Use cached CanvasKit instance if available (#26059)

* If CanvasKit has already been initialized, don't load it again.

This could be useful for hot restart or for DartPad.

* Don't redownload script if it is already loaded.

* trailing whitespace

* Touch up comments

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

@@ -16,8 +16,15 @@ late CanvasKit canvasKit;
 /// static APIs.
 ///
 /// See, e.g. [SkPaint].
+///
+/// This also acts as a cache of an initialized CanvasKit instance. We can use
+/// this, for example, to perform a hot restart without needing to redownload
+/// and reinitialize CanvasKit.
+@JS('window.flutterCanvasKit')
+external set windowFlutterCanvasKit(CanvasKit? value);
+
 @JS('window.flutterCanvasKit')
-external set windowFlutterCanvasKit(CanvasKit value);
+external CanvasKit? get windowFlutterCanvasKit;
 
 @JS()
 @anonymous
@@ -141,7 +148,7 @@ class ColorSpace {}
 @anonymous
 class SkWebGLContextOptions {
   external factory SkWebGLContextOptions({
-    required int anitalias,
+    required int antialias,
     // WebGL version: 1 or 2.
     required int majorVersion,
   });

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

@@ -87,20 +87,23 @@ String canvasKitWasmModuleUrl(String file) => canvasKitBuildUrl + file;
 /// This calls `CanvasKitInit` and assigns the global [canvasKit] object.
 Future<void> initializeCanvasKit() {
   final Completer<void> canvasKitCompleter = Completer<void>();
-  late StreamSubscription<html.Event> loadSubscription;
-  loadSubscription = domRenderer.canvasKitScript!.onLoad.listen((_) {
-    loadSubscription.cancel();
-    final CanvasKitInitPromise canvasKitInitPromise =
-        CanvasKitInit(CanvasKitInitOptions(
-      locateFile: js.allowInterop(
-          (String file, String unusedBase) => canvasKitWasmModuleUrl(file)),
-    ));
-    canvasKitInitPromise.then(js.allowInterop((CanvasKit ck) {
-      canvasKit = ck;
-      windowFlutterCanvasKit = canvasKit;
-      canvasKitCompleter.complete();
-    }));
-  });
+  if (windowFlutterCanvasKit != null) {
+    canvasKit = windowFlutterCanvasKit!;
+    canvasKitCompleter.complete();
+  } else {
+    domRenderer.canvasKitLoaded!.then((_) {
+      final CanvasKitInitPromise canvasKitInitPromise =
+          CanvasKitInit(CanvasKitInitOptions(
+        locateFile: js.allowInterop(
+            (String file, String unusedBase) => canvasKitWasmModuleUrl(file)),
+      ));
+      canvasKitInitPromise.then(js.allowInterop((CanvasKit ck) {
+        canvasKit = ck;
+        windowFlutterCanvasKit = canvasKit;
+        canvasKitCompleter.complete();
+      }));
+    });
+  }
 
   /// Add a Skia scene host.
   skiaSceneHost = html.Element.tag('flt-scene');

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

@@ -210,7 +210,7 @@ class Surface {
         SkWebGLContextOptions(
           // Default to no anti-aliasing. Paint commands can be explicitly
           // anti-aliased by setting their `Paint` object's `antialias` property.
-          anitalias: 0,
+          antialias: 0,
           majorVersion: webGLVersion,
         ),
       );

lib/web_ui/lib/src/engine/dom_renderer.dart

@@ -48,6 +48,9 @@ class DomRenderer {
   html.ScriptElement? get canvasKitScript => _canvasKitScript;
   html.ScriptElement? _canvasKitScript;
 
+  Future<void>? get canvasKitLoaded => _canvasKitLoaded;
+  Future<void>? _canvasKitLoaded;
+
   /// The element that contains the [sceneElement].
   ///
   /// This element is created and inserted in the HTML DOM once. It is never
@@ -441,7 +444,8 @@ flt-glass-pane * {
 
     _sceneHostElement = createElement('flt-scene-host');
 
-    final html.Element semanticsHostElement = createElement('flt-semantics-host');
+    final html.Element semanticsHostElement =
+        createElement('flt-semantics-host');
     semanticsHostElement.style
       ..position = 'absolute'
       ..transformOrigin = '0 0 0';
@@ -509,11 +513,21 @@ flt-glass-pane * {
       });
     }
 
-    if (useCanvasKit) {
+    // Only reset CanvasKit if it's not already available.
+    if (useCanvasKit && windowFlutterCanvasKit == null) {
       _canvasKitScript?.remove();
       _canvasKitScript = html.ScriptElement();
       _canvasKitScript!.src = canvasKitJavaScriptBindingsUrl;
 
+      Completer<void> canvasKitLoadCompleter = Completer<void>();
+      _canvasKitLoaded = canvasKitLoadCompleter.future;
+
+      late StreamSubscription<html.Event> loadSubscription;
+      loadSubscription = _canvasKitScript!.onLoad.listen((_) {
+        loadSubscription.cancel();
+        canvasKitLoadCompleter.complete(true);
+      });
+
       // TODO(hterkelsen): Rather than this monkey-patch hack, we should
       // build CanvasKit ourselves. See:
       // https://github.com/flutter/flutter/issues/52588
@@ -591,7 +605,8 @@ flt-glass-pane * {
   /// logical pixels. To compensate, we inject an inverse scale at the root
   /// level.
   void updateSemanticsScreenProperties() {
-    _semanticsHostElement!.style.transform = 'scale(${1 / html.window.devicePixelRatio})';
+    _semanticsHostElement!.style.transform =
+        'scale(${1 / html.window.devicePixelRatio})';
   }
 
   /// Called immediately after browser window metrics change.

lib/web_ui/test/canvaskit/hot_restart_test.dart

+// 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 '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('initialize', () {
+    test(
+        're-uses the same initialized instance if it is already set on the window',
+        () async {
+      expect(windowFlutterCanvasKit, isNull);
+
+      DomRenderer();
+      await ui.webOnlyInitializePlatform(
+          assetManager: WebOnlyMockAssetManager());
+      expect(windowFlutterCanvasKit, isNotNull);
+
+      var firstCanvasKitInstance = windowFlutterCanvasKit;
+
+      // Triggers a reset of the CanvasKit script element.
+      DomRenderer();
+      await ui.webOnlyInitializePlatform(
+          assetManager: WebOnlyMockAssetManager());
+      // The instance is the same.
+      expect(firstCanvasKitInstance, windowFlutterCanvasKit);
+    });
+    // TODO: https://github.com/flutter/flutter/issues/60040
+  }, skip: isIosSafari);
+}

lib/web_ui/test/canvaskit/initialization_test.dart

@@ -20,7 +20,8 @@ void testMain() {
 
     test('populates flt-renderer and flt-build-mode', () {
       DomRenderer();
-      expect(html.document.body!.attributes['flt-renderer'], 'canvaskit (requested explicitly)');
+      expect(html.document.body!.attributes['flt-renderer'],
+          'canvaskit (requested explicitly)');
       expect(html.document.body!.attributes['flt-build-mode'], 'debug');
     });
     // TODO: https://github.com/flutter/flutter/issues/60040