessWidget {
+        public _VendorItemShrineOrder(Key key = null, Vendor vendor = null)
+            : base(key: key) {
+            D.assert(vendor != null);
+            this.vendor = vendor;
+        }
+
+        public readonly Vendor vendor;
+
+        public override Widget build(BuildContext context) {
+            ShrineTheme theme = ShrineTheme.of(context);
+            return new Column(
+                mainAxisSize: MainAxisSize.min,
+                crossAxisAlignment: CrossAxisAlignment.stretch,
+                children: new List<Widget> {
+                    new SizedBox(
+                        height: 24.0f,
+                        child: new Align(
+                            alignment: Alignment.bottomLeft,
+                            child: new Text(this.vendor.name, style: theme.vendorTitleStyle)
+                        )
+                    ),
+                    new SizedBox(height: 16.0f),
+                    new Text(this.vendor.description, style: theme.vendorStyle)
+                }
+            );
+        }
+    }
+
+    class _HeadingLayoutShrineOrder : MultiChildLayoutDelegate {
+        public _HeadingLayoutShrineOrder() {
+        }
+
+        public const string image = "image";
+        public const string icon = "icon";
+        public const string product = "product";
+        public const string vendor = "vendor";
+
+        public override void performLayout(Size size) {
+            const float margin = 56.0f;
+            bool landscape = size.width > size.height;
+            float imageWidth = (landscape ? size.width / 2.0f : size.width) - margin * 2.0f;
+            BoxConstraints imageConstraints = new BoxConstraints(maxHeight: 224.0f, maxWidth: imageWidth);
+            Size imageSize = this.layoutChild(image, imageConstraints);
+            const float imageY = 0.0f;
+            this.positionChild(image, new Offset(margin, imageY));
+
+            float productWidth = landscape ? size.width / 2.0f : size.width - margin;
+            BoxConstraints productConstraints = new BoxConstraints(maxWidth: productWidth);
+            Size productSize = this.layoutChild(product, productConstraints);
+            float productX = landscape ? size.width / 2.0f : margin;
+            float productY = landscape ? 0.0f : imageY + imageSize.height + 16.0f;
+            this.positionChild(product, new Offset(productX, productY));
+
+            Size iconSize = this.layoutChild(icon, BoxConstraints.loose(size));
+            this.positionChild(icon, new Offset(productX - iconSize.width - 16.0f, productY + 8.0f));
+
+            float vendorWidth = landscape ? size.width - margin : productWidth;
+            this.layoutChild(vendor, new BoxConstraints(maxWidth: vendorWidth));
+            float vendorX = landscape ? margin : productX;
+            float vendorY = productY + productSize.height + 16.0f;
+            this.positionChild(vendor, new Offset(vendorX, vendorY));
+        }
+
+        public override bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) {
+            return true;
+        }
+    }
+
+    class _HeadingShrineOrder : StatelessWidget {
+        public _HeadingShrineOrder(
+            Key key = null,
+            Product product = null,
+            int? quantity = null,
+            ValueChanged<int> quantityChanged = null
+        ) : base(key: key) {
+            D.assert(product != null);
+            D.assert(quantity != null && quantity >= 0 && quantity <= 5);
+            this.product = product;
+            this.quantity = quantity;
+            this.quantityChanged = quantityChanged;
+        }
+
+        public readonly Product product;
+        public readonly int? quantity;
+        public readonly ValueChanged<int> quantityChanged;
+
+        public override Widget build(BuildContext context) {
+            Size screenSize = MediaQuery.of(context).size;
+            return new SizedBox(
+                height: (screenSize.height - Constants.kToolbarHeight) * 1.35f,
+                child: new Material(
+                    type: MaterialType.card,
+                    elevation: 0.0f,
+                    child: new Padding(
+                        padding: EdgeInsets.only(left: 16.0f, top: 18.0f, right: 16.0f, bottom: 24.0f),
+                        child: new CustomMultiChildLayout(
+                            layoutDelegate: new _HeadingLayoutShrineOrder(),
+                            children: new List<Widget> {
+                                new LayoutId(
+                                    id: _HeadingLayoutShrineOrder.image,
+                                    child: new Hero(
+                                        tag: this.product.tag,
+                                        child: Image.asset(this.product.imageAsset,
+                                            fit: BoxFit.contain,
+                                            alignment: Alignment.center
+                                        )
+                                    )
+                                ),
+                                new LayoutId(
+                                    id: _HeadingLayoutShrineOrder.icon,
+                                    child: new Icon(
+                                        Icons.info_outline,
+                                        size: 24.0f,
+                                        color: new Color(0xFFFFE0E0)
+                                    )
+                                ),
+                                new LayoutId(
+                                    id: _HeadingLayoutShrineOrder.product,
+                                    child: new _ProductItemShrineOrder(
+                                        product: this.product,
+                                        quantity: this.quantity,
+                                        onChanged: this.quantityChanged
+                                    )
+                                ),
+                                new LayoutId(
+                                    id: _HeadingLayoutShrineOrder.vendor,
+                                    child: new _VendorItemShrineOrder(vendor: this.product.vendor)
+                                )
+                            }
+                        )
+                    )
+                )
+            );
+        }
+    }
+
+    public class OrderPage : StatefulWidget {
+        public OrderPage(
+            Key key = null,
+            Order order = null,
+            List<Product> products = null,
+            Dictionary<Product, Order> shoppingCart = null
+        ) : base(key: key) {
+            D.assert(order != null);
+            D.assert(products != null && products.isNotEmpty());
+            D.assert(shoppingCart != null);
+            this.order = order;
+            this.products = products;
+            this.shoppingCart = shoppingCart;
+        }
+
+        public readonly Order order;
+        public readonly List<Product> products;
+        public readonly Dictionary<Product, Order> shoppingCart;
+
+        public override State createState() {
+            return new _OrderPageState();
+        }
+    }
+
+    class _OrderPageState : State<OrderPage> {
+        GlobalKey<ScaffoldState> scaffoldKey;
+
+        public override void initState() {
+            base.initState();
+            this.scaffoldKey = GlobalKey<ScaffoldState>.key(debugLabel: $"Shrine Order {this.widget.order}");
+        }
+
+        public Order currentOrder {
+            get { return ShrineOrderRoute.of(this.context).order; }
+            set { ShrineOrderRoute.of(this.context).order = value; }
+        }
+
+        void updateOrder(int? quantity = null, bool? inCart = null) {
+            Order newOrder = this.currentOrder.copyWith(quantity: quantity, inCart: inCart);
+            if (this.currentOrder != newOrder) {
+                this.setState(() => {
+                    this.widget.shoppingCart[newOrder.product] = newOrder;
+                    this.currentOrder = newOrder;
+                });
+            }
+        }
+
+        void showSnackBarMessage(string message) {
+            this.scaffoldKey.currentState.showSnackBar(new SnackBar(content: new Text(message)));
+        }
+
+        public override Widget build(BuildContext context) {
+            return new ShrinePage(
+                scaffoldKey: this.scaffoldKey,
+                products: this.widget.products,
+                shoppingCart: this.widget.shoppingCart,
+                floatingActionButton: new FloatingActionButton(
+                    onPressed: () => {
+                        this.updateOrder(inCart: true);
+                        int n = this.currentOrder.quantity;
+                        string item = this.currentOrder.product.name;
+                        string message = n == 1 ? $"is one {item} item" : $"are {n} {item} items";
+                        this.showSnackBarMessage(
+                            $"There {message} in the shopping cart."
+                        );
+                    },
+                    backgroundColor: new Color(0xFF16F0F0),
+                    tooltip: "Add to cart",
+                    child: new Icon(
+                        Icons.add_shopping_cart,
+                        color: Colors.black
+                    )
+                ),
+                body: new CustomScrollView(
+                    slivers: new List<Widget> {
+                        new SliverToBoxAdapter(
+                            child: new _HeadingShrineOrder(
+                                product: this.widget.order.product,
+                                quantity: this.currentOrder.quantity,
+                                quantityChanged: (int value) => { this.updateOrder(quantity: value); }
+                            )
+                        ),
+                        new SliverSafeArea(
+                            top: false,
+                            minimum: EdgeInsets.fromLTRB(8.0f, 32.0f, 8.0f, 8.0f),
+                            sliver: new SliverGrid(
+                                gridDelegate: new SliverGridDelegateWithMaxCrossAxisExtent(
+                                    maxCrossAxisExtent: 248.0f,
+                                    mainAxisSpacing: 8.0f,
+                                    crossAxisSpacing: 8.0f
+                                ),
+                                layoutDelegate: new SliverChildListDelegate(
+                                    this.widget.products
+                                        .FindAll((Product product) => product != this.widget.order.product)
+                                        .Select<Product, Widget>((Product product) => {
+                                            return new Card(
+                                                elevation: 1.0f,
+                                                child: Image.asset(
+                                                    product.imageAsset,
+                                                    fit: BoxFit.contain
+                                                )
+                                            );
+                                        }).ToList()
+                                )
+                            )
+                        )
+                    }
+                )
+            );
+        }
+    }
+
+    public class ShrineOrderRoute : ShrinePageRoute<Order> {
+        public ShrineOrderRoute(
+            Order order = null,
+            WidgetBuilder builder = null,
+            RouteSettings settings = null
+        ) : base(builder: builder, settings: settings) {
+            D.assert(order != null);
+            this.order = order;
+        }
+
+        public Order order;
+
+        public new Order currentResult {
+            get { return this.order; }
+        }
+
+        public new static ShrineOrderRoute of(BuildContext context) {
+            return (ShrineOrderRoute) ModalRoute.of(context);
+        }
+    }
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