try fix ParentDataMixin issue

com.unity.uiwidgets/Runtime/rendering/object.cs

 using Rect = Unity.UIWidgets.ui.Rect;
 
 namespace Unity.UIWidgets.rendering {
-    public class ParentData {
+    public interface IParentData {
+        void detach();
+    }
+    
+    public class ParentData : IParentData {
         public virtual void detach() {
         }
 
         ChildType child { get; set; }
     }
 
-    public interface ContainerParentDataMixin<ChildType> where ChildType : RenderObject {
+    public interface ContainerParentDataMixin<ChildType> : IParentData where ChildType : RenderObject {
         ChildType previousSibling { get; set; }
         ChildType nextSibling { get; set; }
     }

com.unity.uiwidgets/Runtime/rendering/sliver_multi_box_adaptor.cs

 using Unity.UIWidgets.ui;
 
 namespace Unity.UIWidgets.rendering {
-    public class KeepAliveParentDataMixin : ParentData {
-        public bool keepAlive = false;
+    public interface IKeepAliveParentDataMixin : IParentData {
+        bool keptAlive { get; }
+        
+        bool keepAlive { get; set; }
+    }
+    
+    public class KeepAliveParentDataMixin : ParentData, IKeepAliveParentDataMixin { 
+        bool _keepAlive = false;
+
+        public bool keepAlive {
+            get { return _keepAlive; }
+            set { _keepAlive = value; }
+        }
-        bool keptAlive { get; }
+        public bool keptAlive { get; }
     }
 
     public interface RenderSliverBoxChildManager {
         bool debugAssertChildListLocked();
     }
 
-    public class SliverMultiBoxAdaptorParentData : ContainerParentDataMixinSliverLogicalParentData<RenderBox> {
+    public class SliverMultiBoxAdaptorParentData : ContainerParentDataMixinSliverLogicalParentData<RenderBox>, IKeepAliveParentDataMixin {
-        public bool keepAlive = false;
+        bool _keepAlive = false;
+
+        public bool keepAlive {
+            get { return _keepAlive; }
+            set { _keepAlive = value; }
+        }
 
         public bool keptAlive {
             get { return _keptAlive; }

com.unity.uiwidgets/Runtime/rendering/sliver_padding.cs

     
     public abstract class RenderSliverEdgeInsetsPadding : RenderObjectWithChildMixinRenderSliver<RenderSliver> {
 
-        EdgeInsets resolvedPadding { get; }
-
-
+        protected virtual EdgeInsets resolvedPadding { get; }
+        
         float?  beforePadding {
             get {
                 D.assert(constraints != null);
             if (!(child.parentData is SliverPhysicalParentData)) 
                 child.parentData = new SliverPhysicalParentData();
         }
-        protected override void performLayout() { 
+        protected override void performLayout() {
             SliverConstraints constraints = this.constraints; 
             D.assert(resolvedPadding != null);
             float? beforePadding = this.beforePadding;
             this.child = child;
         }
 
-        EdgeInsets resolvedPadding {
+        protected override EdgeInsets resolvedPadding {
             get { return  _resolvedPadding;}
         }
         EdgeInsets _resolvedPadding;

com.unity.uiwidgets/Runtime/widgets/framework.cs

         }
     }
 
-    public abstract class ParentDataWidget<T> : ParentDataWidget where T : ParentData {
+    public abstract class ParentDataWidget<T> : ParentDataWidget where T : IParentData {
         public ParentDataWidget(Key key = null, Widget child = null)
             : base(key: key, child: child) {
         }
         
 
         public override bool debugIsValidRenderObject(RenderObject renderObject) {
-            D.assert(typeof(T) != typeof(ParentData));
+            D.assert(typeof(T) != typeof(IParentData));
             return renderObject.parentData is T;
         }
 

com.unity.uiwidgets/Runtime/widgets/sliver.cs

         }
     }*/
 
-    public class KeepAlive : ParentDataWidget<KeepAliveParentDataMixin> {
+    public class KeepAlive : ParentDataWidget<IKeepAliveParentDataMixin> {
         public KeepAlive(
             Key key = null,
             bool keepAlive = true,
         public readonly bool keepAlive;
 
         public override void applyParentData(RenderObject renderObject) {
-            D.assert(renderObject.parentData is SliverMultiBoxAdaptorParentData);
-           // SliverMultiBoxAdaptorParentData parentData = (SliverMultiBoxAdaptorParentData) renderObject.parentData;
-            KeepAliveParentDataMixin parentData = (KeepAliveParentDataMixin) renderObject.parentData ;
+            D.assert(renderObject.parentData is IKeepAliveParentDataMixin);
+            IKeepAliveParentDataMixin parentData = (IKeepAliveParentDataMixin) renderObject.parentData ;
-            if (((KeepAliveParentDataMixin)parentData).keepAlive != keepAlive) {
+            if (((IKeepAliveParentDataMixin)parentData).keepAlive != keepAlive) {
                 parentData.keepAlive = keepAlive;
                 var targetParent = renderObject.parent;
                 if (targetParent is RenderObject && !keepAlive) {