[web] Move scene DOM management to DomManager (flutter#47460)

- Move scene insertion logic to `DomManager`.
- Add TODOs in `Renderer` subclasses (cc @harryterkelsen).

Part of flutter/flutter#134443

lib/web_ui/lib/src/engine/canvaskit/renderer.dart

@@ -68,7 +68,9 @@ class CanvasKitRenderer implements Renderer {
     // added eagerly during initialization here and never touched, unless the
     // system is reset due to hot restart or in a test.
     _sceneHost = createDomElement('flt-scene');
-    embedder.addSceneToSceneHost(_sceneHost);
+    // TODO(harryterkelsen): Do this operation on the appropriate Flutter View.
+    final EngineFlutterView implicitView = EnginePlatformDispatcher.instance.implicitView!;
+    implicitView.dom.setScene(_sceneHost!);
   }
 
   @override

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

@@ -39,30 +39,12 @@ class FlutterViewEmbedder {
     reset();
   }
 
-  DomElement get _sceneHostElement => window.dom.sceneHost;
-
   /// A child element of body outside the shadowroot that hosts
   /// global resources such svg filters and clip paths when using webkit.
   DomElement? _resourcesHost;
 
   DomElement get _semanticsHostElement => window.dom.semanticsHost;
 
-  /// The last scene element rendered by the [render] method.
-  DomElement? get sceneElement => _sceneElement;
-  DomElement? _sceneElement;
-
-  /// Don't unnecessarily move DOM nodes around. If a DOM node is
-  /// already in the right place, skip DOM mutation. This is both faster and
-  /// more correct, because moving DOM nodes loses internal state, such as
-  /// text selection.
-  void addSceneToSceneHost(DomElement? sceneElement) {
-    if (sceneElement != _sceneElement) {
-      _sceneElement?.remove();
-      _sceneElement = sceneElement;
-      _sceneHostElement.append(sceneElement!);
-    }
-  }
-
   DomElement get _flutterViewElement => window.dom.rootElement;
   DomShadowRoot get _glassPaneShadow => window.dom.renderingHost;
 

lib/web_ui/lib/src/engine/html/renderer.dart

@@ -20,8 +20,6 @@ class HtmlRenderer implements Renderer {
 
   late final HtmlFontCollection _fontCollection = HtmlFontCollection();
 
-  late FlutterViewEmbedder _viewEmbedder;
-
   @override
   HtmlFontCollection get fontCollection => _fontCollection;
 
@@ -38,9 +36,7 @@ class HtmlRenderer implements Renderer {
   }
 
   @override
-  void reset(FlutterViewEmbedder embedder) {
-    _viewEmbedder = embedder;
-  }
+  void reset(FlutterViewEmbedder embedder) {}
 
   @override
   ui.Paint createPaint() => SurfacePaint();
@@ -328,7 +324,8 @@ class HtmlRenderer implements Renderer {
 
   @override
   void renderScene(ui.Scene scene) {
-    _viewEmbedder.addSceneToSceneHost((scene as SurfaceScene).webOnlyRootElement);
+    final EngineFlutterView implicitView = EnginePlatformDispatcher.instance.implicitView!;
+    implicitView.dom.setScene((scene as SurfaceScene).webOnlyRootElement!);
     frameTimingsOnRasterFinish();
   }
 

lib/web_ui/lib/src/engine/skwasm/skwasm_impl/renderer.dart

@@ -406,7 +406,9 @@ class SkwasmRenderer implements Renderer {
 
   @override
   void reset(FlutterViewEmbedder embedder) {
-    embedder.addSceneToSceneHost(sceneView.sceneElement);
+    // TODO(harryterkelsen): Do this operation on the appropriate Flutter View.
+    final EngineFlutterView implicitView = EnginePlatformDispatcher.instance.implicitView!;
+    implicitView.dom.setScene(sceneView.sceneElement);
   }
 
   static final Map<String, Future<ui.FragmentProgram>> _programs = <String, Future<ui.FragmentProgram>>{};

lib/web_ui/lib/src/engine/view_embedder/dom_manager.dart

@@ -178,6 +178,25 @@ class DomManager {
 
   /// This is where accessibility announcements are inserted.
   final DomElement announcementsHost;
+
+  DomElement? _lastSceneElement;
+
+  /// Inserts the [sceneElement] into the DOM and removes the existing scene (if
+  /// any).
+  ///
+  /// The [sceneElement] is inserted  as a child of the <flt-scene-host> element
+  /// inside the [renderingHost].
+  ///
+  /// If the [sceneElement] has already been inserted, this method does nothing
+  /// to avoid unnecessary DOM mutations. This is both faster and more correct,
+  /// because moving DOM nodes loses internal state, such as text selection.
+  void setScene(DomElement sceneElement) {
+    if (sceneElement != _lastSceneElement) {
+      _lastSceneElement?.remove();
+      _lastSceneElement = sceneElement;
+      sceneHost.append(sceneElement);
+    }
+  }
 }
 
 DomShadowRoot _attachShadowRoot(DomElement element) {

lib/web_ui/test/canvaskit/embedded_views_test.dart

@@ -16,9 +16,8 @@ import 'test_data.dart';
 EngineFlutterWindow get implicitView =>
     EnginePlatformDispatcher.instance.implicitView!;
 
-DomElement get platformViewsHost {
-  return EnginePlatformDispatcher.instance.implicitView!.dom.platformViewsHost;
-}
+DomElement get platformViewsHost => implicitView.dom.platformViewsHost;
+DomElement get sceneElement => implicitView.dom.sceneHost.querySelector('flt-scene')!;
 
 void main() {
   internalBootstrapBrowserTest(() => testMain);
@@ -49,8 +48,7 @@ void testMain() {
       // as a child of the glassPane, and the slot lives in the glassPane
       // shadow root. The slot is the one that has pointer events auto.
       final DomElement contents = platformViewsHost.querySelector('#view-0')!;
-      final DomElement slot =
-          flutterViewEmbedder.sceneElement!.querySelector('slot')!;
+      final DomElement slot = sceneElement.querySelector('slot')!;
       final DomElement contentsHost = contents.parent!;
       final DomElement slotHost = slot.parent!;
 
@@ -82,41 +80,27 @@ void testMain() {
       rasterizer.draw(sb.build().layerTree);
 
       expect(
-        flutterViewEmbedder.sceneElement!
-            .querySelectorAll('#sk_path_defs')
-            .single,
+        sceneElement.querySelectorAll('#sk_path_defs').single,
         isNotNull,
       );
       expect(
-        flutterViewEmbedder.sceneElement!
+        sceneElement
             .querySelectorAll('#sk_path_defs')
             .single
             .querySelectorAll('clipPath')
             .single,
         isNotNull,
       );
       expect(
-        flutterViewEmbedder.sceneElement!
-            .querySelectorAll('flt-clip')
-            .single
-            .style
-            .clipPath,
+        sceneElement.querySelectorAll('flt-clip').single.style.clipPath,
         'url("#svgClip1")',
       );
       expect(
-        flutterViewEmbedder.sceneElement!
-            .querySelectorAll('flt-clip')
-            .single
-            .style
-            .width,
+        sceneElement.querySelectorAll('flt-clip').single.style.width,
         '100%',
       );
       expect(
-        flutterViewEmbedder.sceneElement!
-            .querySelectorAll('flt-clip')
-            .single
-            .style
-            .height,
+        sceneElement.querySelectorAll('flt-clip').single.style.height,
         '100%',
       );
     });
@@ -140,8 +124,8 @@ void testMain() {
       rasterizer.draw(sb.build().layerTree);
 
       // Transformations happen on the slot element.
-      final DomElement slotHost = flutterViewEmbedder.sceneElement!
-          .querySelector('flt-platform-view-slot')!;
+      final DomElement slotHost =
+          sceneElement.querySelector('flt-platform-view-slot')!;
 
       expect(
         slotHost.style.transform,
@@ -163,8 +147,8 @@ void testMain() {
       sb.addPlatformView(0, offset: const ui.Offset(3, 4), width: 5, height: 6);
       CanvasKitRenderer.instance.rasterizer.draw(sb.build().layerTree);
 
-      final DomElement slotHost = flutterViewEmbedder.sceneElement!
-          .querySelector('flt-platform-view-slot')!;
+      final DomElement slotHost =
+          sceneElement.querySelector('flt-platform-view-slot')!;
       final DomCSSStyleDeclaration style = slotHost.style;
 
       expect(style.transform, 'matrix(1, 0, 0, 1, 3, 4)');
@@ -206,8 +190,8 @@ void testMain() {
       CanvasKitRenderer.instance.rasterizer.draw(sb.build().layerTree);
 
       // Transformations happen on the slot element.
-      DomElement slotHost = flutterViewEmbedder.sceneElement!
-          .querySelector('flt-platform-view-slot')!;
+      DomElement slotHost =
+          sceneElement.querySelector('flt-platform-view-slot')!;
 
       expect(
         getTransformChain(slotHost),
@@ -227,8 +211,7 @@ void testMain() {
       CanvasKitRenderer.instance.rasterizer.draw(sb.build().layerTree);
 
       // Transformations happen on the slot element.
-      slotHost = flutterViewEmbedder.sceneElement!
-          .querySelector('flt-platform-view-slot')!;
+      slotHost = sceneElement.querySelector('flt-platform-view-slot')!;
 
       expect(
         getTransformChain(slotHost),
@@ -256,8 +239,8 @@ void testMain() {
       CanvasKitRenderer.instance.rasterizer.draw(sb.build().layerTree);
 
       // Transformations happen on the slot element.
-      final DomElement slotHost = flutterViewEmbedder.sceneElement!
-          .querySelector('flt-platform-view-slot')!;
+      final DomElement slotHost =
+          sceneElement.querySelector('flt-platform-view-slot')!;
 
       expect(
         getTransformChain(slotHost),
@@ -283,8 +266,8 @@ void testMain() {
       CanvasKitRenderer.instance.rasterizer.draw(sb.build().layerTree);
 
       // Transformations happen on the slot element.
-      final DomElement slotHost = flutterViewEmbedder.sceneElement!
-          .querySelector('flt-platform-view-slot')!;
+      final DomElement slotHost =
+          sceneElement.querySelector('flt-platform-view-slot')!;
 
       expect(
         getTransformChain(slotHost),
@@ -736,8 +719,7 @@ void testMain() {
         rasterizer.draw(sb.build().layerTree);
       }
 
-      final DomNode skPathDefs =
-          flutterViewEmbedder.sceneElement!.querySelector('#sk_path_defs')!;
+      final DomNode skPathDefs = sceneElement.querySelector('#sk_path_defs')!;
 
       expect(skPathDefs.childNodes, hasLength(0));
 
@@ -1000,8 +982,7 @@ void _expectSceneMatches(
   String? reason,
 }) {
   // Convert the scene elements to its corresponding array of _EmbeddedViewMarker
-  final List<_EmbeddedViewMarker> sceneElements = flutterViewEmbedder
-      .sceneElement!.children
+  final List<_EmbeddedViewMarker> sceneElements = sceneElement.children
       .where((DomElement element) => element.tagName != 'svg')
       .map((DomElement element) =>
           _tagToViewMarker[element.tagName.toLowerCase()]!)

lib/web_ui/test/engine/view_embedder/dom_manager_test.dart

@@ -117,6 +117,27 @@ void doTests() {
       expect(style!.tagName, equalsIgnoringCase('style'));
       expect(style.parentNode, domManager.renderingHost);
     });
+
+    test('setScene', () {
+      final DomManager domManager = DomManager(devicePixelRatio: 3.0);
+
+      final DomElement sceneHost =
+          domManager.renderingHost.querySelector('flt-scene-host')!;
+
+      final DomElement scene1 = createDomElement('flt-scene');
+      domManager.setScene(scene1);
+      expect(sceneHost.children, <DomElement>[scene1]);
+
+      // Insert the same scene again.
+      domManager.setScene(scene1);
+      expect(sceneHost.children, <DomElement>[scene1]);
+
+      // Insert a different scene.
+      final DomElement scene2 = createDomElement('flt-scene');
+      domManager.setScene(scene2);
+      expect(sceneHost.children, <DomElement>[scene2]);
+      expect(scene1.parent, isNull);
+    });
   });
 }