Maintain dirtiness and use retained engine layers (#23434)

For #21756

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

@@ -198,6 +198,7 @@ class PaintingContext extends ClipContext {
         return true;
       }());
     }
+    assert(child._layer != null);
     child._layer.offset = offset;
     appendLayer(child._layer);
   }
@@ -488,7 +489,7 @@ class PaintingContext extends ClipContext {
   /// ancestor render objects that this render object will include a composited
   /// layer, which, for example, causes them to use composited clips.
   void pushOpacity(Offset offset, int alpha, PaintingContextCallback painter) {
-    pushLayer(OpacityLayer(alpha: alpha), painter, offset);
+    pushLayer(OpacityLayer(alpha: alpha, offset: offset), painter, Offset.zero);
   }
 
   @override

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

@@ -192,8 +192,7 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
     Timeline.startSync('Compositing', arguments: timelineWhitelistArguments);
     try {
       final ui.SceneBuilder builder = ui.SceneBuilder();
-      layer.addToScene(builder);
-      final ui.Scene scene = builder.build();
+      final ui.Scene scene = layer.buildScene(builder);
       if (automaticSystemUiAdjustment)
         _updateSystemChrome();
       ui.window.render(scene);

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

@@ -12,6 +12,7 @@ import 'dart:ui' as ui
     show
         window,
         ClipOp,
+        EngineLayer,
         Image,
         ImageByteFormat,
         Paragraph,
@@ -54,8 +55,8 @@ class _ProxyLayer extends Layer {
   final Layer _layer;
 
   @override
-  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
-    _layer.addToScene(builder, layerOffset);
+  ui.EngineLayer addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
+    return _layer.addToScene(builder, layerOffset);
   }
 
   @override
@@ -312,8 +313,9 @@ 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 = Offset.zero]) {
+  ui.EngineLayer addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     addChildrenToScene(builder, layerOffset);
+    return null; // this does not have an engine layer.
   }
 }
 
@@ -588,7 +590,7 @@ 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());
+    repaintBoundary.layer.buildScene(ui.SceneBuilder());
 
     return data.containerLayer.toImage(renderBounds, pixelRatio: pixelRatio);
   }
@@ -2226,9 +2228,9 @@ class _InspectorOverlayLayer extends Layer {
   double _textPainterMaxWidth;
 
   @override
-  void addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
+  ui.EngineLayer addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
     if (!selection.active)
-      return;
+      return null;
 
     final RenderObject selected = selection.current;
     final List<_TransformedRect> candidates = <_TransformedRect>[];
@@ -2251,6 +2253,7 @@ class _InspectorOverlayLayer extends Layer {
       _picture = _buildPicture(state);
     }
     builder.addPicture(layerOffset, _picture);
+    return null; // this does not have an engine layer.
   }
 
   ui.Picture _buildPicture(_InspectorOverlayRenderState state) {

packages/flutter/test/rendering/layers_test.dart

@@ -40,4 +40,73 @@ void main() {
     expect(boundary.layer, isNotNull);
     expect(boundary.layer.attached, isTrue); // this time it did again!
   });
+
+  test('layer subtree dirtiness is correctly computed', () {
+    final ContainerLayer a = ContainerLayer();
+    final ContainerLayer b = ContainerLayer();
+    final ContainerLayer c = ContainerLayer();
+    final ContainerLayer d = ContainerLayer();
+    final ContainerLayer e = ContainerLayer();
+    final ContainerLayer f = ContainerLayer();
+    final ContainerLayer g = ContainerLayer();
+
+    final PictureLayer h = PictureLayer(Rect.zero);
+    final PictureLayer i = PictureLayer(Rect.zero);
+    final PictureLayer j = PictureLayer(Rect.zero);
+
+    // The tree is like the following where b and j are dirty:
+    //        a____
+    //       /     \
+    //   (x)b___    c
+    //     / \  \   |
+    //    d   e  f  g
+    //   / \        |
+    //  h   i       j(x)
+    a.append(b);
+    a.append(c);
+    b.append(d);
+    b.append(e);
+    b.append(f);
+    d.append(h);
+    d.append(i);
+    c.append(g);
+    g.append(j);
+
+    a.debugMarkClean();
+    b.markNeedsAddToScene();  // ignore: invalid_use_of_protected_member
+    c.debugMarkClean();
+    d.debugMarkClean();
+    e.debugMarkClean();
+    f.debugMarkClean();
+    g.debugMarkClean();
+    h.debugMarkClean();
+    i.debugMarkClean();
+    j.markNeedsAddToScene();  // ignore: invalid_use_of_protected_member
+
+    a.updateSubtreeNeedsAddToScene();
+
+    expect(a.debugSubtreeNeedsAddToScene, true);
+    expect(b.debugSubtreeNeedsAddToScene, true);
+    expect(c.debugSubtreeNeedsAddToScene, true);
+    expect(g.debugSubtreeNeedsAddToScene, true);
+    expect(j.debugSubtreeNeedsAddToScene, true);
+
+    expect(d.debugSubtreeNeedsAddToScene, false);
+    expect(e.debugSubtreeNeedsAddToScene, false);
+    expect(f.debugSubtreeNeedsAddToScene, false);
+    expect(h.debugSubtreeNeedsAddToScene, false);
+    expect(i.debugSubtreeNeedsAddToScene, false);
+  });
+
+  test('leader and follower layers are always dirty', () {
+    final LayerLink link = LayerLink();
+    final LeaderLayer leaderLayer = LeaderLayer(link: link);
+    final FollowerLayer followerLayer = FollowerLayer(link: link);
+    leaderLayer.debugMarkClean();
+    followerLayer.debugMarkClean();
+    leaderLayer.updateSubtreeNeedsAddToScene();
+    followerLayer.updateSubtreeNeedsAddToScene();
+    expect(leaderLayer.debugSubtreeNeedsAddToScene, true);
+    expect(followerLayer.debugSubtreeNeedsAddToScene, true);
+  });
 }