Initial support for the fling gesture detector

Added FlingGestureRecognizer and exposed it in the GestureDetector class. FlingGestureRecognizer is based on the Android/Chromium VelocityTracker class which computes a velocity vector for for a list of X,Y,Time tuples.

The Scrollable classes now use the FlingGestureRecognizer. Dismissable and Drawer still need to be updated

packages/flutter/lib/gestures/constants.dart

@@ -19,5 +19,5 @@ const double kDoubleTapTouchSlop = kTouchSlop;  // Logical pixels
 const double kPagingTouchSlop = kTouchSlop * 2.0;  // Logical pixels
 const double kDoubleTapSlop = 100.0;  // Logical pixels
 const double kWindowTouchSlop = 16.0;  // Logical pixels
-const double kMinFlingVelocity = 50.0;  // TODO(abarth): Which units is this in?
-const double kMaxFlingVelocity = 8000.0;  // TODO(abarth): Which units is this in?
+const double kMinFlingVelocity = 50.0;  // Logical pixels / second
+const double kMaxFlingVelocity = 8000.0;  // Logical pixels / second

packages/flutter/lib/gestures/fling.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:sky' as sky;
+
+import 'package:sky/gestures/arena.dart';
+import 'package:sky/gestures/recognizer.dart';
+import 'package:sky/gestures/constants.dart';
+
+// Fling velocities are logical pixels per second.
+typedef void GestureFlingCallback(sky.Offset velocity);
+
+int _eventTime(sky.PointerEvent event) => (event.timeStamp * 1000.0).toInt(); // microseconds
+
+bool _isFlingGesture(sky.GestureVelocity velocity) {
+  double velocitySquared = velocity.x * velocity.x + velocity.y * velocity.y;
+  return velocity.isValid &&
+    velocitySquared > kMinFlingVelocity * kMinFlingVelocity &&
+    velocitySquared < kMaxFlingVelocity * kMaxFlingVelocity;
+}
+
+class FlingGestureRecognizer extends GestureRecognizer {
+  FlingGestureRecognizer({ PointerRouter router, this.onFling })
+    : super(router: router);
+
+  GestureFlingCallback onFling;
+  sky.VelocityTracker _velocityTracker = new sky.VelocityTracker();
+  int _primaryPointer;
+
+  void addPointer(sky.PointerEvent event) {
+    startTrackingPointer(event.pointer);
+    if (_primaryPointer == null)
+      _primaryPointer = event.pointer;
+  }
+
+  void handleEvent(sky.PointerEvent event) {
+    if (event.pointer == _primaryPointer) {
+      if (event.type == 'pointermove') {
+        _velocityTracker.addPosition(_eventTime(event), _primaryPointer, event.x, event.y);
+      } else if (event.type == 'pointerup') {
+        sky.GestureVelocity velocity = _velocityTracker.getVelocity(_primaryPointer);
+        if (_isFlingGesture(velocity)) {
+          resolve(GestureDisposition.accepted);
+          if (onFling != null)
+            onFling(new sky.Offset(velocity.x, velocity.y));
+        }
+      }
+    }
+    stopTrackingIfPointerNoLongerDown(event);
+  }
+
+  void didStopTrackingLastPointer() {
+    _primaryPointer = null;
+    _velocityTracker.reset();
+  }
+
+  void dispose() {
+    super.dispose();
+    _velocityTracker.reset();
+    _primaryPointer = null;
+  }
+}

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

@@ -4,6 +4,7 @@
 
 import 'dart:sky' as sky;
 
+import 'package:sky/gestures/fling.dart';
 import 'package:sky/gestures/long_press.dart';
 import 'package:sky/gestures/recognizer.dart';
 import 'package:sky/gestures/scroll.dart';
@@ -27,7 +28,8 @@ class GestureDetector extends StatefulComponent {
     this.onHorizontalDragEnd,
     this.onPanStart,
     this.onPanUpdate,
-    this.onPanEnd
+    this.onPanEnd,
+    this.onFling
   }) : super(key: key);
 
   Widget child;
@@ -47,6 +49,8 @@ class GestureDetector extends StatefulComponent {
   GesturePanUpdateCallback onPanUpdate;
   GesturePanEndCallback onPanEnd;
 
+  GestureFlingCallback onFling;
+
   void syncConstructorArguments(GestureDetector source) {
     child = source.child;
     onTap = source.onTap;
@@ -61,6 +65,7 @@ class GestureDetector extends StatefulComponent {
     onPanStart = source.onPanStart;
     onPanUpdate = source.onPanUpdate;
     onPanEnd = source.onPanEnd;
+    onFling = source.onFling;
     _syncGestureListeners();
   }
 
@@ -108,6 +113,13 @@ class GestureDetector extends StatefulComponent {
     return _pan;
   }
 
+  FlingGestureRecognizer _fling;
+  FlingGestureRecognizer _ensureFling() {
+    if (_fling == null)
+      _fling = new FlingGestureRecognizer(router: _router);
+    return _fling;
+  }
+
   void didMount() {
     super.didMount();
     _syncGestureListeners();
@@ -121,6 +133,7 @@ class GestureDetector extends StatefulComponent {
     _verticalDrag = _ensureDisposed(_verticalDrag);
     _horizontalDrag = _ensureDisposed(_horizontalDrag);
     _pan = _ensureDisposed(_pan);
+    _fling = _ensureDisposed(_fling);
   }
 
   void _syncGestureListeners() {
@@ -130,6 +143,7 @@ class GestureDetector extends StatefulComponent {
     _syncVerticalDrag();
     _syncHorizontalDrag();
     _syncPan();
+    _syncFling();
   }
 
   void _syncTap() {
@@ -186,6 +200,13 @@ class GestureDetector extends StatefulComponent {
     }
   }
 
+  void _syncFling() {
+    if (onFling == null)
+      _fling = _ensureDisposed(_fling);
+    else
+      _ensureFling().onFling = onFling;
+  }
+
   GestureRecognizer _ensureDisposed(GestureRecognizer recognizer) {
     recognizer?.dispose();
     return null;
@@ -204,6 +225,8 @@ class GestureDetector extends StatefulComponent {
       _horizontalDrag.addPointer(event);
     if (_pan != null)
       _pan.addPointer(event);
+    if (_fling != null)
+      _fling.addPointer(event);
     return EventDisposition.processed;
   }
 

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

@@ -85,10 +85,10 @@ abstract class Scrollable extends StatefulComponent {
       onVerticalDragEnd: scrollDirection == ScrollDirection.vertical ? _maybeSettleScrollOffset : null,
       onHorizontalDragUpdate: scrollDirection == ScrollDirection.horizontal ? scrollBy : null,
       onHorizontalDragEnd: scrollDirection == ScrollDirection.horizontal ? _maybeSettleScrollOffset : null,
+      onFling: _handleFling,
       child: new Listener(
         child: buildContent(),
         onPointerDown: _handlePointerDown,
-        onGestureFlingStart: _handleFlingStart,
         onGestureFlingCancel: _handleFlingCancel,
         onWheel: _handleWheel
       )
@@ -158,12 +158,11 @@ abstract class Scrollable extends StatefulComponent {
     _startToEndAnimation();
   }
 
-  // Return the event's velocity in pixels/second.
-  double _eventVelocity(sky.GestureEvent event) {
-    double velocity = scrollDirection == ScrollDirection.horizontal
-      ? -event.velocityX
-      : -event.velocityY;
-    return velocity.clamp(_kMinFlingVelocity, _kMaxFlingVelocity) / _kMillisecondsPerSecond;
+  double _scrollVelocity(sky.Offset velocity) {
+    double scrollVelocity = scrollDirection == ScrollDirection.horizontal
+      ? -velocity.dx
+      : -velocity.dy;
+    return scrollVelocity.clamp(_kMinFlingVelocity, _kMaxFlingVelocity) / _kMillisecondsPerSecond;
   }
 
   EventDisposition _handlePointerDown(_) {
@@ -171,9 +170,8 @@ abstract class Scrollable extends StatefulComponent {
     return EventDisposition.processed;
   }
 
-  EventDisposition _handleFlingStart(sky.GestureEvent event) {
-    _startToEndAnimation(velocity: _eventVelocity(event));
-    return EventDisposition.processed;
+  void _handleFling(Offset velocity) {
+    _startToEndAnimation(velocity: _scrollVelocity(velocity));
   }
 
   void _maybeSettleScrollOffset() {
@@ -559,13 +557,12 @@ class PageableList<T> extends ScrollableList<T> {
       .clamp(scrollBehavior.minScrollOffset, scrollBehavior.maxScrollOffset);
   }
 
-  EventDisposition _handleFlingStart(sky.GestureEvent event) {
-    double velocity = _eventVelocity(event);
-    double newScrollOffset = _snapScrollOffset(scrollOffset + velocity.sign * itemExtent)
+  void _handleFling(sky.Offset velocity) {
+    double scrollVelocity = _scrollVelocity(velocity);
+    double newScrollOffset = _snapScrollOffset(scrollOffset + scrollVelocity.sign * itemExtent)
       .clamp(_snapScrollOffset(scrollOffset - itemExtent / 2.0),
              _snapScrollOffset(scrollOffset + itemExtent / 2.0));
     scrollTo(newScrollOffset, duration: duration, curve: curve).then(_notifyPageChanged);
-    return EventDisposition.processed;
   }
 
   int get currentPage => (scrollOffset / itemExtent).floor() % itemCount;