Fix _DragAvatar to display in the proper position for Overlays whose...

Fix _DragAvatar to display in the proper position for Overlays whose Point.origin is not the global Point.origin. (#7223)

packages/flutter/lib/src/widgets/drag_target.dart

@@ -267,7 +267,7 @@ class _DraggableState<T> extends State<Draggable<T>> {
       _activeCount += 1;
     });
     final _DragAvatar<T> avatar = new _DragAvatar<T>(
-      overlay: Overlay.of(context, debugRequiredFor: config),
+      overlayState: Overlay.of(context, debugRequiredFor: config),
       data: config.data,
       initialPosition: position,
       dragStartPoint: dragStartPoint,
@@ -406,7 +406,7 @@ typedef void _OnDragEnd(Velocity velocity, Offset offset, bool wasAccepted);
 // eeding this object pointer events even after it has been disposed.
 class _DragAvatar<T> extends Drag {
   _DragAvatar({
-    OverlayState overlay,
+    this.overlayState,
     this.data,
     Point initialPosition,
     this.dragStartPoint: Point.origin,
@@ -414,11 +414,11 @@ class _DragAvatar<T> extends Drag {
     this.feedbackOffset: Offset.zero,
     this.onDragEnd
   }) {
-    assert(overlay != null);
+    assert(overlayState != null);
     assert(dragStartPoint != null);
     assert(feedbackOffset != null);
     _entry = new OverlayEntry(builder: _build);
-    overlay.insert(_entry);
+    overlayState.insert(_entry);
     _position = initialPosition;
     updateDrag(initialPosition);
   }
@@ -428,6 +428,7 @@ class _DragAvatar<T> extends Drag {
   final Widget feedback;
   final Offset feedbackOffset;
   final _OnDragEnd onDragEnd;
+  final OverlayState overlayState;
 
   _DragTargetState<T> _activeTarget;
   List<_DragTargetState<T>> _enteredTargets = <_DragTargetState<T>>[];
@@ -525,9 +526,11 @@ class _DragAvatar<T> extends Drag {
   }
 
   Widget _build(BuildContext context) {
+    RenderBox box = overlayState.context.findRenderObject();
+    Point overlayTopLeft = box.localToGlobal(Point.origin);
     return new Positioned(
-      left: _lastOffset.dx,
-      top: _lastOffset.dy,
+      left: _lastOffset.dx - overlayTopLeft.x,
+      top: _lastOffset.dy - overlayTopLeft.y,
       child: new IgnorePointer(
         child: feedback
       )

packages/flutter/test/widgets/draggable_test.dart

@@ -1223,6 +1223,82 @@ void main() {
     await tester.tap(find.text('X'));
     expect(events, equals(<String>['tap']));
   });
+
+  testWidgets('Drag feedback with child anchor positions correctly', (WidgetTester tester) async {
+    await _testChildAnchorFeedbackPosition(tester: tester);
+  });
+
+  testWidgets('Drag feedback with child anchor within a non-global Overlay positions correctly', (WidgetTester tester) async {
+    await _testChildAnchorFeedbackPosition(tester: tester, left: 100.0, top: 100.0);
+  });
+}
+
+Future<Null> _testChildAnchorFeedbackPosition({WidgetTester tester, double top: 0.0, double left: 0.0}) async {
+  List<int> accepted = <int>[];
+  int dragStartedCount = 0;
+
+  await tester.pumpWidget(new Stack(children: <Widget>[
+    new Positioned(
+      left: left,
+      top: top,
+      right: 0.0,
+      bottom: 0.0,
+      child: new MaterialApp(
+        home: new Column(
+          children: <Widget>[
+            new Draggable<int>(
+              data: 1,
+              child: new Text('Source'),
+              feedback: new Text('Dragging'),
+              onDragStarted: () {
+                ++dragStartedCount;
+              },
+            ),
+            new DragTarget<int>(
+              builder: (BuildContext context, List<int> data, List<dynamic> rejects) {
+                return new Container(height: 100.0, child: new Text('Target'));
+              },
+              onAccept: (int data) {
+                accepted.add(data);
+              }
+            ),
+          ]
+        )
+      )
+    )
+  ]));
+
+  expect(accepted, isEmpty);
+  expect(find.text('Source'), findsOneWidget);
+  expect(find.text('Dragging'), findsNothing);
+  expect(find.text('Target'), findsOneWidget);
+  expect(dragStartedCount, 0);
+
+  Point firstLocation = tester.getCenter(find.text('Source'));
+  TestGesture gesture = await tester.startGesture(firstLocation, pointer: 7);
+  await tester.pump();
+
+  expect(accepted, isEmpty);
+  expect(find.text('Source'), findsOneWidget);
+  expect(find.text('Dragging'), findsOneWidget);
+  expect(find.text('Target'), findsOneWidget);
+  expect(dragStartedCount, 1);
+
+
+  Point secondLocation = tester.getBottomRight(find.text('Target'));
+  await gesture.moveTo(secondLocation);
+  await tester.pump();
+
+  expect(accepted, isEmpty);
+  expect(find.text('Source'), findsOneWidget);
+  expect(find.text('Dragging'), findsOneWidget);
+  expect(find.text('Target'), findsOneWidget);
+  expect(dragStartedCount, 1);
+
+  Point feedbackTopLeft = tester.getTopLeft(find.text('Dragging'));
+  Point sourceTopLeft = tester.getTopLeft(find.text('Source'));
+  Offset dragOffset = secondLocation - firstLocation;
+  expect(feedbackTopLeft, equals(sourceTopLeft + dragOffset));
 }
 
 class DragTargetData { }