com.unity.uiwidgets/Assets/UIWidgets/rendering/sliver_fixed_extent_list.cs

using System;
using UIWidgets.ui;

namespace UIWidgets.rendering {
    public abstract class RenderSliverFixedExtentBoxAdaptor : RenderSliverMultiBoxAdaptor {
        RenderSliverFixedExtentBoxAdaptor(
            RenderSliverBoxChildManager childManager = null
        ) : base(childManager: childManager) {
        }

        public abstract double itemExtent { get; }

        public double indexToLayoutOffset(double itemExtent, int index) {
            return itemExtent * index;
        }

        public int getMinChildIndexForScrollOffset(double scrollOffset, double itemExtent) {
            return itemExtent > 0.0 ? Math.Max(0, (int) (scrollOffset / itemExtent)) : 0;
        }

        public int getMaxChildIndexForScrollOffset(double scrollOffset, double itemExtent) {
            return itemExtent > 0.0 ? Math.Max(0, (int) Math.Ceiling(scrollOffset / itemExtent) - 1) : 0;
        }

        public double estimateMaxScrollOffset(SliverConstraints constraints,
            int firstIndex = 0,
            int lastIndex = 0,
            double leadingScrollOffset = 0.0,
            double trailingScrollOffset = 0.0
        ) {
            return this.childManager.estimateMaxScrollOffset(
                constraints,
                firstIndex: firstIndex,
                lastIndex: lastIndex,
                leadingScrollOffset: leadingScrollOffset,
                trailingScrollOffset: trailingScrollOffset
            );
        }

        public double computeMaxScrollOffset(SliverConstraints constraints, double itemExtent) {
            return this.childManager.childCount * itemExtent;
        }


        public override void performLayout() {
            this.childManager.didStartLayout();
            this.childManager.setDidUnderflow(false);

            double itemExtent = this.itemExtent;

            double scrollOffset = this.constraints.scrollOffset + this.constraints.cacheOrigin;
            double remainingExtent = this.constraints.remainingCacheExtent;
            double targetEndScrollOffset = scrollOffset + remainingExtent;

            BoxConstraints childConstraints = this.constraints.asBoxConstraints(
                minExtent: itemExtent,
                maxExtent: itemExtent
            );

            int firstIndex = this.getMinChildIndexForScrollOffset(scrollOffset, itemExtent);
            int? targetLastIndex = !double.IsInfinity(targetEndScrollOffset)
                ? this.getMaxChildIndexForScrollOffset(targetEndScrollOffset, itemExtent)
                : (int?) 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.Value).clamp(0, this.childCount);
                this.collectGarbage(leadingGarbage, trailingGarbage);
            } else {
                this.collectGarbage(0, 0);
            }

            if (this.firstChild == null) {
                if (!this.addInitialChild(index: firstIndex,
                    layoutOffset: this.indexToLayoutOffset(itemExtent, firstIndex))) {
                    double max = this.computeMaxScrollOffset(this.constraints, itemExtent);
                    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) {
                RenderBox child = this.insertAndLayoutLeadingChild(childConstraints);
                if (child == null) {
                    this.geometry = new SliverGeometry(scrollOffsetCorrection: index * itemExtent);
                    return;
                }

                var childParentData = (SliverMultiBoxAdaptorParentData) child.parentData;
                childParentData.layoutOffset = this.indexToLayoutOffset(itemExtent, index);
                trailingChildWithLayout = trailingChildWithLayout ?? child;
            }

            if (trailingChildWithLayout == null) {
                this.firstChild.layout(childConstraints);
                var childParentData = (SliverMultiBoxAdaptorParentData) this.firstChild.parentData;
                childParentData.layoutOffset = this.indexToLayoutOffset(itemExtent, firstIndex);
                trailingChildWithLayout = this.firstChild;
            }

            while (targetLastIndex == null || this.indexOf(trailingChildWithLayout) < targetLastIndex) {
                RenderBox child = this.childAfter(trailingChildWithLayout);
                if (child == null) {
                    child = this.insertAndLayoutChild(childConstraints, after: trailingChildWithLayout);
                    if (child == null) {
                        break;
                    }
                } else {
                    child.layout(childConstraints);
                }

                trailingChildWithLayout = child;
                var childParentData = (SliverMultiBoxAdaptorParentData) child.parentData;
                childParentData.layoutOffset = this.indexToLayoutOffset(itemExtent, childParentData.index);
            }

            int lastIndex = this.indexOf(this.lastChild);
            double leadingScrollOffset = this.indexToLayoutOffset(itemExtent, firstIndex);
            double trailingScrollOffset = this.indexToLayoutOffset(itemExtent, lastIndex + 1);

            double estimatedMaxScrollOffset = this.estimateMaxScrollOffset(
                this.constraints,
                firstIndex: firstIndex,
                lastIndex: lastIndex,
                leadingScrollOffset: leadingScrollOffset,
                trailingScrollOffset: trailingScrollOffset
            );

            double paintExtent = this.calculatePaintOffset(
                this.constraints,
                from: leadingScrollOffset,
                to: trailingScrollOffset
            );

            double cacheExtent = this.calculateCacheOffset(
                this.constraints,
                from: leadingScrollOffset,
                to: trailingScrollOffset
            );

            double targetEndScrollOffsetForPaint =
                this.constraints.scrollOffset + this.constraints.remainingPaintExtent;
            int? targetLastIndexForPaint = !double.IsInfinity(targetEndScrollOffsetForPaint)
                ? this.getMaxChildIndexForScrollOffset(targetEndScrollOffsetForPaint, itemExtent)
                : (int?) null;
            this.geometry = new SliverGeometry(
                scrollExtent: estimatedMaxScrollOffset,
                paintExtent: paintExtent,
                cacheExtent: cacheExtent,
                maxPaintExtent: estimatedMaxScrollOffset,
                hasVisualOverflow: (targetLastIndexForPaint != null && lastIndex >= targetLastIndexForPaint)
                                   || this.constraints.scrollOffset > 0.0
            );
            if (estimatedMaxScrollOffset == trailingScrollOffset) {
                this.childManager.setDidUnderflow(true);
            }

            this.childManager.didFinishLayout();
        }
    }
}