Inject contents of a platform view in the FlutterView of the current …

…renderer, right before injecting its slot.

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

@@ -143,6 +143,10 @@ class HtmlViewEmbedder {
   }
 
   void _compositeWithParams(int viewId, EmbeddedViewParams params) {
+    // Ensure platform view with `viewId` is injected into the `rasterizer.view`
+    // before rendering its shadow DOM `slot`.
+    rasterizer.view.platformViewMessageHandler.injectPlatformView(viewId);
+
     // If we haven't seen this viewId yet, cache it for clips/transforms.
     final ViewClipChain clipChain = _viewClipChains.putIfAbsent(viewId, () {
       return ViewClipChain(view: createPlatformViewSlot(viewId));

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

@@ -3,7 +3,9 @@
 // found in the LICENSE file.
 
 import '../dom.dart';
+import '../platform_dispatcher.dart';
 import '../platform_views/slots.dart';
+import '../window.dart';
 import 'surface.dart';
 
 /// A surface containing a platform view, which is an HTML element.
@@ -18,6 +20,11 @@ class PersistedPlatformView extends PersistedLeafSurface {
 
   @override
   DomElement createElement() {
+    // Ensure platform view with `viewId` is injected into the `implicitView`
+    // before rendering its shadow DOM `slot`.
+    final EngineFlutterView implicitView = EnginePlatformDispatcher.instance.implicitView!;
+    implicitView.platformViewMessageHandler.injectPlatformView(viewId);
+
     return createPlatformViewSlot(viewId);
   }
 

lib/web_ui/lib/src/engine/platform_views/content_manager.dart

@@ -65,6 +65,11 @@ class PlatformViewManager {
     return _contents.containsKey(viewId);
   }
 
+  /// Returns the pre-rendered contents of [viewId], to inject them into the DOM.
+  DomElement getContents(int viewId) {
+    return _contents[viewId]!;
+  }
+
   /// Returns the HTML element created by a registered factory for [viewId].
   ///
   /// Throws an [AssertionError] if [viewId] hasn't been rendered before.

lib/web_ui/lib/src/engine/platform_views/message_handler.dart

@@ -88,16 +88,27 @@ class PlatformViewMessageHandler {
       return;
     }
 
-    final DomElement content = _contentManager.renderContent(
+    // Ensure the DomElement of the view is *created*, so programmers can
+    // access it through `_contentManager.getViewById` (maybe not the DOM!).
+    _contentManager.renderContent(
       platformViewType,
       platformViewId,
       params,
     );
 
+    callback(_codec.encodeSuccessEnvelope(null));
+  }
+
+  /// Injects a platform view with [viewId] into this handler's `platformViewsContainer`.
+  void injectPlatformView(int viewId) {
     // For now, we don't need anything fancier. If needed, this can be converted
     // to a PlatformViewStrategy class for each web-renderer backend?
-    _platformViewsContainer.append(content);
-    callback(_codec.encodeSuccessEnvelope(null));
+    final DomElement pv = _contentManager.getContents(viewId);
+    // If pv is a descendant of _platformViewsContainer -> noop
+    if (_platformViewsContainer.contains(pv)) {
+      return;
+    }
+    _platformViewsContainer.append(pv);
   }
 
   /// Handle a `dispose` Platform View message.