Have the scale gesture callback uses details objects (#6204)

examples/flutter_gallery/lib/demo/grid_list_demo.dart

@@ -78,29 +78,29 @@ class _GridPhotoViewerState extends State<GridPhotoViewer> with SingleTickerProv
     });
   }
 
-  void _handleOnScaleStart(Point focalPoint) {
+  void _handleOnScaleStart(ScaleStartDetails details) {
     setState(() {
       _lastScale = 1.0;
-      _lastFocalPoint = focalPoint;
+      _lastFocalPoint = details.focalPoint;
       // The fling animation stops if an input gesture starts.
       _controller.stop();
     });
   }
 
-  void _handleOnScaleUpdate(double scale, Point focalPoint) {
+  void _handleOnScaleUpdate(ScaleUpdateDetails details) {
     setState(() {
-      _scale = (_scale + (scale - _lastScale)).clamp(1.0, 3.0);
-      _lastScale = scale;
-      _focalPoint = _clampFocalPoint(_focalPoint + (_lastFocalPoint - focalPoint));
-      _lastFocalPoint = focalPoint;
+      _scale = (_scale + (details.scale - _lastScale)).clamp(1.0, 3.0);
+      _lastScale = details.scale;
+      _focalPoint = _clampFocalPoint(_focalPoint + (_lastFocalPoint - details.focalPoint));
+      _lastFocalPoint = details.focalPoint;
     });
   }
 
-  void _handleOnScaleEnd(Velocity flingVelocity) {
-    final double magnitude = flingVelocity.pixelsPerSecond.distance;
+  void _handleOnScaleEnd(ScaleEndDetails details) {
+    final double magnitude = details.velocity.pixelsPerSecond.distance;
     if (magnitude < _kMinFlingVelocity)
       return;
-    final Offset direction = flingVelocity.pixelsPerSecond / magnitude;
+    final Offset direction = details.velocity.pixelsPerSecond / magnitude;
     final RenderBox box = context.findRenderObject();
     final double distance = (Point.origin & box.size).shortestSide;
     _flingAnimation = new Tween<Point>(

examples/layers/widgets/gestures.dart

@@ -79,21 +79,21 @@ class _GestureDemoState extends State<GestureDemo> {
   bool _doubleTapEnabled = true;
   bool _longPressEnabled = true;
 
-  void _handleScaleStart(Point focalPoint) {
+  void _handleScaleStart(ScaleStartDetails details) {
     setState(() {
-      _startingFocalPoint = focalPoint;
+      _startingFocalPoint = details.focalPoint;
       _previousOffset = _offset;
       _previousZoom = _zoom;
     });
   }
 
-  void _handleScaleUpdate(double scale, Point focalPoint) {
+  void _handleScaleUpdate(ScaleUpdateDetails details) {
     setState(() {
-      _zoom = (_previousZoom * scale);
+      _zoom = (_previousZoom * details.scale);
 
       // Ensure that item under the focal point stays in the same place despite zooming
       Offset normalizedOffset = (_startingFocalPoint.toOffset() - _previousOffset) / _previousZoom;
-      _offset = focalPoint.toOffset() - normalizedOffset * _zoom;
+      _offset = details.focalPoint.toOffset() - normalizedOffset * _zoom;
     });
   }
 

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

@@ -27,16 +27,63 @@ enum ScaleState {
   started,
 }
 
+/// Details for [GestureScaleStartCallback].
+class ScaleStartDetails {
+  /// Creates details for [GestureScaleStartCallback].
+  ///
+  /// The [focalPoint] argument must not be null.
+  ScaleStartDetails({ this.focalPoint: Point.origin }) {
+    assert(focalPoint != null);
+  }
+
+  /// The initial focal point of the pointers in contact with the screen.
+  /// Reported in global coordinates.
+  final Point focalPoint;
+}
+
+/// Details for [GestureScaleUpdateCallback].
+class ScaleUpdateDetails {
+  /// Creates details for [GestureScaleUpdateCallback].
+  ///
+  /// The [focalPoint] and [scale] arguments must not be null. The [scale]
+  /// argument must be greater than or equal to zero.
+  ScaleUpdateDetails({ this.focalPoint: Point.origin, this.scale: 1.0 }) {
+    assert(focalPoint != null);
+    assert(scale != null && scale >= 0.0);
+  }
+
+  /// The focal point of the pointers in contact with the screen. Reported in
+  /// global coordinates.
+  final Point focalPoint;
+
+  /// The scale implied by the pointers in contact with the screen. A value
+  /// greater than or equal to zero.
+  final double scale;
+}
+
+/// Details for [GestureScaleEndCallback].
+class ScaleEndDetails {
+  /// Creates details for [GestureScaleEndCallback].
+  ///
+  /// The [velocity] argument must not be null.
+  ScaleEndDetails({ this.velocity: Velocity.zero }) {
+    assert(velocity != null);
+  }
+
+  /// The velocity of the last pointer to be lifted off of the screen.
+  final Velocity velocity;
+}
+
 /// Signature for when the pointers in contact with the screen have established
 /// a focal point and initial scale of 1.0.
-typedef void GestureScaleStartCallback(Point focalPoint);
+typedef void GestureScaleStartCallback(ScaleStartDetails details);
 
 /// Signature for when the pointers in contact with the screen have indicated a
 /// new focal point and/or scale.
-typedef void GestureScaleUpdateCallback(double scale, Point focalPoint);
+typedef void GestureScaleUpdateCallback(ScaleUpdateDetails details);
 
 /// Signature for when the pointers are no longer in contact with the screen.
-typedef void GestureScaleEndCallback(Velocity flingVelocity);
+typedef void GestureScaleEndCallback(ScaleEndDetails details);
 
 bool _isFlingGesture(Velocity velocity) {
   assert(velocity != null);
@@ -133,9 +180,9 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
             final Offset pixelsPerSecond = velocity.pixelsPerSecond;
             if (pixelsPerSecond.distanceSquared > kMaxFlingVelocity * kMaxFlingVelocity)
               velocity = new Velocity(pixelsPerSecond: (pixelsPerSecond / pixelsPerSecond.distance) * kMaxFlingVelocity);
-            onEnd(velocity);
+            onEnd(new ScaleEndDetails(velocity: velocity));
           } else {
-            onEnd(Velocity.zero);
+            onEnd(new ScaleEndDetails(velocity: Velocity.zero));
           }
         }
         _state = ScaleState.accepted;
@@ -153,11 +200,11 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     if (_state == ScaleState.accepted && !configChanged) {
       _state = ScaleState.started;
       if (onStart != null)
-        onStart(focalPoint);
+        onStart(new ScaleStartDetails(focalPoint: focalPoint));
     }
 
     if (_state == ScaleState.started && onUpdate != null)
-      onUpdate(_scaleFactor, focalPoint);
+      onUpdate(new ScaleUpdateDetails(scale: _scaleFactor, focalPoint: focalPoint));
   }
 
   @override

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

@@ -26,6 +26,9 @@ export 'package:flutter/gestures.dart' show
   GestureScaleStartCallback,
   GestureScaleUpdateCallback,
   GestureScaleEndCallback,
+  ScaleStartDetails,
+  ScaleUpdateDetails,
+  ScaleEndDetails,
   TapDownDetails,
   TapUpDetails,
   Velocity;

packages/flutter/test/gestures/scale_test.dart

@@ -16,19 +16,19 @@ void main() {
 
     bool didStartScale = false;
     Point updatedFocalPoint;
-    scale.onStart = (Point focalPoint) {
+    scale.onStart = (ScaleStartDetails details) {
       didStartScale = true;
-      updatedFocalPoint = focalPoint;
+      updatedFocalPoint = details.focalPoint;
     };
 
     double updatedScale;
-    scale.onUpdate = (double scale, Point focalPoint) {
-      updatedScale = scale;
-      updatedFocalPoint = focalPoint;
+    scale.onUpdate = (ScaleUpdateDetails details) {
+      updatedScale = details.scale;
+      updatedFocalPoint = details.focalPoint;
     };
 
     bool didEndScale = false;
-    scale.onEnd = (Velocity flingVelocity) {
+    scale.onEnd = (ScaleEndDetails details) {
       didEndScale = true;
     };