Initial framework support for iOS platform views. (#23412)

This PR adds the full framework stack (layer->render object->widget, and
service) for embedding iOS views with minimal functionality.

I allowed myself to throw the entire framework stack in one PR since we're mostly
mirroring the structure we already established for embedded Android views, so this PR
is a little bigger than usual. I'm happy to break it down to the
different pieces of the stack if reviewers prefer.

Specifically this PR adds:
* A UiKitView widget for embedding a UIView in the widget tree.
* A RenderUiKitView which is the render object for showing a
  UIView.
* A PlatformViewLayer which denotes the position of a UIView in the
  layer tree.
* The iOS platform_views system channel client code in services/platform_views.dart
* Splits the fake platform views controller to an Android and iOS
  controllers.

TBD in following PRs:
* Plumb the layout direction all the way to the platform view (currently
  there is still no engine support for it).
* Integrate with gesture arenas (engine support is still missing as
  well).

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

@@ -357,6 +357,40 @@ class TextureLayer extends Layer {
   S find<S>(Offset regionOffset) => null;
 }
 
+/// A layer that shows an embedded [UIView](https://developer.apple.com/documentation/uikit/uiview)
+/// on iOS.
+class PlatformViewLayer extends Layer {
+  /// Creates a platform view layer.
+  ///
+  /// The `rect` and `viewId` parameters must not be null.
+  PlatformViewLayer({
+    @required this.rect,
+    @required this.viewId,
+  }): assert(rect != null), assert(viewId != null);
+
+  /// Bounding rectangle of this layer in the global coordinate space.
+  final Rect rect;
+
+  /// The unique identifier of the UIView displayed on this layer.
+  ///
+  /// A UIView with this identifier must have been created by [PlatformViewsServices.initUiKitView].
+  final int viewId;
+
+  @override
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
+    final Rect shiftedRect = rect.shift(layerOffset);
+    builder.addPlatformView(
+      viewId,
+      offset: shiftedRect.topLeft,
+      width: shiftedRect.width,
+      height: shiftedRect.height,
+    );
+  }
+
+  @override
+  S find<S>(Offset regionOffset) => null;
+}
+
 /// A layer that indicates to the compositor that it should display
 /// certain performance statistics within it.
 ///

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

@@ -43,8 +43,10 @@ enum _PlatformViewState {
 /// [RenderAndroidView] is responsible for sizing, displaying and passing touch events to an
 /// Android [View](https://developer.android.com/reference/android/view/View).
 ///
+/// {@template flutter.rendering.platformView.layout}
 /// The render object's layout behavior is to fill all available space, the parent of this object must
 /// provide bounded layout constraints.
+/// {@endtemplate}
 ///
 /// RenderAndroidView participates in Flutter's [GestureArena]s, and dispatches touch events to the
 /// Android view iff it won the arena. Specific gestures that should be dispatched to the Android
@@ -233,6 +235,81 @@ class RenderAndroidView extends RenderBox {
   }
 }
 
+/// This is work in progress, not yet ready to be used, and requires a custom engine build. A render object for an iOS UIKit UIView.
+///
+/// [RenderUiKitView] is responsible for sizing and displaying an iOS
+/// [UIView](https://developer.apple.com/documentation/uikit/uiview).
+///
+/// UIViews are added as sub views of the FlutterView and are composited by Quartz.
+///
+/// {@macro flutter.rendering.platformView.layout}
+///
+/// See also:
+///  * [UiKitView] which is a widget that is used to show a UIView.
+///  * [PlatformViewsService] which is a service for controlling platform views.
+class RenderUiKitView extends RenderBox {
+  /// Creates a render object for an iOS UIView.
+  ///
+  /// The `viewId` and `hitTestBehavior` parameters must not be null.
+  RenderUiKitView({
+    @required int viewId,
+    @required this.hitTestBehavior,
+  }) : assert(viewId != null),
+       assert(hitTestBehavior != null),
+       _viewId = viewId;
+
+
+  /// The unique identifier of the UIView controlled by this controller.
+  ///
+  /// Typically generated by [PlatformViewsRegistry.getNextPlatformViewId], the UIView
+  /// must have been created by calling [PlatformViewsService.initUiKitView].
+  int get viewId => _viewId;
+  int _viewId;
+  set viewId(int viewId) {
+    assert(viewId != null);
+    _viewId = viewId;
+    markNeedsPaint();
+  }
+
+  /// 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;
+
+  @override
+  bool get alwaysNeedsCompositing => true;
+
+  @override
+  bool get isRepaintBoundary => true;
+
+  @override
+  void performResize() {
+    size = constraints.biggest;
+  }
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    context.addLayer(PlatformViewLayer(
+      rect: offset & size,
+      viewId: _viewId,
+    ));
+  }
+
+  @override
+  bool hitTest(HitTestResult result, { Offset position }) {
+    if (hitTestBehavior == PlatformViewHitTestBehavior.transparent || !size.contains(position))
+      return false;
+    result.add(BoxHitTestEntry(this, position));
+    return hitTestBehavior == PlatformViewHitTestBehavior.opaque;
+  }
+
+  @override
+  bool hitTestSelf(Offset position) => hitTestBehavior != PlatformViewHitTestBehavior.transparent;
+}
+
 class _AndroidViewGestureRecognizer extends OneSequenceGestureRecognizer {
   _AndroidViewGestureRecognizer(this.dispatcher, this.gestureRecognizerFactories) {
     team = GestureArenaTeam();

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

@@ -91,6 +91,46 @@ class PlatformViewsService {
       onPlatformViewCreated,
     );
   }
+
+  // TODO(amirh): reference the iOS plugin API for registering a UIView factory once it lands.
+  /// This is work in progress, not yet ready to be used, and requires a custom engine build. Creates a controller for a new iOS UIView.
+  ///
+  /// `id` is an unused unique identifier generated with [platformViewsRegistry].
+  ///
+  /// `viewType` is the identifier of the iOS view type to be created, a
+  /// factory for this view type must have been registered on the platform side.
+  /// Platform view factories are typically registered by plugin code.
+  ///
+  /// The `id, `viewType, and `layoutDirection` parameters must not be null.
+  /// If `creationParams` is non null then `cretaionParamsCodec` must not be null.
+  static Future<UiKitViewController> initUiKitView({
+    @required int id,
+    @required String viewType,
+    @required TextDirection layoutDirection,
+    dynamic creationParams,
+    MessageCodec<dynamic> creationParamsCodec,
+  }) async {
+    assert(id != null);
+    assert(viewType != null);
+    assert(layoutDirection != null);
+    assert(creationParams == null || creationParamsCodec != null);
+
+    // TODO(amirh): pass layoutDirection once the system channel supports it.
+    final Map<String, dynamic> args = <String, dynamic> {
+      'id': id,
+      'viewType': viewType,
+    };
+    if (creationParams != null) {
+      final ByteData paramsByteData = creationParamsCodec.encodeMessage(creationParams);
+      args['params'] = Uint8List.view(
+        paramsByteData.buffer,
+        0,
+        paramsByteData.lengthInBytes,
+      );
+    }
+    await SystemChannels.platform_views.invokeMethod('create', args);
+    return UiKitViewController._(id, layoutDirection);
+  }
 }
 
 /// Properties of an Android pointer.
@@ -455,8 +495,7 @@ class AndroidViewController {
   ///
   /// The first time a size is set triggers the creation of the Android view.
   Future<void> setSize(Size size) async {
-    if (_state == _AndroidViewState.disposed)
-      throw FlutterError('trying to size a disposed Android View. View id: $id');
+    assert(_state != _AndroidViewState.disposed, 'trying to size a disposed Android View. View id: $id');
 
     assert(size != null);
     assert(!size.isEmpty);
@@ -473,8 +512,7 @@ class AndroidViewController {
 
   /// Sets the layout direction for the Android view.
   Future<void> setLayoutDirection(TextDirection layoutDirection) async {
-    if (_state == _AndroidViewState.disposed)
-      throw FlutterError('trying to set a layout direction for a disposed Android View. View id: $id');
+    assert(_state != _AndroidViewState.disposed,'trying to set a layout direction for a disposed UIView. View id: $id');
 
     if (layoutDirection == _layoutDirection)
       return;
@@ -544,3 +582,48 @@ class AndroidViewController {
     _state = _AndroidViewState.created;
   }
 }
+
+/// Controls an iOS UIView.
+///
+/// Typically created with [PlatformViewsService.initUiKitView].
+class UiKitViewController {
+  UiKitViewController._(
+    this.id,
+    TextDirection layoutDirection,
+  ) : assert(id != null),
+      assert(layoutDirection != null),
+      _layoutDirection = layoutDirection;
+
+
+  /// The unique identifier of the iOS view controlled by this controller.
+  ///
+  /// This identifer is typically generated by [PlatformViewsRegistry.getNextPlatformViewId].
+  final int id;
+
+  bool _debugDisposed = false;
+
+  TextDirection _layoutDirection;
+
+  /// Sets the layout direction for the Android view.
+  Future<void> setLayoutDirection(TextDirection layoutDirection) async {
+    assert(!_debugDisposed, 'trying to set a layout direction for a disposed Android View. View id: $id');
+
+    if (layoutDirection == _layoutDirection)
+      return;
+
+    assert(layoutDirection != null);
+    _layoutDirection = layoutDirection;
+
+    // TODO(amirh): invoke the iOS platform views channel direction method once available.
+  }
+
+  /// Disposes the view.
+  ///
+  /// The [UiKitViewController] object is unusable after calling this.
+  /// The `id` of the platform view cannot be reused after the view is
+  /// disposed.
+  Future<void> dispose() async {
+    _debugDisposed = true;
+    await SystemChannels.platform_views.invokeMethod('dispose', id);
+  }
+}

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

@@ -21,8 +21,10 @@ import 'framework.dart';
 /// The embedded Android view is painted just like any other Flutter widget and transformations
 /// apply to it as well.
 ///
-/// The widget fill all available space, the parent of this object must provide bounded layout
+/// {@template flutter.widgets.platformViews.layout}
+/// The widget fills all available space, the parent of this object must provide bounded layout
 /// constraints.
+/// {@endtemplate}
 ///
 /// AndroidView participates in Flutter's [GestureArena]s, and dispatches touch events to the
 /// Android view iff it won the arena. Specific gestures that should be dispatched to the Android
@@ -41,19 +43,23 @@ import 'framework.dart';
 ///   }
 /// ```
 ///
-/// The Android view's lifetime is the same as the lifetime of the [State] object for this widget.
+/// {@template flutter.widgets.platformViews.lifetime}
+/// The platform view's lifetime is the same as the lifetime of the [State] object for this widget.
 /// When the [State] is disposed the platform view (and auxiliary resources) are lazily
 /// released (some resources are immediately released and some by platform garbage collector).
 /// A stateful widget's state is disposed when the widget is removed from the tree or when it is
 /// moved within the tree. If the stateful widget has a key and it's only moved relative to its siblings,
 /// or it has a [GlobalKey] and it's moved within the tree, it will not be disposed.
+/// {@endtemplate}
 class AndroidView extends StatefulWidget {
   /// Creates a widget that embeds an Android view.
   ///
+  /// {@template flutter.widgets.platformViews.constructorParams}
   /// The `viewType` and `hitTestBehavior` parameters must not be null.
+  /// {@endtemplate}
   /// If `creationParams` is not null then `creationParamsCodec` must not be null.
   AndroidView({ // ignore: prefer_const_constructors_in_immutables
-                // TODO(aam): Remove lint ignore above once dartbug.com/34297 is fixed
+                // TODO(aam): Remove lint ignore above once https://dartbug.com/34297 is fixed
     Key key,
     @required this.viewType,
     this.onPlatformViewCreated,
@@ -68,25 +74,32 @@ class AndroidView extends StatefulWidget {
        super(key: key);
 
   /// The unique identifier for Android view type to be embedded by this widget.
+  ///
   /// A [PlatformViewFactory](/javadoc/io/flutter/plugin/platform/PlatformViewFactory.html)
   /// for this type must have been registered.
   ///
   /// See also: [AndroidView] for an example of registering a platform view factory.
   final String viewType;
 
-  /// Callback to invoke after the Android view has been created.
+  /// {@template flutter.widgets.platformViews.createdParam}
+  /// Callback to invoke after the platform view has been created.
   ///
   /// May be null.
+  /// {@endtemplate}
   final PlatformViewCreatedCallback onPlatformViewCreated;
 
+  /// {@template flutter.widgets.platformViews.hittestParam}
   /// How this widget should behave during hit testing.
   ///
   /// This defaults to [PlatformViewHitTestBehavior.opaque].
+  /// {@endtemplate}
   final PlatformViewHitTestBehavior hitTestBehavior;
 
+  /// {@template flutter.widgets.platformViews.directionParam}
   /// The text direction to use for the embedded view.
   ///
   /// If this is null, the ambient [Directionality] is used instead.
+  /// {@endtemplate}
   final TextDirection layoutDirection;
 
   /// Which gestures should be forwarded to the Android view.
@@ -157,7 +170,50 @@ class AndroidView extends StatefulWidget {
   final MessageCodec<dynamic> creationParamsCodec;
 
   @override
-  State createState() => _AndroidViewState();
+  State<AndroidView> createState() => _AndroidViewState();
+}
+
+// TODO(amirh): describe the embedding mechanism.
+/// This is work in progress, not yet ready to be used, and requires a custom engine build. Embeds an iOS view in the Widget hierarchy.
+///
+/// Embedding iOS views is an expensive operation and should be avoided when a Flutter
+/// equivalent is possible.
+///
+/// {@macro flutter.widgets.platformViews.layout}
+///
+/// {@macro flutter.widgets.platformViews.lifetime}
+class UiKitView extends StatefulWidget {
+  /// Creates a widget that embeds an iOS view.
+  ///
+  /// {@macro flutter.widgets.platformViews.constructorParams}
+  UiKitView({ // ignore: prefer_const_constructors_in_immutables
+    // TODO(aam): Remove lint ignore above once https://dartbug.com/34297 is fixed
+    Key key,
+    @required this.viewType,
+    this.onPlatformViewCreated,
+    this.hitTestBehavior = PlatformViewHitTestBehavior.opaque,
+    this.layoutDirection,
+  }) : assert(viewType != null),
+        assert(hitTestBehavior != null),
+        super(key: key);
+
+  // TODO(amirh): reference the iOS API doc once avaliable.
+  /// The unique identifier for iOS view type to be embedded by this widget.
+  ///
+  /// A PlatformViewFactory for this type must have been registered.
+  final String viewType;
+
+  /// {@macro flutter.widgets.platformViews.createdParam}
+  final PlatformViewCreatedCallback onPlatformViewCreated;
+
+  /// {@macro flutter.widgets.platformViews.hittestParam}
+  final PlatformViewHitTestBehavior hitTestBehavior;
+
+  /// {@macro flutter.widgets.platformViews.directionParam}
+  final TextDirection layoutDirection;
+
+  @override
+  State<UiKitView> createState() => _UiKitViewState();
 }
 
 class _AndroidViewState extends State<AndroidView> {
@@ -183,19 +239,17 @@ class _AndroidViewState extends State<AndroidView> {
       return;
     }
     _initialized = true;
-    _layoutDirection = _findLayoutDirection();
     _createNewAndroidView();
   }
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    _initializeOnce();
-
     final TextDirection newLayoutDirection = _findLayoutDirection();
     final bool didChangeLayoutDirection = _layoutDirection != newLayoutDirection;
     _layoutDirection = newLayoutDirection;
 
+    _initializeOnce();
     if (didChangeLayoutDirection) {
       // The native view will update asynchronously, in the meantime we don't want
       // to block the framework. (so this is intentionally not awaiting).
@@ -246,6 +300,97 @@ class _AndroidViewState extends State<AndroidView> {
   }
 }
 
+class _UiKitViewState extends State<UiKitView> {
+  int _id;
+  UiKitViewController _controller;
+  TextDirection _layoutDirection;
+  bool _initialized = false;
+
+  static final Set<Factory<OneSequenceGestureRecognizer>> _emptyRecognizersSet =
+    Set<Factory<OneSequenceGestureRecognizer>>();
+
+  @override
+  Widget build(BuildContext context) {
+    if (_controller == null) {
+      return const SizedBox.expand();
+    }
+    return _UiKitPlatformView(
+      viewId: _id,
+      hitTestBehavior: widget.hitTestBehavior,
+    );
+  }
+
+  void _initializeOnce() {
+    if (_initialized) {
+      return;
+    }
+    _initialized = true;
+    _createNewUiKitView();
+  }
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    final TextDirection newLayoutDirection = _findLayoutDirection();
+    final bool didChangeLayoutDirection = _layoutDirection != newLayoutDirection;
+    _layoutDirection = newLayoutDirection;
+
+    _initializeOnce();
+    if (didChangeLayoutDirection) {
+      // The native view will update asynchronously, in the meantime we don't want
+      // to block the framework. (so this is intentionally not awaiting).
+      _controller?.setLayoutDirection(_layoutDirection);
+    }
+  }
+
+  @override
+  void didUpdateWidget(UiKitView oldWidget) {
+    super.didUpdateWidget(oldWidget);
+
+    final TextDirection newLayoutDirection = _findLayoutDirection();
+    final bool didChangeLayoutDirection = _layoutDirection != newLayoutDirection;
+    _layoutDirection = newLayoutDirection;
+
+    if (widget.viewType != oldWidget.viewType) {
+      _controller?.dispose();
+      _createNewUiKitView();
+      return;
+    }
+
+    if (didChangeLayoutDirection) {
+      _controller?.setLayoutDirection(_layoutDirection);
+    }
+  }
+
+  TextDirection _findLayoutDirection() {
+    assert(widget.layoutDirection != null || debugCheckHasDirectionality(context));
+    return widget.layoutDirection ?? Directionality.of(context);
+  }
+
+  @override
+  void dispose() {
+    _controller?.dispose();
+    super.dispose();
+  }
+
+  Future<void> _createNewUiKitView() async {
+    _id = platformViewsRegistry.getNextPlatformViewId();
+    final UiKitViewController controller = await PlatformViewsService.initUiKitView(
+      id: _id,
+      viewType: widget.viewType,
+      layoutDirection: _layoutDirection,
+    );
+    if (!mounted) {
+      controller.dispose();
+      return;
+    }
+    if (widget.onPlatformViewCreated != null) {
+      widget.onPlatformViewCreated(_id);
+    }
+    setState(() { _controller = controller; });
+  }
+}
+
 class _AndroidPlatformView extends LeafRenderObjectWidget {
   const _AndroidPlatformView({
     Key key,
@@ -276,3 +421,30 @@ class _AndroidPlatformView extends LeafRenderObjectWidget {
     renderObject.updateGestureRecognizers(gestureRecognizers);
   }
 }
+
+class _UiKitPlatformView extends LeafRenderObjectWidget {
+  const _UiKitPlatformView({
+    Key key,
+    @required this.viewId,
+    @required this.hitTestBehavior,
+  }) : assert(viewId != null),
+       assert(hitTestBehavior != null),
+       super(key: key);
+
+  final int viewId;
+  final PlatformViewHitTestBehavior hitTestBehavior;
+
+  @override
+  RenderObject createRenderObject(BuildContext context) {
+    return RenderUiKitView(
+      viewId: viewId,
+      hitTestBehavior: hitTestBehavior,
+    );
+  }
+
+  @override
+  void updateRenderObject(BuildContext context, RenderUiKitView renderObject) {
+    renderObject.viewId = viewId;
+    renderObject.hitTestBehavior = hitTestBehavior;
+  }
+}

packages/flutter/test/services/fake_platform_views.dart

@@ -6,21 +6,19 @@ import 'dart:async';
 import 'dart:typed_data';
 import 'package:collection/collection.dart';
 
-import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
 import 'package:flutter/services.dart';
 
-class FakePlatformViewsController {
-  FakePlatformViewsController(this.targetPlatform) : assert(targetPlatform != null) {
+class FakeAndroidPlatformViewsController {
+  FakeAndroidPlatformViewsController() {
     SystemChannels.platform_views.setMockMethodCallHandler(_onMethodCall);
   }
 
-  final TargetPlatform targetPlatform;
 
-  Iterable<FakePlatformView> get views => _views.values;
-  final Map<int, FakePlatformView> _views = <int, FakePlatformView>{};
+  Iterable<FakeAndroidPlatformView> get views => _views.values;
+  final Map<int, FakeAndroidPlatformView> _views = <int, FakeAndroidPlatformView>{};
 
-  final Map<int, List<FakeMotionEvent>> motionEvents = <int, List<FakeMotionEvent>>{};
+  final Map<int, List<FakeAndroidMotionEvent>> motionEvents = <int, List<FakeAndroidMotionEvent>>{};
 
   final Set<String> _registeredViewTypes = Set<String>();
 
@@ -33,12 +31,6 @@ class FakePlatformViewsController {
   }
 
   Future<dynamic> _onMethodCall(MethodCall call) {
-    if (targetPlatform == TargetPlatform.android)
-      return _onMethodCallAndroid(call);
-    return Future<dynamic>.sync(() => null);
-  }
-
-  Future<dynamic> _onMethodCallAndroid(MethodCall call) {
     switch(call.method) {
       case 'create':
         return _create(call);
@@ -75,7 +67,7 @@ class FakePlatformViewsController {
         message: 'Trying to create a platform view of unregistered type: $viewType',
       );
 
-    _views[id] = FakePlatformView(id, viewType, Size(width, height), layoutDirection, creationParams);
+    _views[id] = FakeAndroidPlatformView(id, viewType, Size(width, height), layoutDirection, creationParams);
     final int textureId = _textureCounter++;
     return Future<int>.sync(() => textureId);
   }
@@ -129,9 +121,9 @@ class FakePlatformViewsController {
     }
 
     if (!motionEvents.containsKey(id))
-      motionEvents[id] = <FakeMotionEvent> [];
+      motionEvents[id] = <FakeAndroidMotionEvent> [];
 
-    motionEvents[id].add(FakeMotionEvent(action, pointerIds, pointerOffsets));
+    motionEvents[id].add(FakeAndroidMotionEvent(action, pointerIds, pointerOffsets));
     return Future<dynamic>.sync(() => null);
   }
 
@@ -152,9 +144,77 @@ class FakePlatformViewsController {
   }
 }
 
-class FakePlatformView {
+class FakeIosPlatformViewsController {
+  FakeIosPlatformViewsController() {
+    SystemChannels.platform_views.setMockMethodCallHandler(_onMethodCall);
+  }
+
+
+  Iterable<FakeUiKitView> get views => _views.values;
+  final Map<int, FakeUiKitView> _views = <int, FakeUiKitView>{};
 
-  FakePlatformView(this.id, this.type, this.size, this.layoutDirection, [this.creationParams]);
+  final Set<String> _registeredViewTypes = Set<String>();
+
+  // When this completer is non null, the 'create' method channel call will be
+  // delayed until it completes.
+  Completer<void> creationDelay;
+
+  void registerViewType(String viewType) {
+    _registeredViewTypes.add(viewType);
+  }
+
+  Future<dynamic> _onMethodCall(MethodCall call) {
+    switch(call.method) {
+      case 'create':
+        return _create(call);
+      case 'dispose':
+        return _dispose(call);
+    }
+    return Future<dynamic>.sync(() => null);
+    }
+
+  Future<dynamic> _create(MethodCall call) async {
+    if (creationDelay != null)
+      await creationDelay.future;
+    final Map<dynamic, dynamic> args = call.arguments;
+    final int id = args['id'];
+    final String viewType = args['viewType'];
+
+    if (_views.containsKey(id)) {
+      throw PlatformException(
+        code: 'error',
+        message: 'Trying to create an already created platform view, view id: $id',
+      );
+    }
+
+    if (!_registeredViewTypes.contains(viewType)) {
+      throw PlatformException(
+        code: 'error',
+        message: 'Trying to create a platform view of unregistered type: $viewType',
+      );
+    }
+
+    _views[id] = FakeUiKitView(id, viewType);
+    return Future<int>.sync(() => null);
+  }
+
+  Future<dynamic> _dispose(MethodCall call) {
+    final int id = call.arguments;
+
+    if (!_views.containsKey(id)) {
+      throw PlatformException(
+        code: 'error',
+        message: 'Trying to dispose a platform view with unknown id: $id',
+      );
+    }
+
+    _views.remove(id);
+    return Future<dynamic>.sync(() => null);
+  }
+}
+
+class FakeAndroidPlatformView {
+  FakeAndroidPlatformView(this.id, this.type, this.size, this.layoutDirection, [this.creationParams]);
 
   final int id;
   final String type;
@@ -164,9 +224,9 @@ class FakePlatformView {
 
   @override
   bool operator ==(dynamic other) {
-    if (other is! FakePlatformView)
+    if (other.runtimeType != FakeAndroidPlatformView)
       return false;
-    final FakePlatformView typedOther = other;
+    final FakeAndroidPlatformView typedOther = other;
     return id == typedOther.id &&
         type == typedOther.type &&
         creationParams == typedOther.creationParams &&
@@ -178,12 +238,12 @@ class FakePlatformView {
 
   @override
   String toString() {
-    return 'FakePlatformView(id: $id, type: $type, size: $size, layoutDirection: $layoutDirection, creationParams: $creationParams)';
+    return 'FakeAndroidPlatformView(id: $id, type: $type, size: $size, layoutDirection: $layoutDirection, creationParams: $creationParams)';
   }
 }
 
-class FakeMotionEvent {
-  const FakeMotionEvent(this.action, this.pointerIds, this.pointers);
+class FakeAndroidMotionEvent {
+  const FakeAndroidMotionEvent(this.action, this.pointerIds, this.pointers);
 
   final int action;
   final List<Offset> pointers;
@@ -192,9 +252,9 @@ class FakeMotionEvent {
 
   @override
   bool operator ==(dynamic other) {
-    if (other is! FakeMotionEvent)
+    if (other is! FakeAndroidMotionEvent)
       return false;
-    final FakeMotionEvent typedOther = other;
+    final FakeAndroidMotionEvent typedOther = other;
     const ListEquality<Offset> offsetsEq = ListEquality<Offset>();
     const ListEquality<int> pointersEq = ListEquality<int>();
     return pointersEq.equals(pointerIds, typedOther.pointerIds) &&
@@ -207,6 +267,30 @@ class FakeMotionEvent {
 
   @override
   String toString() {
-    return 'FakeMotionEvent(action: $action, pointerIds: $pointerIds, pointers: $pointers)';
+    return 'FakeAndroidMotionEvent(action: $action, pointerIds: $pointerIds, pointers: $pointers)';
+  }
+}
+
+class FakeUiKitView {
+  FakeUiKitView(this.id, this.type);
+
+  final int id;
+  final String type;
+
+  @override
+  bool operator ==(dynamic other) {
+    if (other.runtimeType != FakeUiKitView)
+      return false;
+    final FakeUiKitView typedOther = other;
+    return id == typedOther.id &&
+      type == typedOther.type;
+  }
+
+  @override
+  int get hashCode => hashValues(id, type);
+
+  @override
+  String toString() {
+    return 'FakeIosPlatformView(id: $id, type: $type)';
   }
 }

packages/flutter/test/services/platform_views_test.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
 import 'package:flutter/services.dart';
 import '../flutter_test_alternative.dart';
@@ -10,11 +9,11 @@ import '../flutter_test_alternative.dart';
 import 'fake_platform_views.dart';
 
 void main() {
-  FakePlatformViewsController viewsController;
 
   group('Android', () {
+    FakeAndroidPlatformViewsController viewsController;
     setUp(() {
-      viewsController = FakePlatformViewsController(TargetPlatform.android);
+      viewsController = FakeAndroidPlatformViewsController();
     });
 
     test('create Android view of unregistered type', () async {
@@ -38,9 +37,9 @@ void main() {
           .setSize(const Size(200.0, 300.0));
       expect(
           viewsController.views,
-          unorderedEquals(<FakePlatformView>[
-            FakePlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionLtr),
-            FakePlatformView(1, 'webview', const Size(200.0, 300.0), AndroidViewController.kAndroidLayoutDirectionRtl),
+          unorderedEquals(<FakeAndroidPlatformView>[
+            FakeAndroidPlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionLtr),
+            FakeAndroidPlatformView(1, 'webview', const Size(200.0, 300.0), AndroidViewController.kAndroidLayoutDirectionRtl),
           ]));
     });
 
@@ -65,11 +64,11 @@ void main() {
           PlatformViewsService.initAndroidView(id: 1, viewType: 'webview', layoutDirection: TextDirection.ltr);
       await viewController.setSize(const Size(200.0, 300.0));
 
-      viewController.dispose();
+      await viewController.dispose();
       expect(
           viewsController.views,
-          unorderedEquals(<FakePlatformView>[
-            FakePlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionLtr),
+          unorderedEquals(<FakeAndroidPlatformView>[
+            FakeAndroidPlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionLtr),
           ]));
     });
 
@@ -94,9 +93,9 @@ void main() {
       await viewController.setSize(const Size(500.0, 500.0));
       expect(
           viewsController.views,
-          unorderedEquals(<FakePlatformView>[
-            FakePlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionLtr),
-            FakePlatformView(1, 'webview', const Size(500.0, 500.0), AndroidViewController.kAndroidLayoutDirectionLtr),
+          unorderedEquals(<FakeAndroidPlatformView>[
+            FakeAndroidPlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionLtr),
+            FakeAndroidPlatformView(1, 'webview', const Size(500.0, 500.0), AndroidViewController.kAndroidLayoutDirectionLtr),
           ]));
     });
 
@@ -129,8 +128,8 @@ void main() {
       await viewController.setSize(const Size(100.0, 100.0));
       expect(
           viewsController.views,
-          unorderedEquals(<FakePlatformView>[
-            FakePlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionLtr),
+          unorderedEquals(<FakeAndroidPlatformView>[
+            FakeAndroidPlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionLtr),
           ]));
     });
 
@@ -142,9 +141,88 @@ void main() {
       await viewController.setLayoutDirection(TextDirection.rtl);
       expect(
           viewsController.views,
-          unorderedEquals(<FakePlatformView>[
-            FakePlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionRtl),
+          unorderedEquals(<FakeAndroidPlatformView>[
+            FakeAndroidPlatformView(0, 'webview', const Size(100.0, 100.0), AndroidViewController.kAndroidLayoutDirectionRtl),
+          ]));
+    });
+  });
+
+  group('iOS', ()
+  {
+    FakeIosPlatformViewsController viewsController;
+    setUp(() {
+      viewsController = FakeIosPlatformViewsController();
+    });
+
+    test('create iOS view of unregistered type', () async {
+      expect(
+            () {
+          return PlatformViewsService.initUiKitView(
+            id: 0,
+            viewType: 'web',
+            layoutDirection: TextDirection.ltr,
+          );
+        },
+        throwsA(isInstanceOf<PlatformException>()),
+      );
+    });
+
+    test('create iOS views', () async {
+      viewsController.registerViewType('webview');
+      await PlatformViewsService.initUiKitView(
+          id: 0, viewType: 'webview', layoutDirection: TextDirection.ltr);
+      await PlatformViewsService.initUiKitView(
+          id: 1, viewType: 'webview', layoutDirection: TextDirection.rtl);
+      expect(
+        viewsController.views,
+        unorderedEquals(<FakeUiKitView>[
+          FakeUiKitView(0, 'webview'),
+          FakeUiKitView(1, 'webview'),
+        ]),
+      );
+    });
+
+    test('reuse iOS view id', () async {
+      viewsController.registerViewType('webview');
+      await PlatformViewsService.initUiKitView(
+        id: 0,
+        viewType: 'webview',
+        layoutDirection: TextDirection.ltr,
+      );
+      expect(
+            () => PlatformViewsService.initUiKitView(
+            id: 0, viewType: 'web', layoutDirection: TextDirection.ltr),
+        throwsA(isInstanceOf<PlatformException>()),
+      );
+    });
+
+    test('dispose iOS view', () async {
+      viewsController.registerViewType('webview');
+      await PlatformViewsService.initUiKitView(
+          id: 0, viewType: 'webview', layoutDirection: TextDirection.ltr);
+      final UiKitViewController viewController = await PlatformViewsService.initUiKitView(
+          id: 1, viewType: 'webview', layoutDirection: TextDirection.ltr);
+
+      viewController.dispose();
+      expect(
+          viewsController.views,
+          unorderedEquals(<FakeUiKitView>[
+            FakeUiKitView(0, 'webview'),
           ]));
     });
+
+    test('dispose inexisting iOS view', () async {
+      viewsController.registerViewType('webview');
+      await PlatformViewsService.initUiKitView(id: 0, viewType: 'webview', layoutDirection: TextDirection.ltr);
+      final UiKitViewController viewController = await PlatformViewsService.initUiKitView(
+          id: 1, viewType: 'webview', layoutDirection: TextDirection.ltr);
+      await viewController.dispose();
+      expect(
+          () async {
+            await viewController.dispose();
+          },
+          throwsA(isInstanceOf<PlatformException>()),
+      );
+    });
   });
 }