Add a Velocity class to be explicit about units

We were using an Offset, which represented pixels/second, but it wasn't
clear to clients whether that was pixels/ms. Now we use a Velocity class
that is explict about the units.

Fixes #1510
Fixes #785

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

@@ -16,19 +16,19 @@ enum DragState {
 
 typedef void GestureDragStartCallback(Point globalPosition);
 typedef void GestureDragUpdateCallback(double delta);
-typedef void GestureDragEndCallback(Offset velocity);
+typedef void GestureDragEndCallback(Velocity velocity);
 
 typedef void GesturePanStartCallback(Point globalPosition);
 typedef void GesturePanUpdateCallback(Offset delta);
-typedef void GesturePanEndCallback(Offset velocity);
+typedef void GesturePanEndCallback(Velocity velocity);
 
 typedef void _GesturePolymorphicUpdateCallback<T>(T delta);
 
-bool _isFlingGesture(Offset velocity) {
+bool _isFlingGesture(Velocity velocity) {
   assert(velocity != null);
-  double velocitySquared = velocity.dx * velocity.dx + velocity.dy * velocity.dy;
-  return velocitySquared > kMinFlingVelocity * kMinFlingVelocity
-      && velocitySquared < kMaxFlingVelocity * kMaxFlingVelocity;
+  final double speedSquared = velocity.pixelsPerSecond.distanceSquared;
+  return speedSquared > kMinFlingVelocity * kMinFlingVelocity
+      && speedSquared < kMaxFlingVelocity * kMaxFlingVelocity;
 }
 
 abstract class _DragGestureRecognizer<T extends dynamic> extends OneSequenceGestureRecognizer {
@@ -110,11 +110,11 @@ abstract class _DragGestureRecognizer<T extends dynamic> extends OneSequenceGest
       VelocityTracker tracker = _velocityTrackers[pointer];
       assert(tracker != null);
 
-      Offset velocity = tracker.getVelocity();
+      Velocity velocity = tracker.getVelocity();
       if (velocity != null && _isFlingGesture(velocity))
         onEnd(velocity);
       else
-        onEnd(Offset.zero);
+        onEnd(Velocity.zero);
     }
     _velocityTrackers.clear();
   }

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

@@ -16,7 +16,7 @@ typedef Drag GestureMultiDragStartCallback(Point position);
 
 class Drag {
   void move(Offset offset) { }
-  void end(Offset velocity) { }
+  void end(Velocity velocity) { }
   void cancel() { }
 }
 

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

@@ -208,6 +208,28 @@ class _LeastSquaresVelocityTrackerStrategy extends _VelocityTrackerStrategy {
 
 }
 
+/// A velocity in two dimensions.
+class Velocity {
+  const Velocity({ this.pixelsPerSecond });
+
+  /// A velocity that isn't moving at all.
+  static const Velocity zero = const Velocity(pixelsPerSecond: Offset.zero);
+
+  /// The number of pixels per second of velocity in the x and y directions.
+  final Offset pixelsPerSecond;
+
+  bool operator ==(dynamic other) {
+    if (other is! Velocity)
+      return false;
+    final Velocity typedOther = other;
+    return pixelsPerSecond == typedOther.pixelsPerSecond;
+  }
+
+  int get hashCode => pixelsPerSecond.hashCode;
+
+  String toString() => 'Velocity(${pixelsPerSecond.dx.toStringAsFixed(1)}, ${pixelsPerSecond.dy.toStringAsFixed(1)})';
+}
+
 /// Computes a pointer velocity based on data from PointerMove events.
 ///
 /// The input data is provided by calling addPosition(). Adding data
@@ -248,12 +270,14 @@ class VelocityTracker {
   ///
   /// getVelocity() will return null if no estimate is available or if
   /// the velocity is zero.
-  Offset getVelocity() {
+  Velocity getVelocity() {
     _Estimate estimate = _strategy.getEstimate();
     if (estimate != null && estimate.degree >= 1) {
-      return new Offset( // convert from pixels/ms to pixels/s
+      return new Velocity(
+        pixelsPerSecond: new Offset( // convert from pixels/ms to pixels/s
           estimate.xCoefficients[1] * 1000,
           estimate.yCoefficients[1] * 1000
+        )
       );
     }
     return null;

packages/flutter/lib/src/material/bottom_sheet.dart

@@ -59,11 +59,11 @@ class _BottomSheetState extends State<BottomSheet> {
     config.animationController.value -= delta / (_childHeight ?? delta);
   }
 
-  void _handleDragEnd(Offset velocity) {
+  void _handleDragEnd(Velocity velocity) {
     if (_dismissUnderway)
       return;
-    if (velocity.dy > _kMinFlingVelocity) {
-      double flingVelocity = -velocity.dy / _childHeight;
+    if (velocity.pixelsPerSecond.dy > _kMinFlingVelocity) {
+      double flingVelocity = -velocity.pixelsPerSecond.dy / _childHeight;
       config.animationController.fling(velocity: flingVelocity);
       if (flingVelocity < 0.0)
         config.onClosing();

packages/flutter/lib/src/material/drawer.dart

@@ -135,11 +135,11 @@ class DrawerControllerState extends State<DrawerController> {
     _controller.value += delta / _width;
   }
 
-  void _settle(Offset velocity) {
+  void _settle(Velocity velocity) {
     if (_controller.isDismissed)
       return;
-    if (velocity.dx.abs() >= _kMinFlingVelocity) {
-      _controller.fling(velocity: velocity.dx / _width);
+    if (velocity.pixelsPerSecond.dx.abs() >= _kMinFlingVelocity) {
+      _controller.fling(velocity: velocity.pixelsPerSecond.dx / _width);
     } else if (_controller.value < 0.5) {
       close();
     } else {

packages/flutter/lib/src/material/slider.dart

@@ -145,7 +145,7 @@ class _RenderSlider extends RenderConstrainedBox {
     }
   }
 
-  void _handleDragEnd(Offset velocity) {
+  void _handleDragEnd(Velocity velocity) {
     if (_active) {
       _active = false;
       _currentDragValue = 0.0;

packages/flutter/lib/src/material/switch.dart

@@ -156,7 +156,7 @@ class _RenderSwitch extends RenderToggleable {
     }
   }
 
-  void _handleDragEnd(Offset velocity) {
+  void _handleDragEnd(Velocity velocity) {
     if (position.value >= 0.5)
       positionController.forward();
     else

packages/flutter/lib/src/material/time_picker.dart

@@ -425,7 +425,7 @@ class _DialState extends State<_Dial> {
     _notifyOnChangedIfNeeded();
   }
 
-  void _handlePanEnd(Offset velocity) {
+  void _handlePanEnd(Velocity velocity) {
     assert(_dragging);
     _dragging = false;
     _position = null;

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

@@ -182,9 +182,9 @@ class _DismissableState extends State<Dismissable> {
       _dismissController.value = _dragExtent.abs() / _size.width;
   }
 
-  bool _isFlingGesture(Offset velocity) {
-    double vx = velocity.dx;
-    double vy = velocity.dy;
+  bool _isFlingGesture(Velocity velocity) {
+    double vx = velocity.pixelsPerSecond.dx;
+    double vy = velocity.pixelsPerSecond.dy;
     if (_directionIsYAxis) {
       if (vy.abs() - vx.abs() < _kMinFlingVelocityDelta)
         return false;
@@ -211,7 +211,7 @@ class _DismissableState extends State<Dismissable> {
     return false;
   }
 
-  void _handleDragEnd(Offset velocity) {
+  void _handleDragEnd(Velocity velocity) {
     if (!_isActive || _dismissController.isAnimating)
       return;
     setState(() {
@@ -219,7 +219,7 @@ class _DismissableState extends State<Dismissable> {
       if (_dismissController.isCompleted) {
         _startResizeAnimation();
       } else if (_isFlingGesture(velocity)) {
-        double flingVelocity = _directionIsYAxis ? velocity.dy : velocity.dx;
+        double flingVelocity = _directionIsYAxis ? velocity.pixelsPerSecond.dy : velocity.pixelsPerSecond.dx;
         _dragExtent = flingVelocity.sign;
         _dismissController.fling(velocity: flingVelocity.abs() * _kFlingVelocityScale);
       } else if (_dismissController.value > _kDismissCardThreshold) {

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

@@ -396,7 +396,7 @@ class _DragAvatar<T> extends Drag {
     _position += offset;
     update(_position);
   }
-  void end(Offset velocity) {
+  void end(Velocity velocity) {
     finish(_DragEndKind.dropped);
   }
   void cancel() {

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

@@ -25,7 +25,8 @@ export 'package:flutter/gestures.dart' show
   GesturePanEndCallback,
   GestureScaleStartCallback,
   GestureScaleUpdateCallback,
-  GestureScaleEndCallback;
+  GestureScaleEndCallback,
+  Velocity;
 
 /// A widget that detects gestures.
 ///
@@ -414,7 +415,7 @@ class _GestureSemantics extends OneChildRenderObjectWidget {
       if (onHorizontalDragUpdate != null)
         onHorizontalDragUpdate(delta);
       if (onHorizontalDragEnd != null)
-        onHorizontalDragEnd(Offset.zero);
+        onHorizontalDragEnd(Velocity.zero);
     } else {
       assert(_watchingPans);
       if (onPanStart != null)
@@ -422,7 +423,7 @@ class _GestureSemantics extends OneChildRenderObjectWidget {
       if (onPanUpdate != null)
         onPanUpdate(new Offset(delta, 0.0));
       if (onPanEnd != null)
-        onPanEnd(Offset.zero);
+        onPanEnd(Velocity.zero);
     }
   }
 
@@ -433,7 +434,7 @@ class _GestureSemantics extends OneChildRenderObjectWidget {
       if (onVerticalDragUpdate != null)
         onVerticalDragUpdate(delta);
       if (onVerticalDragEnd != null)
-        onVerticalDragEnd(Offset.zero);
+        onVerticalDragEnd(Velocity.zero);
     } else {
       assert(_watchingPans);
       if (onPanStart != null)
@@ -441,7 +442,7 @@ class _GestureSemantics extends OneChildRenderObjectWidget {
       if (onPanUpdate != null)
         onPanUpdate(new Offset(0.0, delta));
       if (onPanEnd != null)
-        onPanEnd(Offset.zero);
+        onPanEnd(Velocity.zero);
     }
   }
 

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

@@ -450,8 +450,8 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
     scrollBy(pixelOffsetToScrollOffset(delta));
   }
 
-  Future _handleDragEnd(Offset velocity) {
-    double scrollVelocity = pixelDeltaToScrollOffset(velocity) / Duration.MILLISECONDS_PER_SECOND;
+  Future _handleDragEnd(Velocity velocity) {
+    double scrollVelocity = pixelDeltaToScrollOffset(velocity.pixelsPerSecond) / Duration.MILLISECONDS_PER_SECOND;
     // The gesture velocity properties are pixels/second, config min,max limits are pixels/ms
     return fling(scrollVelocity.clamp(-kMaxFlingVelocity, kMaxFlingVelocity)).then((_) {
       dispatchOnScrollEnd();

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

@@ -58,7 +58,7 @@ class _SemanticsDebuggerState extends State<SemanticsDebugger> {
       _lastPointerDownLocation = null;
     });
   }
-  void _handlePanEnd(Offset velocity) {
+  void _handlePanEnd(Velocity velocity) {
     assert(_lastPointerDownLocation != null);
     _SemanticsDebuggerListener.instance.handlePanEnd(_lastPointerDownLocation, velocity);
     setState(() {
@@ -329,16 +329,18 @@ class _SemanticsDebuggerListener implements mojom.SemanticsListener {
   void handleLongPress(Point position) {
     _server.longPress(_hitTest(position, (_SemanticsDebuggerEntry entry) => entry.canBeLongPressed)?.id ?? 0);
   }
-  void handlePanEnd(Point position, Offset velocity) {
-    if (velocity.dx.abs() == velocity.dy.abs())
+  void handlePanEnd(Point position, Velocity velocity) {
+    double vx = velocity.pixelsPerSecond.dx;
+    double vy = velocity.pixelsPerSecond.dy;
+    if (vx.abs() == vy.abs())
       return;
-    if (velocity.dx.abs() > velocity.dy.abs()) {
-      if (velocity.dx.sign < 0)
+    if (vx.abs() > vy.abs()) {
+      if (vx.sign < 0)
         _server.scrollLeft(_hitTest(position, (_SemanticsDebuggerEntry entry) => entry.canBeScrolledHorizontally)?.id ?? 0);
       else
         _server.scrollRight(_hitTest(position, (_SemanticsDebuggerEntry entry) => entry.canBeScrolledHorizontally)?.id ?? 0);
     } else {
-      if (velocity.dy.sign < 0)
+      if (vy.sign < 0)
         _server.scrollUp(_hitTest(position, (_SemanticsDebuggerEntry entry) => entry.canBeScrolledVertically)?.id ?? 0);
       else
         _server.scrollDown(_hitTest(position, (_SemanticsDebuggerEntry entry) => entry.canBeScrolledVertically)?.id ?? 0);

packages/flutter/test/gestures/scroll_test.dart

@@ -30,7 +30,7 @@ void main() {
     };
 
     bool didEndPan = false;
-    pan.onEnd = (Offset velocity) {
+    pan.onEnd = (Velocity velocity) {
       didEndPan = true;
     };
 

packages/flutter/test/gestures/velocity_tracker_test.dart

@@ -12,10 +12,10 @@ bool _withinTolerance(double actual, double expected) {
   return diff.abs() < kTolerance;
 }
 
-bool _checkVelocity(Offset actual, Offset expected) {
+bool _checkVelocity(Velocity actual, Offset expected) {
   return (actual != null)
-      && _withinTolerance(actual.dx, expected.dx)
-      && _withinTolerance(actual.dy, expected.dy);
+      && _withinTolerance(actual.pixelsPerSecond.dx, expected.dx)
+      && _withinTolerance(actual.pixelsPerSecond.dy, expected.dy);
 }
 
 void main() {

packages/flutter/test/widget/gesture_detector_test.dart

@@ -22,7 +22,7 @@ void main() {
         onVerticalDragUpdate: (double scrollDelta) {
           updatedDragDelta = scrollDelta;
         },
-        onVerticalDragEnd: (Offset velocity) {
+        onVerticalDragEnd: (Velocity velocity) {
           didEndDrag = true;
         },
         child: new Container(
@@ -73,9 +73,9 @@ void main() {
 
       Widget widget = new GestureDetector(
         onVerticalDragUpdate: (double delta) { dragDistance += delta; },
-        onVerticalDragEnd: (Offset velocity) { gestureCount += 1; },
+        onVerticalDragEnd: (Velocity velocity) { gestureCount += 1; },
         onHorizontalDragUpdate: (_) { fail("gesture should not match"); },
-        onHorizontalDragEnd: (Offset velocity) { fail("gesture should not match"); },
+        onHorizontalDragEnd: (Velocity velocity) { fail("gesture should not match"); },
         child: new Container(
           decoration: const BoxDecoration(
             backgroundColor: const Color(0xFF00FF00)