Improve cooperation between scale and drag gestures (#9298)

Now the scale gesture will accept if its focal point moves more than the pan
slop. This change lets it compete with a drag gesture (e.g., a containing scrol
view) in the same way that the pan gesture does.

Fixes #8735

examples/flutter_gallery/lib/demo/material/grid_list_demo.dart

@@ -121,24 +121,20 @@ class _GridPhotoViewerState extends State<GridPhotoViewer> with SingleTickerProv
 
   @override
   Widget build(BuildContext context) {
-    return new LayoutBuilder(
-      builder: (BuildContext context, BoxConstraints constraints) {
     return new GestureDetector(
       onScaleStart: _handleOnScaleStart,
       onScaleUpdate: _handleOnScaleUpdate,
       onScaleEnd: _handleOnScaleEnd,
+      child: new ClipRect(
         child: new Transform(
           transform: new Matrix4.identity()
             ..translate(_offset.dx, _offset.dy)
             ..scale(_scale),
-            child: new ClipRect(
           child: new Image.asset(config.photo.assetName, fit: BoxFit.cover),
         ),
       ),
     );
   }
-    );
-  }
 }
 
 class GridDemoPhotoItem extends StatelessWidget {

packages/flutter/lib/src/gestures/scale.dart

@@ -9,7 +9,7 @@ import 'recognizer.dart';
 import 'velocity_tracker.dart';
 
 /// The possible states of a [ScaleGestureRecognizer].
-enum ScaleState {
+enum _ScaleState {
   /// The recognizer is ready to start recognizing a gesture.
   ready,
 
@@ -39,6 +39,9 @@ class ScaleStartDetails {
   /// The initial focal point of the pointers in contact with the screen.
   /// Reported in global coordinates.
   final Point focalPoint;
+
+  @override
+  String toString() => 'ScaleStartDetails(focalPoint: $focalPoint)';
 }
 
 /// Details for [GestureScaleUpdateCallback].
@@ -59,6 +62,9 @@ class ScaleUpdateDetails {
   /// The scale implied by the pointers in contact with the screen. A value
   /// greater than or equal to zero.
   final double scale;
+
+  @override
+  String toString() => 'ScaleUpdateDetails(focalPoint: $focalPoint, scale: $scale)';
 }
 
 /// Details for [GestureScaleEndCallback].
@@ -72,6 +78,9 @@ class ScaleEndDetails {
 
   /// The velocity of the last pointer to be lifted off of the screen.
   final Velocity velocity;
+
+  @override
+  String toString() => 'ScaleEndDetails(velocity: $velocity)';
 }
 
 /// Signature for when the pointers in contact with the screen have established
@@ -110,8 +119,10 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
   /// The pointers are no longer in contact with the screen.
   GestureScaleEndCallback onEnd;
 
-  ScaleState _state = ScaleState.ready;
+  _ScaleState _state = _ScaleState.ready;
 
+  Point _initialFocalPoint;
+  Point _currentFocalPoint;
   double _initialSpan;
   double _currentSpan;
   Map<int, Point> _pointerLocations;
@@ -123,8 +134,8 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
   void addPointer(PointerEvent event) {
     startTrackingPointer(event.pointer);
     _velocityTrackers[event.pointer] = new VelocityTracker();
-    if (_state == ScaleState.ready) {
+    if (_state == _ScaleState.ready) {
-      _state = ScaleState.possible;
+      _state = _ScaleState.possible;
       _initialSpan = 0.0;
       _currentSpan = 0.0;
       _pointerLocations = <int, Point>{};
@@ -133,44 +144,50 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
 
   @override
   void handleEvent(PointerEvent event) {
-    assert(_state != ScaleState.ready);
+    assert(_state != _ScaleState.ready);
-    bool configChanged = false;
+    bool didChangeConfiguration = false;
+    bool shouldStartIfAccepted = false;
     if (event is PointerMoveEvent) {
       final VelocityTracker tracker = _velocityTrackers[event.pointer];
       assert(tracker != null);
       tracker.addPosition(event.timeStamp, event.position);
       _pointerLocations[event.pointer] = event.position;
+      shouldStartIfAccepted = true;
     } else if (event is PointerDownEvent) {
-      configChanged = true;
       _pointerLocations[event.pointer] = event.position;
-    } else if (event is PointerUpEvent) {
+      didChangeConfiguration = true;
-      configChanged = true;
+      shouldStartIfAccepted = true;
+    } else if (event is PointerUpEvent || event is PointerCancelEvent) {
       _pointerLocations.remove(event.pointer);
+      didChangeConfiguration = true;
     }
 
-    _update(configChanged, event.pointer);
+    _update();
-
+    if (!didChangeConfiguration || _reconfigure(event.pointer))
+      _advanceStateMachine(shouldStartIfAccepted);
     stopTrackingIfPointerNoLongerDown(event);
   }
 
-  void _update(bool configChanged, int pointer) {
+  void _update() {
     final int count = _pointerLocations.keys.length;
 
     // Compute the focal point
     Point focalPoint = Point.origin;
     for (int pointer in _pointerLocations.keys)
       focalPoint += _pointerLocations[pointer].toOffset();
-    focalPoint = new Point(focalPoint.x / count, focalPoint.y / count);
+    _currentFocalPoint = count > 0 ? new Point(focalPoint.x / count, focalPoint.y / count) : Point.origin;
 
     // Span is the average deviation from focal point
     double totalDeviation = 0.0;
     for (int pointer in _pointerLocations.keys)
-      totalDeviation += (focalPoint - _pointerLocations[pointer]).distance;
+      totalDeviation += (_currentFocalPoint - _pointerLocations[pointer]).distance;
     _currentSpan = count > 0 ? totalDeviation / count : 0.0;
+  }
 
-    if (configChanged) {
+  bool _reconfigure(int pointer) {
+    _initialFocalPoint = _currentFocalPoint;
     _initialSpan = _currentSpan;
-      if (_state == ScaleState.started) {
+    if (_state == _ScaleState.started) {
       if (onEnd != null) {
         final VelocityTracker tracker = _velocityTrackers[pointer];
         assert(tracker != null);
@@ -185,52 +202,69 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
           invokeCallback<Null>('onEnd', () => onEnd(new ScaleEndDetails(velocity: Velocity.zero))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
         }
       }
-        _state = ScaleState.accepted;
+      _state = _ScaleState.accepted;
+      return false;
     }
+    return true;
   }
 
-    if (_state == ScaleState.ready)
+  void _advanceStateMachine(bool shouldStartIfAccepted) {
-      _state = ScaleState.possible;
+    if (_state == _ScaleState.ready)
+      _state = _ScaleState.possible;
 
-    if (_state == ScaleState.possible &&
+    if (_state == _ScaleState.possible) {
-        (_currentSpan - _initialSpan).abs() > kScaleSlop) {
+      final double spanDelta = (_currentSpan - _initialSpan).abs();
+      final double focalPointDelta = (_currentFocalPoint - _initialFocalPoint).distance;
+      if (spanDelta > kScaleSlop || focalPointDelta > kPanSlop)
+        resolve(GestureDisposition.accepted);
+    } else if (_state.index >= _ScaleState.accepted.index) {
       resolve(GestureDisposition.accepted);
     }
 
-    if (_state == ScaleState.accepted && !configChanged) {
+    if (_state == _ScaleState.accepted && shouldStartIfAccepted) {
-      _state = ScaleState.started;
+      _state = _ScaleState.started;
-      if (onStart != null)
+      _dispatchOnStartCallbackIfNeeded();
-        invokeCallback<Null>('onStart', () => onStart(new ScaleStartDetails(focalPoint: focalPoint))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
     }
 
-    if (_state == ScaleState.started && onUpdate != null)
+    if (_state == _ScaleState.started && onUpdate != null)
-      invokeCallback<Null>('onUpdate', () => onUpdate(new ScaleUpdateDetails(scale: _scaleFactor, focalPoint: focalPoint))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<Null>('onUpdate', () => onUpdate(new ScaleUpdateDetails(scale: _scaleFactor, focalPoint: _currentFocalPoint))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+  }
+
+  void _dispatchOnStartCallbackIfNeeded() {
+    assert(_state == _ScaleState.started);
+    if (onStart != null)
+      invokeCallback<Null>('onStart', () => onStart(new ScaleStartDetails(focalPoint: _currentFocalPoint))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
   }
 
   @override
   void acceptGesture(int pointer) {
-    if (_state != ScaleState.accepted) {
+    if (_state == _ScaleState.possible) {
-      _state = ScaleState.accepted;
+      _state = _ScaleState.started;
-      _update(false, pointer);
+      _dispatchOnStartCallbackIfNeeded();
     }
   }
 
+  @override
+  void rejectGesture(int pointer) {
+    stopTrackingPointer(pointer);
+  }
+
   @override
   void didStopTrackingLastPointer(int pointer) {
     switch(_state) {
-      case ScaleState.possible:
+      case _ScaleState.possible:
         resolve(GestureDisposition.rejected);
         break;
-      case ScaleState.ready:
+      case _ScaleState.ready:
         assert(false);  // We should have not seen a pointer yet
         break;
-      case ScaleState.accepted:
+      case _ScaleState.accepted:
         break;
-      case ScaleState.started:
+      case _ScaleState.started:
         assert(false);  // We should be in the accepted state when user is done
         break;
     }
-    _state = ScaleState.ready;
+    _state = _ScaleState.ready;
   }
 
   @override

packages/flutter/test/gestures/scale_test.dart

@@ -184,4 +184,81 @@ void main() {
     scale.dispose();
     tap.dispose();
   });
+
+  testGesture('Scale gesture competes with drag', (GestureTester tester) {
+    final ScaleGestureRecognizer scale = new ScaleGestureRecognizer();
+    final HorizontalDragGestureRecognizer drag = new HorizontalDragGestureRecognizer();
+
+    final List<String> log = <String>[];
+
+    scale.onStart = (ScaleStartDetails details) { log.add('scale-start'); };
+    scale.onUpdate = (ScaleUpdateDetails details) { log.add('scale-update'); };
+    scale.onEnd = (ScaleEndDetails details) { log.add('scale-end'); };
+
+    drag.onStart = (DragStartDetails details) { log.add('drag-start'); };
+    drag.onEnd = (DragEndDetails details) { log.add('drag-end'); };
+
+    final TestPointer pointer1 = new TestPointer(1);
+
+    final PointerDownEvent down = pointer1.down(const Point(10.0, 10.0));
+    scale.addPointer(down);
+    drag.addPointer(down);
+
+    tester.closeArena(1);
+    expect(log, isEmpty);
+
+    // Vertical moves are scales.
+    tester.route(down);
+    expect(log, isEmpty);
+
+    tester.route(pointer1.move(const Point(10.0, 30.0)));
+    expect(log, equals(<String>['scale-start', 'scale-update']));
+    log.clear();
+
+    final TestPointer pointer2 = new TestPointer(2);
+    final PointerDownEvent down2 = pointer2.down(const Point(10.0, 20.0));
+    scale.addPointer(down2);
+    drag.addPointer(down2);
+
+    tester.closeArena(2);
+    expect(log, isEmpty);
+
+    // Second pointer joins scale even though it moves horizontally.
+    tester.route(down2);
+    expect(log, <String>['scale-end']);
+    log.clear();
+
+    tester.route(pointer2.move(const Point(30.0, 20.0)));
+    expect(log, equals(<String>['scale-start', 'scale-update']));
+    log.clear();
+
+    tester.route(pointer1.up());
+    expect(log, equals(<String>['scale-end']));
+    log.clear();
+
+    tester.route(pointer2.up());
+    expect(log, isEmpty);
+    log.clear();
+
+    // Horizontal moves are drags.
+    final TestPointer pointer3 = new TestPointer(3);
+    final PointerDownEvent down3 = pointer3.down(const Point(30.0, 30.0));
+    scale.addPointer(down3);
+    drag.addPointer(down3);
+    tester.closeArena(3);
+    tester.route(down3);
+
+    expect(log, isEmpty);
+
+    tester.route(pointer3.move(const Point(50.0, 30.0)));
+    expect(log, equals(<String>['scale-start', 'scale-update']));
+    log.clear();
+
+    tester.route(pointer3.up());
+    expect(log, equals(<String>['scale-end']));
+    log.clear();
+
+    scale.dispose();
+    drag.dispose();
+  });
 }