Add optional DragAnchorStrategy to Draggable (#73143)

dev/manual_tests/lib/drag_and_drop.dart

@@ -116,6 +116,7 @@ class ExampleDragSource extends StatelessWidget {
 
     Offset feedbackOffset;
     DragAnchor anchor;
+    DragAnchorStrategy dragAnchorStrategy;
     if (!under) {
       feedback = Transform(
         transform: Matrix4.identity()
@@ -124,9 +125,11 @@ class ExampleDragSource extends StatelessWidget {
       );
       feedbackOffset = const Offset(0.0, -kFingerSize);
       anchor = DragAnchor.pointer;
+      dragAnchorStrategy = pointerDragAnchorStrategy;
     } else {
       feedbackOffset = Offset.zero;
       anchor = DragAnchor.child;
+      dragAnchorStrategy = childDragAnchorStrategy;
     }
 
     if (heavy) {
@@ -143,7 +146,7 @@ class ExampleDragSource extends StatelessWidget {
         child: contents,
         feedback: feedback,
         feedbackOffset: feedbackOffset,
-        dragAnchor: anchor,
+        dragAnchorStrategy: dragAnchorStrategy,
       );
     }
   }

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

@@ -66,6 +66,24 @@ typedef DragTargetLeave<T> = void Function(T? data);
 /// Used by [DragTarget.onMove].
 typedef DragTargetMove<T> = void Function(DragTargetDetails<T> details);
 
+/// Signature for the strategy that determines the drag start point.
+///
+/// Used for the built-in strategies switched via [DragAnchor] and the optinally
+/// injectable [Draggable.dragAnchorStrategy]
+typedef DragAnchorStrategy = Offset Function(Draggable<Object> draggable, BuildContext context, Offset position);
+
+/// The default [DragAnchorStrategy] used when [Draggable.dragAnchor] is not set
+/// or set to [DragAnchor.child]
+Offset childDragAnchorStrategy(Draggable<Object> draggable, BuildContext context, Offset position) {
+  final RenderBox renderObject = context.findRenderObject()! as RenderBox;
+  return renderObject.globalToLocal(position);
+}
+
+/// The [DragAnchorStrategy] used when [Draggable.dragAnchor] set to
+/// [DragAnchor.pointer]
+Offset pointerDragAnchorStrategy(Draggable<Object> draggable, BuildContext context, Offset position) {
+  return Offset.zero;
+}
 /// Where the [Draggable] should be anchored during a drag.
 enum DragAnchor {
   /// Display the feedback anchored at the position of the original child. If
@@ -191,7 +209,12 @@ class Draggable<T extends Object> extends StatefulWidget {
     this.axis,
     this.childWhenDragging,
     this.feedbackOffset = Offset.zero,
+    @Deprecated(
+      'Use dragAnchorStrategy instead. '
+      'This feature was deprecated after v2.1.0-10.0.pre.'
+    )
     this.dragAnchor = DragAnchor.child,
+    this.dragAnchorStrategy,
     this.affinity,
     this.maxSimultaneousDrags,
     this.onDragStarted,
@@ -263,8 +286,23 @@ class Draggable<T extends Object> extends StatefulWidget {
   final Offset feedbackOffset;
 
   /// Where this widget should be anchored during a drag.
+  ///
+  /// This property is overridden by the [dragAnchorStrategy] if the latter is provided.
+  ///
+  /// Defaults to [DragAnchor.child].
+  @Deprecated(
+    'Use dragAnchorStrategy instead. '
+    'This feature was deprecated after v2.1.0-10.0.pre.'
+  )
   final DragAnchor dragAnchor;
 
+  /// A strategy that is used by this draggable to get the the anchor offset when it is dragged.
+  ///
+  /// The anchor offset refers to the distance between the users' fingers and the [feedback] widget when this draggable is dragged.
+  ///
+  /// Defaults to [childDragAnchorStrategy] if the [dragAnchor] is set to [DragAnchor.child] or [pointerDragAnchorStrategy] if the [dragAnchor] is set to [DragAnchor.pointer].
+  final DragAnchorStrategy? dragAnchorStrategy;
+
   /// Whether the semantics of the [feedback] widget is ignored when building
   /// the semantics tree.
   ///
@@ -308,7 +346,7 @@ class Draggable<T extends Object> extends StatefulWidget {
   /// Called when the draggable starts being dragged.
   final VoidCallback? onDragStarted;
 
-  /// Called when the draggable is being dragged.
+  /// Called when the draggable is dragged.
   ///
   /// This function will only be called while this widget is still mounted to
   /// the tree (i.e. [State.mounted] is true), and if this widget has actually moved.
@@ -487,14 +525,17 @@ class _DraggableState<T extends Object> extends State<Draggable<T>> {
     if (widget.maxSimultaneousDrags != null && _activeCount >= widget.maxSimultaneousDrags!)
       return null;
     final Offset dragStartPoint;
-    switch (widget.dragAnchor) {
-      case DragAnchor.child:
-        final RenderBox renderObject = context.findRenderObject()! as RenderBox;
-        dragStartPoint = renderObject.globalToLocal(position);
-        break;
-      case DragAnchor.pointer:
-        dragStartPoint = Offset.zero;
-        break;
+    if (widget.dragAnchorStrategy == null) {
+      switch (widget.dragAnchor) {
+        case DragAnchor.child:
+          dragStartPoint = childDragAnchorStrategy(widget, context, position);
+          break;
+        case DragAnchor.pointer:
+          dragStartPoint = pointerDragAnchorStrategy(widget, context, position);
+          break;
+      }
+    } else {
+      dragStartPoint = widget.dragAnchorStrategy!(widget, context, position);
     }
     setState(() {
       _activeCount += 1;

packages/flutter/test/widgets/draggable_test.dart

@@ -2540,7 +2540,6 @@ void main() {
     expect(find.text('Target'), findsOneWidget);
     expect(onDragCompletedCalled, isFalse);
 
-
     final Offset secondLocation = tester.getCenter(find.text('Target'));
     await gesture.moveTo(secondLocation);
     await tester.pump();
@@ -3019,6 +3018,30 @@ void main() {
     semantics.dispose();
   });
 
+  testWidgets('Drag and drop - when a dragAnchorStrategy is provided it gets called', (WidgetTester tester) async {
+    bool dragAnchorStrategyCalled = false;
+
+    await tester.pumpWidget(MaterialApp(
+      home: Column(
+        children: <Widget>[
+          Draggable<int>(
+            child: const Text('Source'),
+            feedback: const Text('Feedback'),
+            dragAnchorStrategy: (Draggable<Object> widget, BuildContext context, Offset position) {
+              dragAnchorStrategyCalled = true;
+              return const Offset(0, 0);
+            }
+          )
+        ],
+      ),
+    ));
+
+    final Offset location = tester.getCenter(find.text('Source'));
+    await tester.startGesture(location, pointer: 7);
+
+    expect(dragAnchorStrategyCalled, true);
+  });
+
   testWidgets('configurable Draggable hit test behavior', (WidgetTester tester) async {
     const HitTestBehavior hitTestBehavior = HitTestBehavior.deferToChild;