[canvaskit] cache and reuse platform view overlays

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

@@ -30,14 +30,16 @@ class HtmlViewEmbedder {
   /// The root view in the stack of mutator elements for the view id.
   final Map<int?, html.Element?> _rootViews = <int?, html.Element?>{};
 
-  /// The overlay for the view id.
-  final Map<int, Overlay> _overlays = <int, Overlay>{};
+  /// Surfaces used to draw on top of platform views, keyed by platform view ID.
+  ///
+  /// These surfaces are cached in the [OverlayCache] and reused.
+  final Map<int, Surface> _overlays = <int, Surface>{};
 
   /// The views that need to be recomposited into the scene on the next frame.
   final Set<int> _viewsToRecomposite = <int>{};
 
   /// The views that need to be disposed of on the next frame.
-  final Set<int?> _viewsToDispose = <int?>{};
+  final Set<int> _viewsToDispose = <int>{};
 
   /// The list of view ids that should be composited, in order.
   List<int> _compositionOrder = <int>[];
@@ -115,14 +117,15 @@ class HtmlViewEmbedder {
 
   void _dispose(
       MethodCall methodCall, ui.PlatformMessageResponseCallback callback) {
-    int? viewId = methodCall.arguments;
+    final int? viewId = methodCall.arguments;
     const MethodCodec codec = StandardMethodCodec();
-    if (!_views.containsKey(viewId)) {
+    if (viewId == null || !_views.containsKey(viewId)) {
       callback(codec.encodeErrorEnvelope(
         code: 'unknown_view',
         message: 'trying to dispose an unknown view',
         details: 'view id: $viewId',
       ));
+      return;
     }
     _viewsToDispose.add(viewId);
     callback(codec.encodeSuccessEnvelope(null));
@@ -339,9 +342,9 @@ class HtmlViewEmbedder {
 
     for (int i = 0; i < _compositionOrder.length; i++) {
       int viewId = _compositionOrder[i];
-      ensureOverlayInitialized(viewId);
+      _ensureOverlayInitialized(viewId);
       final SurfaceFrame frame =
-          _overlays[viewId]!.surface.acquireFrame(_frameSize);
+          _overlays[viewId]!.acquireFrame(_frameSize);
       final CkCanvas canvas = frame.skiaCanvas;
       canvas.drawPicture(
         _pictureRecorders[viewId]!.endRecording(),
@@ -353,53 +356,127 @@ class HtmlViewEmbedder {
       _compositionOrder.clear();
       return;
     }
+
+    final Set<int> unusedViews = Set<int>.from(_activeCompositionOrder);
     _activeCompositionOrder.clear();
 
     for (int i = 0; i < _compositionOrder.length; i++) {
       int viewId = _compositionOrder[i];
+
+      assert(
+        _views.containsKey(viewId),
+        'Cannot render platform view $viewId. '
+        'It has not been created, or it has been deleted.',
+      );
+
+      unusedViews.remove(viewId);
       html.Element platformViewRoot = _rootViews[viewId]!;
-      html.Element overlay = _overlays[viewId]!.surface.htmlElement!;
+      html.Element overlay = _overlays[viewId]!.htmlElement;
       platformViewRoot.remove();
       skiaSceneHost!.append(platformViewRoot);
       overlay.remove();
       skiaSceneHost!.append(overlay);
       _activeCompositionOrder.add(viewId);
     }
     _compositionOrder.clear();
+
+    for (final int unusedViewId in unusedViews) {
+      _releaseOverlay(unusedViewId);
+    }
   }
 
   void disposeViews() {
     if (_viewsToDispose.isEmpty) {
       return;
     }
 
-    for (int? viewId in _viewsToDispose) {
+    for (final int viewId in _viewsToDispose) {
       final html.Element rootView = _rootViews[viewId]!;
       rootView.remove();
       _views.remove(viewId);
       _rootViews.remove(viewId);
-      if (_overlays[viewId] != null) {
-        final Overlay overlay = _overlays[viewId]!;
-        overlay.surface.htmlElement?.remove();
-        overlay.surface.htmlElement = null;
-        overlay.skSurface?.dispose();
-      }
-      _overlays.remove(viewId);
+      _releaseOverlay(viewId);
       _currentCompositionParams.remove(viewId);
       _clipCount.remove(viewId);
       _viewsToRecomposite.remove(viewId);
     }
     _viewsToDispose.clear();
   }
 
-  void ensureOverlayInitialized(int viewId) {
-    Overlay? overlay = _overlays[viewId];
+  void _releaseOverlay(int viewId) {
+    if (_overlays[viewId] != null) {
+      OverlayCache.instance.releaseOverlay(_overlays[viewId]!);
+      _overlays.remove(viewId);
+    }
+  }
+
+  void _ensureOverlayInitialized(int viewId) {
+    // If there's an active overlay for the view ID, continue using it.
+    Surface? overlay = _overlays[viewId];
     if (overlay != null) {
       return;
     }
-    Surface surface = Surface(this);
-    CkSurface? skSurface = surface.acquireRenderSurface(_frameSize);
-    _overlays[viewId] = Overlay(surface, skSurface);
+
+    // Try reusing a cached overlay created for another platform view.
+    overlay = OverlayCache.instance.reserveOverlay();
+
+    // If nothing to reuse, create a new overlay.
+    if (overlay == null) {
+      overlay = Surface(this);
+    }
+
+    _overlays[viewId] = overlay;
+  }
+}
+
+/// Caches surfaces used to overlay platform views.
+class OverlayCache {
+  static const int kDefaultCacheSize = 5;
+
+  /// The cache singleton.
+  static final OverlayCache instance = OverlayCache(kDefaultCacheSize);
+
+  OverlayCache(this.maximumSize);
+
+  /// The cache will not grow beyond this size.
+  final int maximumSize;
+
+  /// Cached surfaces, available for reuse.
+  final List<Surface> _cache = <Surface>[];
+
+  /// Returns the list of cached surfaces.
+  ///
+  /// Useful in tests.
+  List<Surface> get debugCachedSurfaces => _cache;
+
+  /// Reserves an overlay from the cache, if available.
+  ///
+  /// Returns null if the cache is empty.
+  Surface? reserveOverlay() {
+    if (_cache.isEmpty) {
+      return null;
+    }
+    return _cache.removeLast();
+  }
+
+  /// Returns an overlay back to the cache.
+  ///
+  /// If the cache is full, the overlay is deleted.
+  void releaseOverlay(Surface overlay) {
+    overlay.htmlElement.remove();
+    if (_cache.length < maximumSize) {
+      _cache.add(overlay);
+    } else {
+      overlay.dispose();
+    }
+  }
+
+  int get debugLength => _cache.length;
+
+  void debugClear() {
+    for (final Surface overlay in _cache) {
+      overlay.dispose();
+    }
   }
 }
 
@@ -547,11 +624,3 @@ class MutatorsStack extends Iterable<Mutator> {
   @override
   Iterator<Mutator> get iterator => _mutators.reversed.iterator;
 }
-
-/// Represents a surface overlaying a platform view.
-class Overlay {
-  final Surface surface;
-  final CkSurface? skSurface;
-
-  Overlay(this.surface, this.skSurface);
-}

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

@@ -38,10 +38,27 @@ class Surface {
   Surface(this.viewEmbedder);
 
   CkSurface? _surface;
-  html.Element? htmlElement;
+
+  /// If true, forces a new WebGL context to be created, even if the window
+  /// size is the same. This is used to restore the UI after the browser tab
+  /// goes dormant and loses the GL context.
+  bool _forceNewContext = true;
+  bool get debugForceNewContext => _forceNewContext;
+
   SkGrContext? _grContext;
   int? _skiaCacheBytes;
 
+  /// The root HTML element for this surface.
+  ///
+  /// This element contains the canvas used to draw the UI. Unlike the canvas,
+  /// this element is permanent. It is never replaced or deleted, until this
+  /// surface is disposed of via [dispose].
+  ///
+  /// Conversely, the canvas that lives inside this element can be swapped, for
+  /// example, when the screen size changes, or when the WebGL context is lost
+  /// due to the browser tab becoming dormant.
+  final html.Element htmlElement = html.Element.tag('flt-canvas-container');
+
   /// Specify the GPU resource cache limits.
   void setSkiaResourceCacheMaxBytes(int bytes) {
     _skiaCacheBytes = bytes;
@@ -64,7 +81,7 @@ class Surface {
   ///
   /// The given [size] is in physical pixels.
   SurfaceFrame acquireFrame(ui.Size size) {
-    final CkSurface surface = acquireRenderSurface(size);
+    final CkSurface surface = _createOrUpdateSurfaces(size);
 
     if (surface.context != null) {
       canvasKit.setCurrentContext(surface.context!);
@@ -77,33 +94,29 @@ class Surface {
     return SurfaceFrame(surface, submitCallback);
   }
 
-  CkSurface acquireRenderSurface(ui.Size size) {
-    _createOrUpdateSurfaces(size);
-    return _surface!;
-  }
-
   void addToScene() {
     if (!_addedToScene) {
-      skiaSceneHost!.children.insert(0, htmlElement!);
+      skiaSceneHost!.children.insert(0, htmlElement);
     }
     _addedToScene = true;
   }
 
   ui.Size? _currentSize;
 
-  void _createOrUpdateSurfaces(ui.Size size) {
+  CkSurface _createOrUpdateSurfaces(ui.Size size) {
     if (size.isEmpty) {
       throw CanvasKitError('Cannot create surfaces of empty size.');
     }
 
     // Check if the window is shrinking in size, and if so, don't allocate a
     // new canvas as the previous canvas is big enough to fit everything.
     final ui.Size? previousSize = _currentSize;
-    if (previousSize != null &&
+    if (!_forceNewContext &&
+        previousSize != null &&
         size.width <= previousSize.width &&
         size.height <= previousSize.height) {
       // The existing surface is still reusable.
-      return;
+      return _surface!;
     }
 
     _currentSize = _currentSize == null
@@ -116,14 +129,17 @@ class Surface {
 
     _surface?.dispose();
     _surface = null;
-    htmlElement?.remove();
-    htmlElement = null;
     _addedToScene = false;
 
-    _surface = _wrapHtmlCanvas(_currentSize!);
+    return _surface = _wrapHtmlCanvas(_currentSize!);
   }
 
   CkSurface _wrapHtmlCanvas(ui.Size physicalSize) {
+    // Clear the container, if it's not empty.
+    while (htmlElement.firstChild != null) {
+      htmlElement.firstChild!.remove();
+    }
+
     // If `physicalSize` is not precise, use a slightly bigger canvas. This way
     // we ensure that the rendred picture covers the entire browser window.
     final int pixelWidth = physicalSize.width.ceil();
@@ -146,9 +162,28 @@ class Surface {
       ..width = '${logicalWidth}px'
       ..height = '${logicalHeight}px';
 
-    htmlElement = htmlCanvas;
-    if (webGLVersion == -1 || canvasKitForceCpuOnly) {
-      return _makeSoftwareCanvasSurface(htmlCanvas);
+    // When the browser tab using WebGL goes dormant the browser and/or OS may
+    // decide to clear GPU resources to let other tabs/programs use the GPU.
+    // When this happens, the browser sends the "webglcontextlost" event as a
+    // notification. When we receive this notification we force a new context.
+    //
+    // See also: https://www.khronos.org/webgl/wiki/HandlingContextLost
+    htmlCanvas.addEventListener('webglcontextlost', (event) {
+      print('Flutter: restoring WebGL context.');
+      _forceNewContext = true;
+      // Force the framework to rerender the frame.
+      EnginePlatformDispatcher.instance.invokeOnMetricsChanged();
+      event.stopPropagation();
+      event.preventDefault();
+    }, false);
+    _forceNewContext = false;
+
+    htmlElement.append(htmlCanvas);
+
+    if (webGLVersion == -1) {
+      return _makeSoftwareCanvasSurface(htmlCanvas, 'WebGL support not detected');
+    } else if (canvasKitForceCpuOnly) {
+      return _makeSoftwareCanvasSurface(htmlCanvas, 'CPU rendering forced by application');
     } else {
       // Try WebGL first.
       final int glContext = canvasKit.GetWebGLContext(
@@ -162,7 +197,7 @@ class Surface {
       );
 
       if (glContext == 0) {
-        return _makeSoftwareCanvasSurface(htmlCanvas);
+        return _makeSoftwareCanvasSurface(htmlCanvas, 'Failed to initialize WebGL context');
       }
 
       _grContext = canvasKit.MakeGrContext(glContext);
@@ -183,7 +218,7 @@ class Surface {
       );
 
       if (skSurface == null) {
-        return _makeSoftwareCanvasSurface(htmlCanvas);
+        return _makeSoftwareCanvasSurface(htmlCanvas, 'Failed to initialize WebGL surface');
       }
 
       return CkSurface(skSurface, _grContext, glContext);
@@ -192,9 +227,11 @@ class Surface {
 
   static bool _didWarnAboutWebGlInitializationFailure = false;
 
-  CkSurface _makeSoftwareCanvasSurface(html.CanvasElement htmlCanvas) {
+  CkSurface _makeSoftwareCanvasSurface(html.CanvasElement htmlCanvas, String reason) {
     if (!_didWarnAboutWebGlInitializationFailure) {
-      html.window.console.warn('WARNING: failed to initialize WebGL. Falling back to CPU-only rendering.');
+      html.window.console.warn(
+        'WARNING: Falling back to CPU-only rendering. $reason.'
+      );
       _didWarnAboutWebGlInitializationFailure = true;
     }
     return CkSurface(
@@ -211,6 +248,11 @@ class Surface {
     _surface!.flush();
     return true;
   }
+
+  void dispose() {
+    htmlElement.remove();
+    _surface?.dispose();
+  }
 }
 
 /// A Dart wrapper around Skia's CkSurface.

lib/web_ui/test/canvaskit/common.dart

@@ -38,6 +38,7 @@ void setUpCanvasKitTest() {
   tearDown(() {
     testCollector.cleanUpAfterTest();
     debugResetBrowserSupportsFinalizationRegistry();
+    OverlayCache.instance.debugClear();
   });
 
   tearDownAll(() {