[web] Move EmbeddingStrategy and DimensionsProvider out of FlutterViewEmbedder (flutter/engine#48025)

Part of https://github.com/flutter/flutter/issues/134443
Part of https://github.com/flutter/flutter/issues/117098

engine/src/flutter/lib/web_ui/lib/src/engine/embedder.dart

@@ -14,9 +14,7 @@ import 'platform_dispatcher.dart';
 import 'pointer_binding.dart';
 import 'semantics.dart';
 import 'text_editing/text_editing.dart';
-import 'view_embedder/dimensions_provider/dimensions_provider.dart';
 import 'view_embedder/dom_manager.dart';
-import 'view_embedder/embedding_strategy/embedding_strategy.dart';
 import 'view_embedder/style_manager.dart';
 import 'window.dart';
 
@@ -40,30 +38,10 @@ import 'window.dart';
 /// multiple instances in a multi-view scenario. (One ViewEmbedder per FlutterView).
 class FlutterViewEmbedder {
   /// Creates a FlutterViewEmbedder.
-  ///
-  /// The incoming [hostElement] parameter specifies the root element in the DOM
-  /// into which Flutter will be rendered.
-  ///
-  /// The hostElement is abstracted by an [EmbeddingStrategy] instance, which has
-  /// different behavior depending on the `hostElement` value:
-  ///
-  /// - A `null` `hostElement` will cause Flutter to take over the whole page.
-  /// - A non-`null` `hostElement` will render flutter inside that element.
-  FlutterViewEmbedder({DomElement? hostElement})
-      : _embeddingStrategy =
-            EmbeddingStrategy.create(hostElement: hostElement) {
-    // Configure the EngineWindow so it knows how to measure itself.
-    // TODO(dit): Refactor ownership according to new design, https://github.com/flutter/flutter/issues/117098
-    window.configureDimensionsProvider(DimensionsProvider.create(
-      hostElement: hostElement,
-    ));
-
+  FlutterViewEmbedder() {
     reset();
   }
 
-  /// Abstracts all the DOM manipulations required to embed a Flutter app in an user-supplied `hostElement`.
-  final EmbeddingStrategy _embeddingStrategy;
-
   /// The element that contains the [sceneElement].
   ///
   /// This element is created and inserted in the HTML DOM once. It is never
@@ -139,7 +117,7 @@ class FlutterViewEmbedder {
         : 'requested explicitly';
 
     // Initializes the embeddingStrategy so it can host a single-view Flutter app.
-    _embeddingStrategy.initialize(
+    window.embeddingStrategy.initialize(
       hostElementAttributes: <String, String>{
         'flt-renderer': '${renderer.rendererTag} ($rendererSelection)',
         'flt-build-mode': buildMode,
@@ -158,7 +136,7 @@ class FlutterViewEmbedder {
     //
     // The embeddingStrategy will take care of cleaning up the glassPane on hot
     // restart.
-    _embeddingStrategy.attachGlassPane(_flutterViewElement);
+    window.embeddingStrategy.attachGlassPane(_flutterViewElement);
     _flutterViewElement.appendChild(_glassPaneElement);
 
     if (getJsProperty<Object?>(_glassPaneElement, 'attachShadow') == null) {
@@ -270,7 +248,7 @@ class FlutterViewEmbedder {
         ..style.visibility = 'hidden';
       if (isWebKit) {
         // The resourcesHost *must* be a sibling of the glassPaneElement.
-        _embeddingStrategy.attachResourcesHost(resourcesHost,
+        window.embeddingStrategy.attachResourcesHost(resourcesHost,
             nextTo: _flutterViewElement);
       } else {
         _glassPaneShadow.insertBefore(resourcesHost, _glassPaneShadow.firstChild);
@@ -312,10 +290,9 @@ FlutterViewEmbedder? _flutterViewEmbedder;
 /// Initializes the [FlutterViewEmbedder], if it's not already initialized.
 FlutterViewEmbedder ensureFlutterViewEmbedderInitialized() {
   // FlutterViewEmbedder needs the implicit view to be initialized because it
-  // uses some of its methods e.g. `configureDimensionsProvider`, `onResize`.
+  // uses some of its members e.g. `embeddingStrategy`, `onResize`.
   ensureImplicitViewInitialized();
-  return _flutterViewEmbedder ??=
-      FlutterViewEmbedder(hostElement: configuration.hostElement);
+  return _flutterViewEmbedder ??= FlutterViewEmbedder();
 }
 
 /// Creates a node to host text editing elements and applies a stylesheet

engine/src/flutter/lib/web_ui/lib/src/engine/initialization.dart

@@ -218,7 +218,7 @@ Future<void> initializeEngineUi() async {
   _initializationState = DebugEngineInitializationState.initializingUi;
 
   RawKeyboard.initialize(onMacOs: operatingSystem == OperatingSystem.macOs);
-  ensureImplicitViewInitialized();
+  ensureImplicitViewInitialized(hostElement: configuration.hostElement);
   ensureFlutterViewEmbedderInitialized();
   _initializationState = DebugEngineInitializationState.initialized;
 }

engine/src/flutter/lib/web_ui/lib/src/engine/window.dart

@@ -26,6 +26,7 @@ import 'semantics/accessibility.dart';
 import 'services.dart';
 import 'util.dart';
 import 'view_embedder/dom_manager.dart';
+import 'view_embedder/embedding_strategy/embedding_strategy.dart';
 
 typedef _HandleMessageCallBack = Future<bool> Function();
 
@@ -40,15 +41,25 @@ const int kImplicitViewId = 0;
 /// In addition to everything defined in [ui.FlutterView], this class adds
 /// a few web-specific properties.
 base class EngineFlutterView implements ui.FlutterView {
+  /// Creates a [ui.FlutterView] that can be used in multi-view mode.
+  ///
+  /// The [hostElement] parameter specifies the container in the DOM into which
+  /// the Flutter view will be rendered.
   factory EngineFlutterView(
     int viewId,
     EnginePlatformDispatcher platformDispatcher,
+    DomElement hostElement,
   ) = _EngineFlutterViewImpl;
 
   EngineFlutterView._(
     this.viewId,
     this.platformDispatcher,
-  );
+    // This is nullable to accommodate the legacy `EngineFlutterWindow`. In
+    // multi-view mode, the host element is required for each view (as reflected
+    // by the public `EngineFlutterView` constructor).
+    DomElement? hostElement,
+  )   : embeddingStrategy = EmbeddingStrategy.create(hostElement: hostElement),
+        _dimensionsProvider = DimensionsProvider.create(hostElement: hostElement);
 
   @override
   final int viewId;
@@ -56,6 +67,9 @@ base class EngineFlutterView implements ui.FlutterView {
   @override
   final EnginePlatformDispatcher platformDispatcher;
 
+  /// Abstracts all the DOM manipulations required to embed a Flutter view in a user-supplied `hostElement`.
+  final EmbeddingStrategy embeddingStrategy;
+
   final ViewConfiguration _viewConfiguration = const ViewConfiguration();
 
   @override
@@ -143,19 +157,17 @@ base class EngineFlutterView implements ui.FlutterView {
   @override
   double get devicePixelRatio => display.devicePixelRatio;
 
-  late DimensionsProvider _dimensionsProvider;
-  void configureDimensionsProvider(DimensionsProvider dimensionsProvider) {
-    _dimensionsProvider = dimensionsProvider;
-  }
+  final DimensionsProvider _dimensionsProvider;
 
   Stream<ui.Size?> get onResize => _dimensionsProvider.onResize;
 }
 
 final class _EngineFlutterViewImpl extends EngineFlutterView {
   _EngineFlutterViewImpl(
-    int viewId,
-    EnginePlatformDispatcher platformDispatcher,
-  ) : super._(viewId, platformDispatcher) {
+    super.viewId,
+    super.platformDispatcher,
+    super.hostElement,
+  ) : super._() {
     platformDispatcher.registerView(this);
     registerHotRestartListener(() {
       // TODO(harryterkelsen): What should we do about this in multi-view?
@@ -168,9 +180,10 @@ final class _EngineFlutterViewImpl extends EngineFlutterView {
 /// The Web implementation of [ui.SingletonFlutterWindow].
 final class EngineFlutterWindow extends EngineFlutterView implements ui.SingletonFlutterWindow {
   EngineFlutterWindow(
-    int viewId,
-    EnginePlatformDispatcher platformDispatcher,
-  ) : super._(viewId, platformDispatcher) {
+    super.viewId,
+    super.platformDispatcher,
+    super.hostElement,
+  ) : super._() {
     platformDispatcher.registerView(this);
     if (ui_web.isCustomUrlStrategySet) {
       _browserHistory = createHistoryForExistingState(ui_web.urlStrategy);
@@ -585,9 +598,14 @@ EngineFlutterWindow? _window;
 
 /// Initializes the [window] (aka the implicit view), if it's not already
 /// initialized.
-EngineFlutterWindow ensureImplicitViewInitialized() {
-  return _window ??=
-      EngineFlutterWindow(kImplicitViewId, EnginePlatformDispatcher.instance);
+EngineFlutterWindow ensureImplicitViewInitialized({
+  DomElement? hostElement,
+}) {
+  return _window ??= EngineFlutterWindow(
+    kImplicitViewId,
+    EnginePlatformDispatcher.instance,
+    hostElement,
+  );
 }
 
 /// The Web implementation of [ui.ViewPadding].

engine/src/flutter/lib/web_ui/test/engine/routing_test.dart

@@ -37,7 +37,7 @@ void testMain() {
 
   setUp(() {
     savedWindow = EnginePlatformDispatcher.instance.implicitView;
-    myWindow = EngineFlutterWindow(0, EnginePlatformDispatcher.instance);
+    myWindow = EngineFlutterWindow(0, EnginePlatformDispatcher.instance, createDomHTMLDivElement());
   });
 
   tearDown(() async {

engine/src/flutter/lib/web_ui/test/engine/surface/platform_view_test.dart

@@ -14,7 +14,11 @@ import '../../common/matchers.dart';
 import '../../common/test_initialization.dart';
 
 const MethodCodec codec = StandardMethodCodec();
-final EngineFlutterWindow window = EngineFlutterWindow(0, EnginePlatformDispatcher.instance);
+final EngineFlutterWindow window = EngineFlutterWindow(
+  0,
+  EnginePlatformDispatcher.instance,
+  createDomHTMLDivElement(),
+);
 
 void main() {
   internalBootstrapBrowserTest(() => testMain);

engine/src/flutter/lib/web_ui/test/engine/window_test.dart

@@ -27,7 +27,7 @@ Future<void> testMain() async {
   late EngineFlutterWindow myWindow;
 
   setUp(() {
-    myWindow = EngineFlutterWindow(99, EnginePlatformDispatcher.instance);
+    myWindow = EngineFlutterWindow(99, EnginePlatformDispatcher.instance, createDomHTMLDivElement());
   });
 
   tearDown(() async {