Public nodes needing paint or layout (#166148)

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This PR adds 2 new `@protected` fields to `PipelineOwner` which list the
RenderObjects currently needing paint or layout for the next frame.

This PR addresses https://github.com/flutter/flutter/issues/166147,
which has further discussion of the motivation.

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

@@ -1088,6 +1088,21 @@ base class PipelineOwner with DiagnosticableTreeMixin {
   bool _shouldMergeDirtyNodes = false;
   List<RenderObject> _nodesNeedingLayout = <RenderObject>[];
 
+  /// The [RenderObject]s representing relayout boundaries which need to be laid out
+  /// in the next [flushLayout] pass.
+  ///
+  /// Relayout boundaries are added when they are marked for layout.
+  /// Subclasses of [PipelineOwner] may use them to invalidate caches or
+  /// otherwise make performance optimizations. Since nodes may be marked for
+  /// layout at any time, they are best checked during [flushLayout].
+  ///
+  /// Relayout boundaries owned by child [PipelineOwner]s are not included here.
+  ///
+  /// Boundaries appear in an arbitrary order, and may appear multiple times.
+  @protected
+  @nonVirtual
+  Iterable<RenderObject> get nodesNeedingLayout => _nodesNeedingLayout;
+
   /// Whether this pipeline is currently in the layout phase.
   ///
   /// Specifically, whether [flushLayout] is currently running.
@@ -1232,6 +1247,19 @@ base class PipelineOwner with DiagnosticableTreeMixin {
 
   List<RenderObject> _nodesNeedingPaint = <RenderObject>[];
 
+  /// The [RenderObject]s which need to be painted in the next [flushPaint] pass.
+  ///
+  /// [RenderObject]s marked with [RenderObject.isRepaintBoundary] are added
+  /// when they are marked needing paint. Subclasses of [PipelineOwner] may use them
+  /// to invalidate caches or otherwise make performance optimizations.
+  /// Since nodes may be marked for layout at any time, they are best checked during
+  /// [flushPaint].
+  ///
+  /// Marked children of child [PipelineOwner]s are not included here.
+  @protected
+  @nonVirtual
+  Iterable<RenderObject> get nodesNeedingPaint => _nodesNeedingPaint;
+
   /// Whether this pipeline is currently in the paint phase.
   ///
   /// Specifically, whether [flushPaint] is currently running.

packages/flutter/test/rendering/object_test.dart

@@ -22,6 +22,37 @@ void main() {
     );
   });
 
+  test('nodesNeedingLayout updated with layout changes', () {
+    final _TestPipelineOwner owner = _TestPipelineOwner();
+    final TestRenderObject renderObject = TestRenderObject()..isRepaintBoundary = true;
+    renderObject.attach(owner);
+    expect(owner.needLayout, isEmpty);
+
+    renderObject.layout(const BoxConstraints.tightForFinite());
+    renderObject.markNeedsLayout();
+    expect(owner.needLayout, contains(renderObject));
+
+    owner.flushLayout();
+    expect(owner.needLayout, isEmpty);
+  });
+
+  test('nodesNeedingPaint updated with paint changes', () {
+    final _TestPipelineOwner owner = _TestPipelineOwner();
+    final TestRenderObject renderObject = TestRenderObject(allowPaintBounds: true)
+      ..isRepaintBoundary = true;
+    final OffsetLayer layer = OffsetLayer();
+    layer.attach(owner);
+    renderObject.attach(owner);
+    expect(owner.needPaint, isEmpty);
+
+    renderObject.markNeedsPaint();
+    renderObject.scheduleInitialPaint(layer);
+    expect(owner.needPaint, contains(renderObject));
+
+    owner.flushPaint();
+    expect(owner.needPaint, isEmpty);
+  });
+
   test('ensure frame is scheduled for markNeedsSemanticsUpdate', () {
     // Initialize all bindings because owner.flushSemantics() requires a window
     final TestRenderObject renderObject = TestRenderObject();
@@ -686,3 +717,10 @@ class TestThrowingRenderObject extends RenderObject {
     return Rect.zero;
   }
 }
+
+final class _TestPipelineOwner extends PipelineOwner {
+  // Make these protected fields visible for testing.
+  Iterable<RenderObject> get needLayout => super.nodesNeedingLayout;
+
+  Iterable<RenderObject> get needPaint => super.nodesNeedingPaint;
+}