AndroidView touch support.

This PR adds 2 features to RenderAndroidView and AndroidView:

1. Hit testing behavior
Adds a `PlatformViewHitTestBehavior` which is similar to
`HitTestBehavior` without the `deferToChild` option (as platform views
don't have child render objects) and with a `transparent` option which
prevents it from forwarding any events to the Android view.

2. MotionEvent recomposing logic
FlutterView and the framework `converter.dart` are working together to
transform each Android MotionEvent object into one or more
`PointerEvent` objects.

This PR adds the reverse logic (in _MotionEventDispatcher which is used
by RenderAndroidView) which turns a stream of PointerEvent objects into
MotionEvent objects.

The correctness of the recomposing logic is tested in an integration
test which will land in a separate PR (the unit test PR is pretty big,
trying to keep as many bite-size PRs for reviewer's convenience)

packages/flutter/lib/src/rendering/platform_view.dart

@@ -6,6 +6,7 @@ import 'dart:async';
 import 'dart:ui';
 
 import 'package:flutter/foundation.dart';
+import 'package:flutter/gestures.dart';
 import 'package:flutter/services.dart';
 
 import 'box.dart';
@@ -13,6 +14,22 @@ import 'layer.dart';
 import 'object.dart';
 
 
+/// How an embedded platform view behave during hit tests.
+enum PlatformViewHitTestBehavior {
+  /// Opaque targets can be hit by hit tests, causing them to both receive
+  /// events within their bounds and prevent targets visually behind them from
+  /// also receiving events.
+  opaque,
+
+  /// Translucent targets both receive events within their bounds and permit
+  /// targets visually behind them to also receive events.
+  translucent,
+
+  /// Transparent targets don't receive events within their bounds and permit
+  /// targets visually behind them to receive events.
+  transparent,
+}
+
 enum _PlatformViewState {
   uninitialized,
   resizing,
@@ -21,7 +38,8 @@ enum _PlatformViewState {
 
 /// A render object for an Android view.
 ///
-/// [RenderAndroidView] is responsible for sizing and displaying an Android [View](https://developer.android.com/reference/android/view/View).
+/// [RenderAndroidView] is responsible for sizing, displaying and passing touch events to an
+/// Android [View](https://developer.android.com/reference/android/view/View).
 ///
 /// The render object's layout behavior is to fill all available space, the parent of this object must
 /// provide bounded layout constraints
@@ -34,8 +52,12 @@ class RenderAndroidView extends RenderBox {
   /// Creates a render object for an Android view.
   RenderAndroidView({
     @required AndroidViewController viewController,
+    @required this.hitTestBehavior,
   }) : assert(viewController != null),
-       _viewController = viewController;
+       assert(hitTestBehavior != null),
+       _viewController = viewController {
+    _motionEventsDispatcher = new _MotionEventsDispatcher(globalToLocal, viewController);
+  }
 
   _PlatformViewState _state = _PlatformViewState.uninitialized;
 
@@ -47,10 +69,17 @@ class RenderAndroidView extends RenderBox {
   /// `viewController` must not be null.
   set viewController(AndroidViewController viewController) {
     assert(_viewController != null);
+    if (_viewController == viewController)
+      return;
     _viewController = viewController;
     _sizePlatformView();
   }
 
+  /// How to behave during hit testing.
+  // The implicit setter is enough here as changing this value will just affect
+  // any newly arriving events there's nothing we need to invalidate.
+  PlatformViewHitTestBehavior hitTestBehavior;
+
   @override
   bool get sizedByParent => true;
 
@@ -60,6 +89,8 @@ class RenderAndroidView extends RenderBox {
   @override
   bool get isRepaintBoundary => true;
 
+  _MotionEventsDispatcher _motionEventsDispatcher;
+
   @override
   void performResize() {
     size = constraints.biggest;
@@ -96,4 +127,160 @@ class RenderAndroidView extends RenderBox {
       textureId: _viewController.textureId,
     ));
   }
+
+  @override
+  bool hitTest(HitTestResult result, { Offset position }) {
+    if (hitTestBehavior == PlatformViewHitTestBehavior.transparent || !size.contains(position))
+      return false;
+    result.add(new BoxHitTestEntry(this, position));
+    return hitTestBehavior == PlatformViewHitTestBehavior.opaque;
+  }
+
+  @override
+  bool hitTestSelf(Offset position) => hitTestBehavior != PlatformViewHitTestBehavior.transparent;
+
+  @override
+  void handleEvent(PointerEvent event, HitTestEntry entry) {
+    _motionEventsDispatcher.handlePointerEvent(event);
+  }
+}
+
+typedef Offset _GlobalToLocal(Offset point);
+
+// Composes a stream of PointerEvent objects into AndroidMotionEvent objects
+// and dispatches them to the associated embedded Android view.
+class _MotionEventsDispatcher {
+  _MotionEventsDispatcher(this.globalToLocal, this.viewController);
+
+  final Map<int, AndroidPointerCoords> pointerPositions = <int, AndroidPointerCoords>{};
+  final Map<int, AndroidPointerProperties> pointerProperties = <int, AndroidPointerProperties>{};
+  final _GlobalToLocal globalToLocal;
+  final AndroidViewController viewController;
+
+  int nextPointerId = 0;
+  int downTimeMillis;
+
+  void handlePointerEvent(PointerEvent event) {
+    if (event is PointerDownEvent) {
+      if (nextPointerId == 0)
+        downTimeMillis = event.timeStamp.inMilliseconds;
+      pointerProperties[event.pointer] = propertiesFor(event, nextPointerId++);
+    }
+    pointerPositions[event.pointer] = coordsFor(event);
+
+    dispatchPointerEvent(event);
+
+    if (event is PointerUpEvent) {
+      pointerPositions.remove(event.pointer);
+      pointerProperties.remove(event.pointer);
+      if (pointerProperties.isEmpty) {
+        nextPointerId = 0;
+        downTimeMillis = null;
+      }
+    }
+    if (event is PointerCancelEvent) {
+      pointerPositions.clear();
+      pointerProperties.clear();
+      nextPointerId = 0;
+      downTimeMillis = null;
+    }
+  }
+
+  void dispatchPointerEvent(PointerEvent event) {
+    final List<int> pointers = pointerPositions.keys.toList();
+    final int pointerIdx = pointers.indexOf(event.pointer);
+    final int numPointers = pointers.length;
+
+    // Android MotionEvent objects can batch information on multiple pointers.
+    // Flutter breaks these such batched events into multiple PointerEvent objects.
+    // When there are multiple active pointers we accumulate the information for all pointers
+    // as we get PointerEvents, and only send it to the embedded Android view when
+    // we see the last pointer. This way we achieve the same batching as Android.
+    if(isSinglePointerAction(event) && pointerIdx < numPointers - 1)
+      return;
+
+    int action;
+    switch(event.runtimeType){
+      case PointerDownEvent:
+        action = numPointers == 1 ? AndroidViewController.kActionDown
+            : AndroidViewController.pointerAction(pointerIdx, AndroidViewController.kActionPointerDown);
+        break;
+      case PointerUpEvent:
+        action = numPointers == 1 ? AndroidViewController.kActionUp
+            : AndroidViewController.pointerAction(pointerIdx, AndroidViewController.kActionPointerUp);
+        break;
+      case PointerMoveEvent:
+        action = AndroidViewController.kActionMove;
+        break;
+      case PointerCancelEvent:
+        action = AndroidViewController.kActionCancel;
+        break;
+      default:
+        return;
+    }
+
+    final AndroidMotionEvent androidMotionEvent = new AndroidMotionEvent(
+        downTime: downTimeMillis,
+        eventTime: event.timeStamp.inMilliseconds,
+        action: action,
+        pointerCount: pointerPositions.length,
+        pointerProperties: pointers.map((int i) => pointerProperties[i]).toList(),
+        pointerCoords: pointers.map((int i) => pointerPositions[i]).toList(),
+        metaState: 0,
+        buttonState: 0,
+        xPrecision: 1.0,
+        yPrecision: 1.0,
+        deviceId: 0,
+        edgeFlags: 0,
+        source: 0,
+        flags: 0
+    );
+    viewController.sendMotionEvent(androidMotionEvent);
+  }
+
+  AndroidPointerCoords coordsFor(PointerEvent event) {
+    final Offset position = globalToLocal(event.position);
+    return new AndroidPointerCoords(
+        orientation: event.orientation,
+        pressure: event.pressure,
+        // Currently the engine omits the pointer size, for now I'm fixing this to 0.33 which is roughly
+        // what I typically see on Android.
+        //
+        // TODO(amirh): Use the original pointer's size.
+        // https://github.com/flutter/flutter/issues/20300
+        size: 0.333,
+        toolMajor: event.radiusMajor,
+        toolMinor: event.radiusMinor,
+        touchMajor: event.radiusMajor,
+        touchMinor: event.radiusMinor,
+        x: position.dx,
+        y: position.dy
+    );
+  }
+
+  AndroidPointerProperties propertiesFor(PointerEvent event, int pointerId) {
+    int toolType = AndroidPointerProperties.kToolTypeUnknown;
+    switch(event.kind) {
+      case PointerDeviceKind.touch:
+        toolType = AndroidPointerProperties.kToolTypeFinger;
+        break;
+      case PointerDeviceKind.mouse:
+        toolType = AndroidPointerProperties.kToolTypeMouse;
+        break;
+      case PointerDeviceKind.stylus:
+        toolType = AndroidPointerProperties.kToolTypeStylus;
+        break;
+      case PointerDeviceKind.invertedStylus:
+        toolType = AndroidPointerProperties.kToolTypeEraser;
+        break;
+      case PointerDeviceKind.unknown:
+        toolType = AndroidPointerProperties.kToolTypeUnknown;
+        break;
+    }
+    return new AndroidPointerProperties(id: pointerId, toolType: toolType);
+  }
+
+  bool isSinglePointerAction(PointerEvent event) =>
+      !(event is PointerDownEvent) && !(event is PointerUpEvent);
 }
+

packages/flutter/lib/src/services/platform_views.dart

@@ -78,6 +78,21 @@ class PlatformViewsService {
 ///
 /// A Dart version of Android's [MotionEvent.PointerProperties](https://developer.android.com/reference/android/view/MotionEvent.PointerProperties).
 class AndroidPointerProperties {
+  /// Value for `toolType` when the tool type is unknown.
+  static const int kToolTypeUnknown = 0;
+
+  /// Value for `toolType` when the tool type is a finger.
+  static const int kToolTypeFinger = 1;
+
+  /// Value for `toolType` when the tool type is a stylus.
+  static const int kToolTypeStylus = 2;
+
+  /// Value for `toolType` when the tool type is a mouse.
+  static const int kToolTypeMouse = 3;
+
+  /// Value for `toolType` when the tool type is an eraser.
+  static const int kToolTypeEraser = 4;
+
   /// Creates an AndroidPointerProperties.
   ///
   /// All parameters must not be null.

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

@@ -39,12 +39,14 @@ import 'framework.dart';
 class AndroidView extends StatefulWidget {
   /// Creates a widget that embeds an Android view.
   ///
-  /// The `viewType` parameter must not be null.
+  /// The `viewType` and `hitTestBehavior` parameters must not be null.
   const AndroidView({
     Key key,
     @required this.viewType,
-    this.onPlatformViewCreated
+    this.onPlatformViewCreated,
+    this.hitTestBehavior = PlatformViewHitTestBehavior.opaque,
   }) : assert(viewType != null),
+       assert(hitTestBehavior != null),
        super(key: key);
 
   /// The unique identifier for Android view type to be embedded by this widget.
@@ -59,6 +61,11 @@ class AndroidView extends StatefulWidget {
   /// May be null.
   final PlatformViewCreatedCallback onPlatformViewCreated;
 
+  /// How this widget should behave during hit testing.
+  ///
+  /// This defaults to [PlatformViewHitTestBehavior.opaque].
+  final PlatformViewHitTestBehavior hitTestBehavior;
+
   @override
   State createState() => new _AndroidViewState();
 }
@@ -69,7 +76,10 @@ class _AndroidViewState extends State<AndroidView> {
 
   @override
   Widget build(BuildContext context) {
-    return new _AndroidPlatformView(controller: _controller);
+    return new _AndroidPlatformView(
+        controller: _controller,
+        hitTestBehavior: widget.hitTestBehavior
+    );
   }
 
   @override
@@ -108,17 +118,21 @@ class _AndroidPlatformView extends LeafRenderObjectWidget {
   const _AndroidPlatformView({
     Key key,
     @required this.controller,
+    @required this.hitTestBehavior,
   }) : assert(controller != null),
+       assert(hitTestBehavior != null),
        super(key: key);
 
   final AndroidViewController controller;
+  final PlatformViewHitTestBehavior hitTestBehavior;
 
   @override
   RenderObject createRenderObject(BuildContext context) =>
-    new RenderAndroidView(viewController: controller);
+    new RenderAndroidView(viewController: controller, hitTestBehavior: hitTestBehavior);
 
   @override
   void updateRenderObject(BuildContext context, RenderAndroidView renderObject) {
     renderObject.viewController = controller;
+    renderObject.hitTestBehavior = hitTestBehavior;
   }
 }

packages/flutter/test/services/fake_platform_views.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'package:collection/collection.dart';
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
@@ -14,7 +15,12 @@ class FakePlatformViewsController {
   }
 
   final TargetPlatform targetPlatform;
+
+  Iterable<FakePlatformView> get views => _views.values;
   final Map<int, FakePlatformView> _views = <int, FakePlatformView>{};
+
+  final Map<int, List<FakeMotionEvent>> motionEvents = <int, List<FakeMotionEvent>>{};
+
   final Set<String> _registeredViewTypes = new Set<String>();
 
   int _textureCounter = 0;
@@ -37,6 +43,8 @@ class FakePlatformViewsController {
         return _dispose(call);
       case 'resize':
         return _resize(call);
+      case 'touch':
+        return _touch(call);
     }
     return new Future<Null>.sync(() => null);
   }
@@ -95,7 +103,28 @@ class FakePlatformViewsController {
     return new Future<Null>.sync(() => null);
   }
 
-  Iterable<FakePlatformView> get views => _views.values;
+  Future<dynamic> _touch(MethodCall call) {
+    final List<dynamic> args = call.arguments;
+    final int id = args[0];
+    final int action = args[3];
+    final List<List<dynamic>> pointerProperties = args[5].cast<List<dynamic>>();
+    final List<List<dynamic>> pointerCoords = args[6].cast<List<dynamic>>();
+    final List<Offset> pointerOffsets = <Offset> [];
+    final List<int> pointerIds = <int> [];
+    for (int i = 0; i < pointerCoords.length; i++) {
+      pointerIds.add(pointerProperties[i][0]);
+      final double x = pointerCoords[i][7];
+      final double y = pointerCoords[i][8];
+      pointerOffsets.add(new Offset(x, y));
+    }
+
+    if (!motionEvents.containsKey(id))
+      motionEvents[id] = <FakeMotionEvent> [];
+
+    motionEvents[id].add(new FakeMotionEvent(action, pointerIds, pointerOffsets));
+    return new Future<Null>.sync(() => null);
+  }
+
 }
 
 class FakePlatformView {
@@ -124,3 +153,32 @@ class FakePlatformView {
     return 'FakePlatformView(id: $id, type: $type, size: $size)';
   }
 }
+
+class FakeMotionEvent {
+  const FakeMotionEvent(this.action, this.pointerIds, this.pointers);
+
+  final int action;
+  final List<Offset> pointers;
+  final List<int> pointerIds;
+
+
+  @override
+  bool operator ==(dynamic other) {
+    if (other is! FakeMotionEvent)
+      return false;
+    final FakeMotionEvent typedOther = other;
+    const ListEquality<Offset> offsetsEq = ListEquality<Offset>();
+    const ListEquality<int> pointersEq = ListEquality<int>();
+    return pointersEq.equals(pointerIds, typedOther.pointerIds) &&
+        action == typedOther.action &&
+        offsetsEq.equals(pointers, typedOther.pointers);
+  }
+
+  @override
+  int get hashCode => hashValues(action, pointers, pointerIds);
+
+  @override
+  String toString() {
+    return 'FakeMotionEvent(action: $action, pointerIds: $pointerIds, pointers: $pointers)';
+  }
+}

packages/flutter/test/widgets/platform_view_test.dart

@@ -2,9 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/gestures.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
-import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
 
 import '../services/fake_platform_views.dart';
 
@@ -160,4 +162,191 @@ void main() {
         ])
     );
   });
+
+  testWidgets('Android view gets touch events', (WidgetTester tester) async {
+    final int currentViewId = platformViewsRegistry.getNextPlatformViewId();
+    final FakePlatformViewsController viewsController = new FakePlatformViewsController(TargetPlatform.android);
+    viewsController.registerViewType('webview');
+    await tester.pumpWidget(
+        const Align(
+            alignment: Alignment.topLeft,
+            child: SizedBox(
+              width: 200.0,
+              height: 100.0,
+              child: AndroidView(viewType: 'webview'),
+            )
+        )
+    );
+
+    final TestGesture gesture = await tester.startGesture(const Offset(50.0, 50.0));
+    await gesture.up();
+
+    expect(
+      viewsController.motionEvents[currentViewId + 1],
+      orderedEquals(<FakeMotionEvent> [
+        const FakeMotionEvent(AndroidViewController.kActionDown, <int> [0], <Offset> [Offset(50.0, 50.0)]),
+        const FakeMotionEvent(AndroidViewController.kActionUp, <int> [0], <Offset> [Offset(50.0, 50.0)]),
+      ])
+    );
+  });
+
+  testWidgets('Android view transparent hit test behavior', (WidgetTester tester) async {
+    final int currentViewId = platformViewsRegistry.getNextPlatformViewId();
+    final FakePlatformViewsController viewsController = new FakePlatformViewsController(TargetPlatform.android);
+    viewsController.registerViewType('webview');
+
+    int numPointerDownsOnParent = 0;
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Stack(
+          children: <Widget> [
+            new Listener(
+              behavior: HitTestBehavior.opaque,
+              onPointerDown: (PointerDownEvent e) { numPointerDownsOnParent++; },
+            ),
+            const Positioned(
+                child: SizedBox(
+                  width: 200.0,
+                  height: 100.0,
+                  child: AndroidView(
+                    viewType: 'webview',
+                    hitTestBehavior: PlatformViewHitTestBehavior.transparent,
+                  ),
+                )
+            ),
+          ],
+        ),
+      ),
+    );
+
+    await tester.startGesture(const Offset(50.0, 50.0));
+
+    expect(
+        viewsController.motionEvents[currentViewId + 1],
+        isNull,
+    );
+    expect(
+      numPointerDownsOnParent,
+      1
+    );
+  });
+
+  testWidgets('Android view translucent hit test behavior', (WidgetTester tester) async {
+    final int currentViewId = platformViewsRegistry.getNextPlatformViewId();
+    final FakePlatformViewsController viewsController = new FakePlatformViewsController(TargetPlatform.android);
+    viewsController.registerViewType('webview');
+
+    int numPointerDownsOnParent = 0;
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Stack(
+          children: <Widget> [
+            new Listener(
+              behavior: HitTestBehavior.opaque,
+              onPointerDown: (PointerDownEvent e) { numPointerDownsOnParent++; },
+            ),
+            const Positioned(
+                child: SizedBox(
+                  width: 200.0,
+                  height: 100.0,
+                  child: AndroidView(
+                    viewType: 'webview',
+                    hitTestBehavior: PlatformViewHitTestBehavior.translucent,
+                  ),
+                )
+            ),
+          ],
+        ),
+      ),
+    );
+
+    await tester.startGesture(const Offset(50.0, 50.0));
+
+    expect(
+      viewsController.motionEvents[currentViewId + 1],
+        orderedEquals(<FakeMotionEvent> [
+          const FakeMotionEvent(AndroidViewController.kActionDown, <int> [0], <Offset> [Offset(50.0, 50.0)]),
+        ])
+    );
+    expect(
+        numPointerDownsOnParent,
+        1
+    );
+  });
+
+  testWidgets('Android view opaque hit test behavior', (WidgetTester tester) async {
+    final int currentViewId = platformViewsRegistry.getNextPlatformViewId();
+    final FakePlatformViewsController viewsController = new FakePlatformViewsController(TargetPlatform.android);
+    viewsController.registerViewType('webview');
+
+    int numPointerDownsOnParent = 0;
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Stack(
+          children: <Widget> [
+            new Listener(
+              behavior: HitTestBehavior.opaque,
+              onPointerDown: (PointerDownEvent e) { numPointerDownsOnParent++; },
+            ),
+            const Positioned(
+                child: SizedBox(
+                  width: 200.0,
+                  height: 100.0,
+                  child: AndroidView(
+                    viewType: 'webview',
+                    hitTestBehavior: PlatformViewHitTestBehavior.opaque,
+                  ),
+                )
+            ),
+          ],
+        ),
+      ),
+    );
+
+    await tester.startGesture(const Offset(50.0, 50.0));
+
+    expect(
+        viewsController.motionEvents[currentViewId + 1],
+        orderedEquals(<FakeMotionEvent> [
+          const FakeMotionEvent(AndroidViewController.kActionDown, <int> [0], <Offset> [Offset(50.0, 50.0)]),
+        ])
+    );
+    expect(
+        numPointerDownsOnParent,
+        0
+    );
+  });
+
+  testWidgets('Android view touch events are in virtual display\'s coordinate system', (WidgetTester tester) async {
+    final int currentViewId = platformViewsRegistry.getNextPlatformViewId();
+    final FakePlatformViewsController viewsController = new FakePlatformViewsController(TargetPlatform.android);
+    viewsController.registerViewType('webview');
+    await tester.pumpWidget(
+        new Align(
+          alignment: Alignment.topLeft,
+          child: new Container(
+            margin: const EdgeInsets.all(10.0),
+            child: const SizedBox(
+              width: 200.0,
+              height: 100.0,
+              child: AndroidView(viewType: 'webview'),
+            ),
+          ),
+        )
+    );
+
+    final TestGesture gesture = await tester.startGesture(const Offset(50.0, 50.0));
+    await gesture.up();
+
+    expect(
+        viewsController.motionEvents[currentViewId + 1],
+        orderedEquals(<FakeMotionEvent> [
+          const FakeMotionEvent(AndroidViewController.kActionDown, <int> [0], <Offset> [Offset(40.0, 40.0)]),
+          const FakeMotionEvent(AndroidViewController.kActionUp, <int> [0], <Offset> [Offset(40.0, 40.0)]),
+        ])
+    );
+  });
 }