fix meterial (9)

fix drag target and add sample

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

             D.assert(!_candidateAvatars.Contains(avatar));
             D.assert(!_rejectedAvatars.Contains(avatar));
 
-            if (avatar.data is _DragAvatar<T> && (widget.onWillAccept == null || widget.onWillAccept(avatar.data))) {
+            if (avatar is _DragAvatar<T> && (widget.onWillAccept == null || widget.onWillAccept(avatar.data))) {
                 setState(() => { _candidateAvatars.Add(avatar); });
                 return true;
             }

Samples/UIWidgetsSamples_2019_4/Assets/WidgetsSample.meta

+fileFormatVersion: 2
+guid: 00ea9ee5829c47a687a61fb521f91107
+timeCreated: 1612237197

Samples/UIWidgetsSamples_2019_4/Assets/WidgetsSample/DragDropSample.cs

+using System.Collections.Generic;
+using uiwidgets;
+using Unity.UIWidgets.engine2;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.rendering;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+using UnityEngine;
+using ui_ = Unity.UIWidgets.widgets.ui_;
+
+namespace UIWidgetsSample {
+    public class DragDropSample : UIWidgetsPanel {
+        protected override void main() {
+            ui_.runApp(new MyApp());
+        }
+        
+        class MyApp : StatelessWidget
+        {
+            public override Widget build(BuildContext context)
+            {
+                return new WidgetsApp(
+                    home: new DragDropApp(),
+                    pageRouteBuilder: (settings, builder) =>
+                        new PageRouteBuilder(
+                            settings: settings,
+                            pageBuilder: (Buildcontext, animation, secondaryAnimation) => builder(context)
+                        )
+                );
+            }
+        }
+
+        class DragDropApp : StatefulWidget {
+            public DragDropApp(Key key = null) : base(key) {
+            }
+
+            public override State createState() {
+                return new DragDropState();
+            }
+        }
+
+        class DragTargetWidget : StatefulWidget {
+            public DragTargetWidget(Key key = null) : base(key) {
+            }
+
+            public override State createState() {
+                return new DragTargetWidgetState();
+            }
+        }
+
+        class DragTargetWidgetState : State<DragTargetWidget> {
+            int value;
+
+            public override Widget build(BuildContext context)
+            {
+                return new Positioned(
+                    left: 40.0f,
+                    bottom: 40.0f,
+                    child: new DragTarget<int>(
+                        onWillAccept: (o) => true,
+                        onAccept: obj => {
+                            Debug.Log("ON ACCEPTED ..." + obj);
+                            this.setState(() => { this.value += obj; });
+                        },
+                        builder: (inner_context2, accepted, rejected) => {
+                            return new Container(
+                                width: 40.0f,
+                                height: 40.0f,
+                                constraints: BoxConstraints.tight(new Size(40, 40)),
+                                color: Colors.red,
+                                child: new Center(child: new Text("" + this.value))
+                            );
+                        }
+                    )
+                );
+            }
+        }
+
+        class DragDropState : State<DragDropApp> {
+            public override Widget build(BuildContext context) {
+                var entries = new List<OverlayEntry>();
+
+                var entry_bg = new OverlayEntry(
+                    inner_context => new Container(
+                        color: Colors.white
+                    ));
+
+                var entry = new OverlayEntry(
+                    inner_context => new Positioned(
+                        left: 0.0f,
+                        bottom: 0.0f,
+                        child: new GestureDetector(
+                            onTap: () => { },
+                            child: new Draggable<int>(
+                                data: 5,
+                                child: new Container(
+                                    color: Colors.blue,
+                                    width: 30.0f,
+                                    height: 30.0f,
+                                    constraints: BoxConstraints.tight(new Size(30, 30)),
+                                    child: new Center(child: new Text("5"))
+                                ),
+                                feedback: new Container(
+                                    color: Colors.green,
+                                    width: 30.0f,
+                                    height: 30.0f),
+                                //maxSimultaneousDrags: 1,
+                                childWhenDragging: new Container(
+                                    color: Colors.black,
+                                    width: 30.0f,
+                                    height: 30.0f,
+                                    constraints: BoxConstraints.tight(new Size(30, 30))
+                                )
+                            )
+                        )
+                    )
+                );
+
+                var entry3 = new OverlayEntry(
+                    inner_context => new Positioned(
+                        left: 0.0f,
+                        bottom: 40.0f,
+                        child: new GestureDetector(
+                            onTap: () => { },
+                            child:
+                            new Draggable<int>(
+                                data: 8,
+                                child: new Container(
+                                    color: Colors.grey,
+                                    width: 30.0f,
+                                    height: 30.0f,
+                                    constraints: BoxConstraints.tight(new Size(30, 30)),
+                                    child: new Center(child: new Text("8")))
+                                ,
+                                feedback: new Container(
+                                    color: Colors.green,
+                                    width: 30.0f,
+                                    height: 30.0f),
+                                maxSimultaneousDrags: 1,
+                                childWhenDragging: new Container(
+                                    color: Colors.black,
+                                    width: 30.0f,
+                                    height: 30.0f,
+                                    constraints: BoxConstraints.tight(new Size(30, 30))
+                                )
+                            )
+                        )
+                    )
+                );
+
+                var entry2 = new OverlayEntry(
+                    inner_context => new DragTargetWidget()
+                );
+
+                entries.Add(entry_bg);
+                entries.Add(entry);
+                entries.Add(entry2);
+                entries.Add(entry3);
+
+                return new Container(
+                    color: Colors.white,
+                    child: new Overlay(
+                        initialEntries: entries
+                    )
+                );
+            }
+        }
+    }
+}

Samples/UIWidgetsSamples_2019_4/Assets/WidgetsSample/DragDropSample.cs.meta

+fileFormatVersion: 2
+guid: f6b796615f43b734d8c9507513773ff3
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: