Add Draggable.onDragStarted (#6343)

Fixes #6084

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

@@ -75,6 +75,7 @@ class Draggable<T> extends StatefulWidget {
     this.dragAnchor: DragAnchor.child,
     this.affinity,
     this.maxSimultaneousDrags,
+    this.onDragStarted,
     this.onDraggableCanceled
   }) : super(key: key) {
     assert(child != null);
@@ -127,6 +128,9 @@ class Draggable<T> extends StatefulWidget {
   /// dragged at a time.
   final int maxSimultaneousDrags;
 
+  /// Called when the draggable starts being dragged.
+  final VoidCallback onDragStarted;
+
   /// Called when the draggable is dropped without being accepted by a [DragTarget].
   final DraggableCanceledCallback onDraggableCanceled;
 
@@ -164,6 +168,7 @@ class LongPressDraggable<T> extends Draggable<T> {
     Offset feedbackOffset: Offset.zero,
     DragAnchor dragAnchor: DragAnchor.child,
     int maxSimultaneousDrags,
+    VoidCallback onDragStarted,
     DraggableCanceledCallback onDraggableCanceled
   }) : super(
     key: key,
@@ -174,6 +179,7 @@ class LongPressDraggable<T> extends Draggable<T> {
     feedbackOffset: feedbackOffset,
     dragAnchor: dragAnchor,
     maxSimultaneousDrags: maxSimultaneousDrags,
+    onDragStarted: onDragStarted,
     onDraggableCanceled: onDraggableCanceled
   );
 
@@ -227,7 +233,7 @@ class _DraggableState<T> extends State<Draggable<T>> {
     setState(() {
       _activeCount += 1;
     });
-    return new _DragAvatar<T>(
+    final _DragAvatar<T> avatar = new _DragAvatar<T>(
       overlay: Overlay.of(context, debugRequiredFor: config),
       data: config.data,
       initialPosition: position,
@@ -242,6 +248,9 @@ class _DraggableState<T> extends State<Draggable<T>> {
         });
       }
     );
+    if (config.onDragStarted != null)
+      config.onDragStarted();
+    return avatar;
   }
 
   @override

packages/flutter/test/widget/draggable_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter/material.dart';
 void main() {
   testWidgets('Drag and drop - control test', (WidgetTester tester) async {
     List<int> accepted = <int>[];
+    int dragStartedCount = 0;
 
     await tester.pumpWidget(new MaterialApp(
       home: new Column(
@@ -15,7 +16,10 @@ void main() {
           new Draggable<int>(
             data: 1,
             child: new Text('Source'),
-            feedback: new Text('Dragging')
+            feedback: new Text('Dragging'),
+            onDragStarted: () {
+              ++dragStartedCount;
+            },
           ),
           new DragTarget<int>(
             builder: (BuildContext context, List<int> data, List<dynamic> rejects) {
@@ -33,6 +37,7 @@ void main() {
     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);
@@ -42,6 +47,7 @@ void main() {
     expect(find.text('Source'), findsOneWidget);
     expect(find.text('Dragging'), findsOneWidget);
     expect(find.text('Target'), findsOneWidget);
+    expect(dragStartedCount, 1);
 
     Point secondLocation = tester.getCenter(find.text('Target'));
     await gesture.moveTo(secondLocation);
@@ -51,6 +57,7 @@ void main() {
     expect(find.text('Source'), findsOneWidget);
     expect(find.text('Dragging'), findsOneWidget);
     expect(find.text('Target'), findsOneWidget);
+    expect(dragStartedCount, 1);
 
     await gesture.up();
     await tester.pump();
@@ -59,6 +66,7 @@ void main() {
     expect(find.text('Source'), findsOneWidget);
     expect(find.text('Dragging'), findsNothing);
     expect(find.text('Target'), findsOneWidget);
+    expect(dragStartedCount, 1);
   });
 
   testWidgets('Drag and drop - dragging over button', (WidgetTester tester) async {