Use H5vcc CanvasKit implementation if it is detected.

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

@@ -25,6 +25,14 @@ import '../profiler.dart';
 /// Entrypoint into the CanvasKit API.
 late CanvasKit canvasKit;
 
+/// Whether to use a CanvasKit implementation provided by a JavaScript
+/// `window.h5vcc.canvasKit` object.
+///
+/// Cobalt may use this object to expose a native implementation of the
+/// CanvasKit bindings. If this exists, use it instead of using the normal
+/// downloaded CanvasKit library.
+final bool useH5vccCanvasKit = h5vcc != null;
+
 /// Sets the [CanvasKit] object on `window` so we can use `@JS()` to bind to
 /// static APIs.
 ///
@@ -39,6 +47,18 @@ external set windowFlutterCanvasKit(CanvasKit? value);
 @JS('window.flutterCanvasKit')
 external CanvasKit? get windowFlutterCanvasKit;
 
+@JS('window.h5vcc')
+external H5vcc? get h5vcc;
+
+@JS('window.h5vcc')
+external set debugH5vccSetter(H5vcc? value);
+
+@JS()
+@anonymous
+abstract class H5vcc {
+  external CanvasKit? get canvasKit;
+}
+
 @JS()
 @anonymous
 class CanvasKit {
@@ -132,6 +152,15 @@ class CanvasKit {
     Object src,
     SkPartialImageInfo info,
   );
+
+  /// Gets a Skia surface from Cobalt's h5vcc object.
+  ///
+  /// This is only applicable when running on Cobalt and when using Cobalt's
+  /// h5vcc CanvasKit bindings.
+  ///
+  /// On Cobalt, this is the only way to get a Skia surface. Other CanvasKit
+  /// Make...Surface methods are not supported.
+  external SkSurface getH5vccSkSurface();
 }
 
 @JS('window.CanvasKitInit')

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

@@ -13,6 +13,7 @@ import '../embedder.dart';
 import '../safe_browser_api.dart';
 import 'canvaskit_api.dart';
 import 'fonts.dart';
+import 'util.dart';
 
 /// Whether to use CanvasKit as the rendering backend.
 final bool useCanvasKit = FlutterConfiguration.flutterWebAutoDetect
@@ -47,6 +48,12 @@ String canvasKitWasmModuleUrl(String canvasKitBase, String file) =>
 Future<void> initializeCanvasKit({String? canvasKitBase}) async {
   if (windowFlutterCanvasKit != null) {
     canvasKit = windowFlutterCanvasKit!;
+  } else if (useH5vccCanvasKit) {
+    if (h5vcc?.canvasKit == null) {
+      throw CanvasKitError('H5vcc CanvasKit implementation not found.');
+    }
+    canvasKit = h5vcc!.canvasKit!;
+    windowFlutterCanvasKit = canvasKit;
   } else {
     canvasKit = await downloadCanvasKit(canvasKitBase: canvasKitBase);
     windowFlutterCanvasKit = canvasKit;

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

@@ -137,6 +137,11 @@ class Surface {
 
   /// Creates a <canvas> and SkSurface for the given [size].
   CkSurface createOrUpdateSurface(ui.Size size) {
+    if (useH5vccCanvasKit) {
+      _surface ??= CkSurface(canvasKit.getH5vccSkSurface(), null);
+      return _surface!;
+    }
+
     if (size.isEmpty) {
       throw CanvasKitError('Cannot create surfaces of empty size.');
     }

lib/web_ui/test/canvaskit/h5vcc_test.dart

@@ -0,0 +1,73 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:html' as html;
+import 'dart:js' as js;
+
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui.dart' as ui;
+
+import 'common.dart';
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+void testMain() {
+  group('H5vcc patched CanvasKit', () {
+    int getH5vccSkSurfaceCalledCount = 0;
+
+    setUpAll(() async {
+      // Set `window.h5vcc` to PatchedH5vcc which uses a downloaded CanvasKit.
+      final CanvasKit downloadedCanvasKit = await downloadCanvasKit();
+      debugH5vccSetter = PatchedH5vcc(downloadedCanvasKit);
+
+      // Monkey-patch the getH5vccSkSurface function of
+      // `window.h5vcc.canvasKit`.
+      js.context['h5vcc']['canvasKit']['getH5vccSkSurface'] = () {
+        getH5vccSkSurfaceCalledCount++;
+
+        // Returns a fake [SkSurface] object with a minimal implementation.
+        return js.JsObject.jsify(<String, dynamic>{
+          'dispose': () {}
+        });
+      };
+    });
+
+    setUpCanvasKitTest();
+
+    setUp(() {
+      getH5vccSkSurfaceCalledCount = 0;
+    });
+
+    test('sets useH5vccCanvasKit', () {
+      expect(useH5vccCanvasKit, true);
+    });
+
+    test('API includes patched getH5vccSkSurface', () {
+      expect(canvasKit.getH5vccSkSurface, isNotNull);
+    });
+
+    test('Surface acquireFrame uses getH5vccSkSurface', () {
+      final Surface surface = SurfaceFactory.instance.getSurface();
+      surface.acquireFrame(ui.Size.zero);
+      expect(getH5vccSkSurfaceCalledCount, 1);
+
+      // No <canvas> element should be created.
+      expect(
+        flutterViewEmbedder.glassPaneElement!.querySelectorAll<html.Element>('canvas'),
+        isEmpty,
+      );
+    });
+  }, testOn: 'chrome');
+}
+
+class PatchedH5vcc implements H5vcc {
+  @override
+  final CanvasKit canvasKit;
+
+  PatchedH5vcc(this.canvasKit);
+}