Don't push offset to leaf layers (#21619)

For retained rendering, we don't want to push the offset down to each leaf layer. Otherwise, changing an offset layer on the very high level could cascade the change to too many leaves, which means that we can't retain them.

To not push the offset downwards, we simply push a TransformLayer when there's an offset. Skia has a fast path for concatenating scale/translation-only matrix so this operation should be fast (no performance regression is measured on Moto G4).

This is our first step towards https://github.com/flutter/flutter/issues/21756

packages/flutter/lib/src/rendering/layer.dart

@@ -104,10 +104,7 @@ abstract class Layer extends AbstractNode with DiagnosticableTreeMixin {
   S find<S>(Offset regionOffset);
 
   /// Override this method to upload this layer to the engine.
-  ///
-  /// The `layerOffset` is the accumulated offset of this layer's parent from the
-  /// origin of the builder's coordinate system.
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset);
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]);
 
   /// The object responsible for creating this layer.
   ///
@@ -171,7 +168,7 @@ class PictureLayer extends Layer {
   bool willChangeHint = false;
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     builder.addPicture(layerOffset, picture, isComplexHint: isComplexHint, willChangeHint: willChangeHint);
   }
 
@@ -237,7 +234,7 @@ class TextureLayer extends Layer {
   final bool freeze;
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     final Rect shiftedRect = rect.shift(layerOffset);
     builder.addTexture(
       textureId,
@@ -305,7 +302,7 @@ class PerformanceOverlayLayer extends Layer {
   final bool checkerboardOffscreenLayers;
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     assert(optionsMask != null);
     builder.addPerformanceOverlay(optionsMask, overlayRect.shift(layerOffset));
     builder.setRasterizerTracingThreshold(rasterizerThreshold);
@@ -454,7 +451,7 @@ class ContainerLayer extends Layer {
   }
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     addChildrenToScene(builder, layerOffset);
   }
 
@@ -465,7 +462,7 @@ class ContainerLayer extends Layer {
   /// to apply effects to the scene using the [SceneBuilder] API, then insert
   /// their children using [addChildrenToScene], then reverse the aforementioned
   /// effects before returning from [addToScene].
-  void addChildrenToScene(ui.SceneBuilder builder, Offset childOffset) {
+  void addChildrenToScene(ui.SceneBuilder builder, [Offset childOffset = Offset.zero]) {
     Layer child = firstChild;
     while (child != null) {
       child.addToScene(builder, childOffset);
@@ -560,8 +557,22 @@ class OffsetLayer extends ContainerLayer {
   }
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
-    addChildrenToScene(builder, offset + layerOffset);
+  void applyTransform(Layer child, Matrix4 transform) {
+    assert(child != null);
+    assert(transform != null);
+    transform.multiply(Matrix4.translationValues(offset.dx, offset.dy, 0.0));
+  }
+
+  @override
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
+    // Skia has a fast path for concatenating scale/translation only matrices.
+    // Hence pushing a translation-only transform layer should be fast. For
+    // retained rendering, we don't want to push the offset down to each leaf
+    // node. Otherwise, changing an offset layer on the very high level could
+    // cascade the change to too many leaves.
+    builder.pushOffset(layerOffset.dx + offset.dx, layerOffset.dy + offset.dy);
+    addChildrenToScene(builder);
+    builder.pop();
   }
 
   @override
@@ -597,7 +608,7 @@ class OffsetLayer extends ContainerLayer {
     );
     transform.scale(pixelRatio, pixelRatio);
     builder.pushTransform(transform.storage);
-    addToScene(builder, Offset.zero);
+    addToScene(builder);
     final ui.Scene scene = builder.build();
     try {
       // Size is rounded up to the next pixel to make sure we don't clip off
@@ -652,7 +663,7 @@ class ClipRectLayer extends ContainerLayer {
   }
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     bool enabled = true;
     assert(() {
       enabled = !debugDisableClipLayers;
@@ -708,7 +719,7 @@ class ClipRRectLayer extends ContainerLayer {
   }
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     bool enabled = true;
     assert(() {
       enabled = !debugDisableClipLayers;
@@ -764,7 +775,7 @@ class ClipPathLayer extends ContainerLayer {
   }
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     bool enabled = true;
     assert(() {
       enabled = !debugDisableClipLayers;
@@ -815,7 +826,7 @@ class TransformLayer extends OffsetLayer {
   bool _inverseDirty = true;
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     _lastEffectiveTransform = transform;
     final Offset totalOffset = offset + layerOffset;
     if (totalOffset != Offset.zero) {
@@ -823,7 +834,7 @@ class TransformLayer extends OffsetLayer {
         ..multiply(_lastEffectiveTransform);
     }
     builder.pushTransform(_lastEffectiveTransform.storage);
-    addChildrenToScene(builder, Offset.zero);
+    addChildrenToScene(builder);
     builder.pop();
   }
 
@@ -876,7 +887,7 @@ class OpacityLayer extends ContainerLayer {
   int alpha;
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     bool enabled = true;
     assert(() {
       enabled = !debugDisableOpacityLayers;
@@ -923,7 +934,7 @@ class ShaderMaskLayer extends ContainerLayer {
   BlendMode blendMode;
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     builder.pushShaderMask(shader, maskRect.shift(layerOffset), blendMode);
     addChildrenToScene(builder, layerOffset);
     builder.pop();
@@ -953,7 +964,7 @@ class BackdropFilterLayer extends ContainerLayer {
   ui.ImageFilter filter;
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     builder.pushBackdropFilter(filter);
     addChildrenToScene(builder, layerOffset);
     builder.pop();
@@ -1024,7 +1035,7 @@ class PhysicalModelLayer extends ContainerLayer {
   }
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     bool enabled = true;
     assert(() {
       enabled = !debugDisablePhysicalShapeLayers;
@@ -1133,12 +1144,12 @@ class LeaderLayer extends ContainerLayer {
   }
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     assert(offset != null);
     _lastOffset = offset + layerOffset;
     if (_lastOffset != Offset.zero)
       builder.pushTransform(Matrix4.translationValues(_lastOffset.dx, _lastOffset.dy, 0.0).storage);
-    addChildrenToScene(builder, Offset.zero);
+    addChildrenToScene(builder);
     if (_lastOffset != Offset.zero)
       builder.pop();
   }
@@ -1337,7 +1348,7 @@ class FollowerLayer extends ContainerLayer {
   }
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     assert(link != null);
     assert(showWhenUnlinked != null);
     if (link.leader == null && !showWhenUnlinked) {
@@ -1349,12 +1360,15 @@ class FollowerLayer extends ContainerLayer {
     _establishTransform();
     if (_lastTransform != null) {
       builder.pushTransform(_lastTransform.storage);
-      addChildrenToScene(builder, Offset.zero);
+      addChildrenToScene(builder);
       builder.pop();
       _lastOffset = unlinkedOffset + layerOffset;
     } else {
       _lastOffset = null;
-      addChildrenToScene(builder, unlinkedOffset + layerOffset);
+      final Matrix4 matrix = Matrix4.translationValues(unlinkedOffset.dx, unlinkedOffset.dy, .0);
+      builder.pushTransform(matrix.storage);
+      addChildrenToScene(builder);
+      builder.pop();
     }
     _inverseDirty = true;
   }
@@ -1363,8 +1377,11 @@ class FollowerLayer extends ContainerLayer {
   void applyTransform(Layer child, Matrix4 transform) {
     assert(child != null);
     assert(transform != null);
-    if (_lastTransform != null)
+    if (_lastTransform != null) {
       transform.multiply(_lastTransform);
+    } else {
+      transform.multiply(Matrix4.translationValues(unlinkedOffset.dx, unlinkedOffset.dy, .0));
+    }
   }
 
   @override

packages/flutter/lib/src/rendering/view.dart

@@ -192,7 +192,7 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
     Timeline.startSync('Compositing', arguments: timelineWhitelistArguments);
     try {
       final ui.SceneBuilder builder = ui.SceneBuilder();
-      layer.addToScene(builder, Offset.zero);
+      layer.addToScene(builder);
       final ui.Scene scene = builder.build();
       if (automaticSystemUiAdjustment)
         _updateSystemChrome();

packages/flutter/lib/src/widgets/widget_inspector.dart

@@ -54,7 +54,7 @@ class _ProxyLayer extends Layer {
   _ProxyLayer(this._layer);
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     _layer.addToScene(builder, layerOffset);
   }
 
@@ -312,7 +312,7 @@ Rect _calculateSubtreeBounds(RenderObject object) {
 /// screenshots render to the scene in the local coordinate system of the layer.
 class _ScreenshotContainerLayer extends OffsetLayer {
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     addChildrenToScene(builder, layerOffset);
   }
 }
@@ -588,7 +588,7 @@ class _ScreenshotPaintingContext extends PaintingContext {
     // We must build the regular scene before we can build the screenshot
     // scene as building the screenshot scene assumes addToScene has already
     // been called successfully for all layers in the regular scene.
-    repaintBoundary.layer.addToScene(ui.SceneBuilder(), Offset.zero);
+    repaintBoundary.layer.addToScene(ui.SceneBuilder());
 
     return data.containerLayer.toImage(renderBounds, pixelRatio: pixelRatio);
   }
@@ -2235,7 +2235,7 @@ class _InspectorOverlayLayer extends Layer {
   double _textPainterMaxWidth;
 
   @override
-  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     if (!selection.active)
       return;