[web] clear surfaces on hot restart (flutter/engine#28675)

* [web] clear surfaces on hot restart

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

@@ -361,6 +361,17 @@ void registerHotRestartListener(ui.VoidCallback listener) {
   _hotRestartListeners.add(listener);
 }
 
+/// Pretends that hot restart is about to happen.
+///
+/// Useful in tests to check that the engine performs appropriate clean-ups,
+/// such as removing static DOM listeners, prior to allowing the Dart runtime
+/// to re-initialize the program.
+void debugEmulateHotRestart() {
+  for (final ui.VoidCallback listener in _hotRestartListeners) {
+    listener();
+  }
+}
+
 /// This method performs one-time initialization of the Web environment that
 /// supports the Flutter framework.
 ///

engine/src/flutter/lib/web_ui/lib/src/engine/canvaskit/surface_factory.dart

@@ -4,19 +4,33 @@
 
 import 'package:meta/meta.dart';
 
+import '../../engine.dart';
 import '../util.dart';
 import 'embedded_views.dart';
 import 'surface.dart';
 
 /// Caches surfaces used to overlay platform views.
 class SurfaceFactory {
-  /// The cache singleton.
-  static final SurfaceFactory instance =
-      SurfaceFactory(HtmlViewEmbedder.maximumSurfaces);
+  /// The lazy-initialized singleton surface factory.
+  ///
+  /// [debugClear] causes this singleton to be reinitialized.
+  static SurfaceFactory get instance =>
+      _instance ??= SurfaceFactory(HtmlViewEmbedder.maximumSurfaces);
+
+  /// Returns the raw (potentially uninitialized) value of the singleton.
+  ///
+  /// Useful in tests for checking the lifecycle of this class.
+  static SurfaceFactory? get debugUninitializedInstance => _instance;
+
+  static SurfaceFactory? _instance;
 
   SurfaceFactory(this.maximumSurfaces)
       : assert(maximumSurfaces >= 1,
-            'The maximum number of surfaces must be at least 1');
+            'The maximum number of surfaces must be at least 1') {
+    if (assertionsEnabled) {
+      registerHotRestartListener(debugClear);
+    }
+  }
 
   /// The base surface to paint on. This is the default surface which will be
   /// painted to. If there are no platform views, then this surface will receive
@@ -168,7 +182,10 @@ class SurfaceFactory {
     for (final Surface surface in _liveSurfaces) {
       surface.dispose();
     }
+    baseSurface.dispose();
+    backupSurface.dispose();
     _liveSurfaces.clear();
     _cache.clear();
+    _instance = null;
   }
 }

engine/src/flutter/lib/web_ui/test/canvaskit/surface_factory_test.dart

@@ -5,6 +5,7 @@
 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 '../matchers.dart';
 import 'common.dart';
@@ -76,6 +77,40 @@ void testMain() {
       expect(factory.isLive(surface), isFalse);
     });
 
+    test('hot restart', () {
+      void expectDisposed(Surface surface) {
+        expect(surface.htmlCanvas!.isConnected, isFalse);
+      }
+
+      final SurfaceFactory originalFactory = SurfaceFactory.instance;
+      expect(SurfaceFactory.debugUninitializedInstance, isNotNull);
+
+      // Cause the surface and its canvas to be attached to the page
+      originalFactory.baseSurface.acquireFrame(const ui.Size(10, 10));
+      originalFactory.baseSurface.addToScene();
+      expect(originalFactory.baseSurface.htmlCanvas!.isConnected, isTrue);
+
+      // Create a few overlay surfaces
+      final List<Surface> overlays = <Surface>[];
+      for (int i = 0; i < 3; i++) {
+        overlays.add(originalFactory.getSurface()
+          ..acquireFrame(const ui.Size(10, 10))
+          ..addToScene());
+      }
+      expect(originalFactory.debugSurfaceCount, 5);
+
+      originalFactory.backupSurface.acquireFrame(const ui.Size(10, 10));
+      originalFactory.backupSurface.addToScene();
+      expect(originalFactory.backupSurface.htmlCanvas!.isConnected, isTrue);
+
+      // Trigger hot restart clean-up logic and check that we indeed clean up.
+      debugEmulateHotRestart();
+      expect(SurfaceFactory.debugUninitializedInstance, isNull);
+      expectDisposed(originalFactory.baseSurface);
+      expectDisposed(originalFactory.backupSurface);
+      overlays.forEach(expectDisposed);
+      expect(originalFactory.debugSurfaceCount, 2);
+    });
     // TODO(hterkelsen): https://github.com/flutter/flutter/issues/60040
   }, skip: isIosSafari);
 }