[canvaskit] support adding leaf layers w/o container layers (flutter/engine#24357)

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

@@ -117,6 +117,17 @@ abstract class ContainerLayer extends Layer {
   }
 }
 
+/// The top-most layer in the layer tree.
+///
+/// This layer does not draw anything. It's only used so we can add leaf layers
+/// to [LayerSceneBuilder] without requiring a [ContainerLayer].
+class RootLayer extends ContainerLayer {
+  @override
+  void paint(PaintContext context) {
+    paintChildren(context);
+  }
+}
+
 class BackdropFilterEngineLayer extends ContainerLayer implements ui.BackdropFilterEngineLayer {
   final ui.ImageFilter _filter;
 

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

@@ -8,9 +8,7 @@ part of engine;
 class LayerScene implements ui.Scene {
   final LayerTree layerTree;
 
-  LayerScene(Layer? rootLayer) : layerTree = LayerTree() {
-    layerTree.rootLayer = rootLayer;
-  }
+  LayerScene(RootLayer rootLayer) : layerTree = LayerTree(rootLayer);
 
   @override
   void dispose() {}
@@ -23,8 +21,12 @@ class LayerScene implements ui.Scene {
 }
 
 class LayerSceneBuilder implements ui.SceneBuilder {
-  Layer? rootLayer;
-  ContainerLayer? currentLayer;
+  LayerSceneBuilder() : rootLayer = RootLayer() {
+    currentLayer = rootLayer;
+  }
+
+  final RootLayer rootLayer;
+  late ContainerLayer currentLayer;
 
   @override
   void addChildScene({
@@ -50,16 +52,13 @@ class LayerSceneBuilder implements ui.SceneBuilder {
     bool isComplexHint = false,
     bool willChangeHint = false,
   }) {
-    currentLayer!.add(PictureLayer(
+    currentLayer.add(PictureLayer(
         picture as CkPicture, offset, isComplexHint, willChangeHint));
   }
 
   @override
   void addRetained(ui.EngineLayer retainedLayer) {
-    if (currentLayer == null) {
-      return;
-    }
-    currentLayer!.add(retainedLayer as Layer);
+    currentLayer.add(retainedLayer as Layer);
   }
 
   @override
@@ -82,7 +81,7 @@ class LayerSceneBuilder implements ui.SceneBuilder {
     double height = 0.0,
     Object? webOnlyPaintedBy,
   }) {
-    currentLayer!.add(PlatformViewLayer(viewId, offset, width, height));
+    currentLayer.add(PlatformViewLayer(viewId, offset, width, height));
   }
 
   @override
@@ -92,10 +91,11 @@ class LayerSceneBuilder implements ui.SceneBuilder {
 
   @override
   void pop() {
-    if (currentLayer == null) {
+    if (currentLayer == rootLayer) {
+      // Don't pop the root layer. It must always be there.
       return;
     }
-    currentLayer = currentLayer!.parent;
+    currentLayer = currentLayer.parent!;
   }
 
   @override
@@ -221,16 +221,7 @@ class LayerSceneBuilder implements ui.SceneBuilder {
   }
 
   T pushLayer<T extends ContainerLayer>(T layer) {
-    if (rootLayer == null) {
-      rootLayer = currentLayer = layer;
-      return layer;
-    }
-
-    if (currentLayer == null) {
-      return layer;
-    }
-
-    currentLayer!.add(layer);
+    currentLayer.add(layer);
     currentLayer = layer;
     return layer;
   }

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

@@ -7,8 +7,10 @@ part of engine;
 
 /// A tree of [Layer]s that, together with a [Size] compose a frame.
 class LayerTree {
+  LayerTree(this.rootLayer);
+
   /// The root of the layer tree.
-  Layer? rootLayer;
+  final RootLayer rootLayer;
 
   /// The size (in physical pixels) of the frame to paint this layer tree into.
   final ui.Size frameSize = ui.window.physicalSize;
@@ -27,7 +29,7 @@ class LayerTree {
       ignoreRasterCache ? null : frame.rasterCache,
       frame.viewEmbedder,
     );
-    rootLayer!.preroll(context, Matrix4.identity());
+    rootLayer.preroll(context, Matrix4.identity());
   }
 
   /// Paints the layer tree into the given [frame].
@@ -48,8 +50,8 @@ class LayerTree {
       ignoreRasterCache ? null : frame.rasterCache,
       frame.viewEmbedder,
     );
-    if (rootLayer!.needsPainting) {
-      rootLayer!.paint(context);
+    if (rootLayer.needsPainting) {
+      rootLayer.paint(context);
     }
   }
 
@@ -59,17 +61,15 @@ class LayerTree {
   ui.Picture flatten() {
     CkPictureRecorder recorder = CkPictureRecorder();
     CkCanvas canvas = recorder.beginRecording(ui.Rect.largest);
-    if (rootLayer != null) {
-      final PrerollContext prerollContext = PrerollContext(null, null);
-      rootLayer!.preroll(prerollContext, Matrix4.identity());
+    final PrerollContext prerollContext = PrerollContext(null, null);
+    rootLayer.preroll(prerollContext, Matrix4.identity());
 
-      CkNWayCanvas internalNodesCanvas = CkNWayCanvas();
-      internalNodesCanvas.addCanvas(canvas);
-      final PaintContext paintContext =
-          PaintContext(internalNodesCanvas, canvas, null, null);
-      if (rootLayer!.needsPainting) {
-        rootLayer!.paint(paintContext);
-      }
+    CkNWayCanvas internalNodesCanvas = CkNWayCanvas();
+    internalNodesCanvas.addCanvas(canvas);
+    final PaintContext paintContext =
+        PaintContext(internalNodesCanvas, canvas, null, null);
+    if (rootLayer.needsPainting) {
+      rootLayer.paint(paintContext);
     }
     return recorder.endRecording();
   }

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

@@ -197,7 +197,7 @@ void testMain() {
 
       // Render the scene once without painting the shadow bounds just to
       // preroll the scene to compute the shadow bounds.
-      buildTestScene(paintShadowBounds: false).rootLayer!.preroll(
+      buildTestScene(paintShadowBounds: false).rootLayer.preroll(
         PrerollContext(
           RasterCache(),
           HtmlViewEmbedder(),

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

@@ -44,12 +44,18 @@ void testMain() {
       sb.addPicture(ui.Offset.zero, picture);
       final LayerTree layerTree = sb.build().layerTree;
       dispatcher.rasterizer!.draw(layerTree);
-      final ClipRectEngineLayer clipRect = layerTree.rootLayer as ClipRectEngineLayer;
+      final ClipRectEngineLayer clipRect = layerTree.rootLayer.debugLayers.single as ClipRectEngineLayer;
       expect(clipRect.paintBounds, ui.Rect.fromLTRB(15, 15, 30, 30));
 
       final TransformEngineLayer transform = clipRect.debugLayers.single as TransformEngineLayer;
       expect(transform.paintBounds, ui.Rect.fromLTRB(0, 0, 30, 30));
     });
+
+    test('can push a leaf layer without a container layer', () async {
+      final CkPictureRecorder recorder = CkPictureRecorder();
+      recorder.beginRecording(ui.Rect.zero);
+      LayerSceneBuilder().addPicture(ui.Offset.zero, recorder.endRecording());
+    });
     // TODO: https://github.com/flutter/flutter/issues/60040
   }, skip: isIosSafari);
 }