Fix iOS last up event velocity calculation - version 3 (#11571)

* Add synthesized property to pointer events

* test

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

@@ -170,7 +170,8 @@ class PointerEventConverter {
               radiusMin: datum.radiusMin,
               radiusMax: datum.radiusMax,
               orientation: datum.orientation,
-              tilt: datum.tilt
+              tilt: datum.tilt,
+              synthesized: true,
             );
             state.lastPosition = position;
           }
@@ -257,7 +258,8 @@ class PointerEventConverter {
               radiusMin: datum.radiusMin,
               radiusMax: datum.radiusMax,
               orientation: datum.orientation,
-              tilt: datum.tilt
+              tilt: datum.tilt,
+              synthesized: true,
             );
             state.lastPosition = position;
           }

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

@@ -113,7 +113,8 @@ abstract class PointerEvent {
     this.radiusMin: 0.0,
     this.radiusMax: 0.0,
     this.orientation: 0.0,
-    this.tilt: 0.0
+    this.tilt: 0.0,
+    this.synthesized: false,
   });
 
   /// Time of event dispatch, relative to an arbitrary timeline.
@@ -235,6 +236,18 @@ abstract class PointerEvent {
   /// the stylus is flat on that surface).
   final double tilt;
 
+  /// We occasionally synthesize PointerEvents that aren't exact translations
+  /// of [ui.PointerData] from the engine to cover small cross-OS discrepancies
+  /// in pointer behaviours.
+  ///
+  /// For instance, on end events, Android always drops any location changes
+  /// that happened between its reporting intervals when emiting the end events.
+  ///
+  /// On iOS, minor incorrect location changes from the previous move events
+  /// can be reported on end events. We synthesize a [PointerEvent] to cover
+  /// the difference between the 2 events in that case.
+  final bool synthesized;
+
   @override
   String toString() => '$runtimeType($position)';
 
@@ -261,7 +274,8 @@ abstract class PointerEvent {
              'radiusMin: $radiusMin, '
              'radiusMax: $radiusMax, '
              'orientation: $orientation, '
-             'tilt: $tilt'
+             'tilt: $tilt, '
+             'synthesized: $synthesized'
            ')';
   }
 }
@@ -365,7 +379,8 @@ class PointerHoverEvent extends PointerEvent {
     double radiusMin: 0.0,
     double radiusMax: 0.0,
     double orientation: 0.0,
-    double tilt: 0.0
+    double tilt: 0.0,
+    bool synthesized: false,
   }) : super(
     timeStamp: timeStamp,
     kind: kind,
@@ -384,7 +399,8 @@ class PointerHoverEvent extends PointerEvent {
     radiusMin: radiusMin,
     radiusMax: radiusMax,
     orientation: orientation,
-    tilt: tilt
+    tilt: tilt,
+    synthesized: synthesized,
   );
 }
 
@@ -463,7 +479,8 @@ class PointerMoveEvent extends PointerEvent {
     double radiusMin: 0.0,
     double radiusMax: 0.0,
     double orientation: 0.0,
-    double tilt: 0.0
+    double tilt: 0.0,
+    bool synthesized: false,
   }) : super(
     timeStamp: timeStamp,
     pointer: pointer,
@@ -484,7 +501,8 @@ class PointerMoveEvent extends PointerEvent {
     radiusMin: radiusMin,
     radiusMax: radiusMax,
     orientation: orientation,
-    tilt: tilt
+    tilt: tilt,
+    synthesized: synthesized,
   );
 }
 

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

@@ -130,7 +130,8 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
     if (event is PointerMoveEvent) {
       final VelocityTracker tracker = _velocityTrackers[event.pointer];
       assert(tracker != null);
-      tracker.addPosition(event.timeStamp, event.position);
+      if (!event.synthesized)
+        tracker.addPosition(event.timeStamp, event.position);
       final Offset delta = event.delta;
       if (_state == _DragState.accepted) {
         if (onUpdate != null) {

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

@@ -62,7 +62,8 @@ abstract class MultiDragPointerState {
 
   void _move(PointerMoveEvent event) {
     assert(_arenaEntry != null);
-    _velocityTracker.addPosition(event.timeStamp, event.position);
+    if (!event.synthesized)
+      _velocityTracker.addPosition(event.timeStamp, event.position);
     if (_client != null) {
       assert(pendingDelta == null);
       // Call client last to avoid reentrancy.

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

@@ -152,7 +152,8 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     if (event is PointerMoveEvent) {
       final VelocityTracker tracker = _velocityTrackers[event.pointer];
       assert(tracker != null);
-      tracker.addPosition(event.timeStamp, event.position);
+      if (!event.synthesized)
+        tracker.addPosition(event.timeStamp, event.position);
       _pointerLocations[event.pointer] = event.position;
       shouldStartIfAccepted = true;
     } else if (event is PointerDownEvent) {

packages/flutter/test/gestures/drag_test.dart

@@ -236,6 +236,38 @@ void main() {
     drag.dispose();
   });
 
+  testGesture('Synthesized pointer events are ignored for velocity tracking', (GestureTester tester) {
+    final HorizontalDragGestureRecognizer drag = new HorizontalDragGestureRecognizer();
+
+    Velocity velocity;
+    drag.onEnd = (DragEndDetails details) {
+      velocity = details.velocity;
+    };
+
+    final TestPointer pointer = new TestPointer(1);
+    final PointerDownEvent down = pointer.down(const Offset(10.0, 25.0), timeStamp: const Duration(milliseconds: 10));
+    drag.addPointer(down);
+    tester.closeArena(1);
+    tester.route(down);
+    tester.route(pointer.move(const Offset(20.0, 25.0), timeStamp: const Duration(milliseconds: 20)));
+    tester.route(pointer.move(const Offset(30.0, 25.0), timeStamp: const Duration(milliseconds: 30)));
+    tester.route(pointer.move(const Offset(40.0, 25.0), timeStamp: const Duration(milliseconds: 40)));
+    tester.route(pointer.move(const Offset(50.0, 25.0), timeStamp: const Duration(milliseconds: 50)));
+    tester.route(new PointerMoveEvent(
+      pointer: 1,
+      // Simulate a small synthesized wobble which would have slowed down the
+      // horizontal velocity from 1 px/ms and introduced a slight vertical velocity.
+      position: const Offset(51.0, 26.0),
+      timeStamp: const Duration(milliseconds: 60),
+      synthesized: true,
+    ));
+    tester.route(pointer.up(timeStamp: const Duration(milliseconds: 20)));
+    expect(velocity.pixelsPerSecond.dx, moreOrLessEquals(1000.0));
+    expect(velocity.pixelsPerSecond.dy, moreOrLessEquals(0.0));
+
+    drag.dispose();
+  });
+
   testGesture('Drag details', (GestureTester tester) {
     expect(new DragDownDetails(), hasOneLineDescription);
     expect(new DragStartDetails(), hasOneLineDescription);