Implement paintsChild on RenderObjects that skip painting on their children (#103768)

Author: dnfield

packages/flutter/lib/src/material/material.dart

@@ -682,7 +682,16 @@ abstract class InkFeature {
     final List<RenderObject> descendants = <RenderObject>[referenceBox];
     RenderObject node = referenceBox;
     while (node != _controller) {
+      final RenderObject childNode = node;
       node = node.parent! as RenderObject;
+      if (!node.paintsChild(childNode)) {
+        // Some node between the reference box and this would skip painting on
+        // the reference box, so bail out early and avoid unnecessary painting.
+        // Some cases where this can happen are the reference box being
+        // offstage, in a fully transparent opacity node, or in a keep alive
+        // bucket.
+        return;
+      }
       descendants.add(node);
     }
     // determine the transform that gets our coordinate system to be like theirs

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

@@ -2701,10 +2701,35 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
   ///
   /// Used by coordinate conversion functions to translate coordinates local to
   /// one render object into coordinates local to another render object.
+  ///
+  /// Some RenderObjects will provide a zeroed out matrix in this method,
+  /// indicating that the child should not paint anything or respond to hit
+  /// tests currently. A parent may supply a non-zero matrix even though it
+  /// does not paint its child currently, for example if the parent is a
+  /// [RenderOffstage] with `offstage` set to true. In both of these cases,
+  /// the parent must return `false` from [paintsChild].
   void applyPaintTransform(covariant RenderObject child, Matrix4 transform) {
     assert(child.parent == this);
   }
 
+  /// Whether the given child would be painted if [paint] were called.
+  ///
+  /// Some RenderObjects skip painting their children if they are configured to
+  /// not produce any visible effects. For example, a [RenderOffstage] with
+  /// its `offstage` property set to true, or a [RenderOpacity] with its opacity
+  /// value set to zero.
+  ///
+  /// In these cases, the parent may still supply a non-zero matrix in
+  /// [applyPaintTransform] to inform callers about where it would paint the
+  /// child if the child were painted at all. Alternatively, the parent may
+  /// supply a zeroed out matrix if it would not otherwise be able to determine
+  /// a valid matrix for the child and thus cannot meaningfully determine where
+  /// the child would paint.
+  bool paintsChild(covariant RenderObject child) {
+    assert(child.parent == this);
+    return true;
+  }
+
   /// Applies the paint transform up the tree to `ancestor`.
   ///
   /// Returns a matrix that maps the local paint coordinate system to the

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

@@ -896,6 +896,12 @@ class RenderOpacity extends RenderProxyBox {
     markNeedsSemanticsUpdate();
   }
 
+  @override
+  bool paintsChild(RenderBox child) {
+    assert(child.parent == this);
+    return _alpha > 0;
+  }
+
   @override
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {
@@ -1014,6 +1020,12 @@ mixin RenderAnimatedOpacityMixin<T extends RenderObject> on RenderObjectWithChil
     }
   }
 
+  @override
+  bool paintsChild(RenderObject child) {
+    assert(child.parent == this);
+    return opacity.value > 0;
+  }
+
   @override
   void paint(PaintingContext context, Offset offset) {
     if (_alpha == 0) {
@@ -2805,9 +2817,15 @@ class RenderFittedBox extends RenderProxyBox {
     );
   }
 
+  @override
+  bool paintsChild(RenderBox child) {
+    assert(child.parent == this);
+    return !size.isEmpty && !child.size.isEmpty;
+  }
+
   @override
   void applyPaintTransform(RenderBox child, Matrix4 transform) {
-    if (size.isEmpty || child.size.isEmpty) {
+    if (!paintsChild(child)) {
       transform.setZero();
     } else {
       _updatePaintData();
@@ -3575,7 +3593,6 @@ class RenderOffstage extends RenderProxyBox {
     return super.computeDryLayout(constraints);
   }
 
-
   @override
   void performResize() {
     assert(offstage);
@@ -3596,6 +3613,12 @@ class RenderOffstage extends RenderProxyBox {
     return !offstage && super.hitTest(result, position: position);
   }
 
+  @override
+  bool paintsChild(RenderBox child) {
+    assert(child.parent == this);
+    return !offstage;
+  }
+
   @override
   void paint(PaintingContext context, Offset offset) {
     if (offstage)

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

@@ -575,19 +575,23 @@ abstract class RenderSliverMultiBoxAdaptor extends RenderSliver
     return childParentData.layoutOffset;
   }
 
+  @override
+  bool paintsChild(RenderBox child) {
+    final SliverMultiBoxAdaptorParentData? childParentData = child.parentData as SliverMultiBoxAdaptorParentData?;
+    return childParentData?.index != null &&
+           !_keepAliveBucket.containsKey(childParentData!.index);
+  }
+
   @override
   void applyPaintTransform(RenderBox child, Matrix4 transform) {
-    final SliverMultiBoxAdaptorParentData childParentData = child.parentData! as SliverMultiBoxAdaptorParentData;
-    if (childParentData.index == null) {
-      // If the child has no index, such as with the prototype of a
-      // SliverPrototypeExtentList, then it is not visible, so we give it a
-      // zero transform to prevent it from painting.
-      transform.setZero();
-    } else if (_keepAliveBucket.containsKey(childParentData.index)) {
-      // It is possible that widgets under kept alive children want to paint
-      // themselves. For example, the Material widget tries to paint all
-      // InkFeatures under its subtree as long as they are not disposed. In
-      // such case, we give it a zero transform to prevent them from painting.
+    if (!paintsChild(child)) {
+      // This can happen if some child asks for the global transform even though
+      // they are not getting painted. In that case, the transform sets set to
+      // zero since [applyPaintTransformForBoxChild] would end up throwing due
+      // to the child not being configured correctly for applying a transform.
+      // There's no assert here because asking for the paint transform is a
+      // valid thing to do even if a child would not be painted, but there is no
+      // meaningful non-zero matrix to use in this case.
       transform.setZero();
     } else {
       applyPaintTransformForBoxChild(child, transform);

packages/flutter/test/material/material_test.dart

@@ -927,4 +927,52 @@ void main() {
       );
     });
   });
+
+  testWidgets('InkFeature skips painting if intermediate node skips', (WidgetTester tester) async {
+    final GlobalKey sizedBoxKey = GlobalKey();
+    final GlobalKey materialKey = GlobalKey();
+    await tester.pumpWidget(Material(
+      key: materialKey,
+      child: Offstage(
+        child: SizedBox(key: sizedBoxKey, width: 20, height: 20),
+      ),
+    ));
+    final MaterialInkController controller = Material.of(sizedBoxKey.currentContext!)!;
+
+    final TrackPaintInkFeature tracker = TrackPaintInkFeature(
+      controller: controller,
+      referenceBox: sizedBoxKey.currentContext!.findRenderObject()! as RenderBox,
+    );
+    controller.addInkFeature(tracker);
+    expect(tracker.paintCount, 0);
+
+    // Force a repaint. Since it's offstage, the ink feture should not get painted.
+    materialKey.currentContext!.findRenderObject()!.paint(PaintingContext(ContainerLayer(), Rect.largest), Offset.zero);
+    expect(tracker.paintCount, 0);
+
+    await tester.pumpWidget(Material(
+      key: materialKey,
+      child: Offstage(
+        offstage: false,
+        child: SizedBox(key: sizedBoxKey, width: 20, height: 20),
+      ),
+    ));
+    // Gets a paint because the global keys have reused the elements and it is
+    // now onstage.
+    expect(tracker.paintCount, 1);
+
+    // Force a repaint again. This time, it gets repainted because it is onstage.
+    materialKey.currentContext!.findRenderObject()!.paint(PaintingContext(ContainerLayer(), Rect.largest), Offset.zero);
+    expect(tracker.paintCount, 2);
+  });
+}
+
+class TrackPaintInkFeature extends InkFeature {
+  TrackPaintInkFeature({required super.controller, required super.referenceBox});
+
+  int paintCount = 0;
+  @override
+  void paintFeature(Canvas canvas, Matrix4 transform) {
+    paintCount += 1;
+  }
 }

packages/flutter/test/rendering/proxy_box_test.dart

@@ -687,6 +687,66 @@ void main() {
     expect(() => pumpFrame(phase: EnginePhase.composite), returnsNormally);
   });
 
+  test('Offstage implements paintsChild correctly', () {
+    final RenderBox box = RenderConstrainedBox(additionalConstraints: const BoxConstraints.tightFor(width: 20));
+    final RenderBox parent = RenderConstrainedBox(additionalConstraints: const BoxConstraints.tightFor(width: 20));
+    final RenderOffstage offstage = RenderOffstage(offstage: false, child: box);
+    parent.adoptChild(offstage);
+
+    expect(offstage.paintsChild(box), true);
+
+    offstage.offstage = true;
+
+    expect(offstage.paintsChild(box), false);
+  });
+
+  test('Opacity implements paintsChild correctly', () {
+    final RenderBox box = RenderConstrainedBox(additionalConstraints: const BoxConstraints.tightFor(width: 20));
+    final RenderBox parent = RenderConstrainedBox(additionalConstraints: const BoxConstraints.tightFor(width: 20));
+    final RenderOpacity opacity = RenderOpacity(child: box);
+    parent.adoptChild(opacity);
+
+    expect(opacity.paintsChild(box), true);
+
+    opacity.opacity = 0;
+
+    expect(opacity.paintsChild(box), false);
+  });
+
+  test('AnimatedOpacity sets paint matrix to zero when alpha == 0', () {
+    final RenderBox box = RenderConstrainedBox(additionalConstraints: const BoxConstraints.tightFor(width: 20));
+    final RenderBox parent = RenderConstrainedBox(additionalConstraints: const BoxConstraints.tightFor(width: 20));
+    final AnimationController opacityAnimation = AnimationController(value: 1, vsync: FakeTickerProvider());
+    final RenderAnimatedOpacity opacity = RenderAnimatedOpacity(opacity: opacityAnimation, child: box);
+    parent.adoptChild(opacity);
+
+    // Make it listen to the animation.
+    opacity.attach(PipelineOwner());
+
+    expect(opacity.paintsChild(box), true);
+
+    opacityAnimation.value = 0;
+
+    expect(opacity.paintsChild(box), false);
+  });
+
+  test('AnimatedOpacity sets paint matrix to zero when alpha == 0 (sliver)', () {
+    final RenderSliver sliver = RenderSliverToBoxAdapter(child: RenderConstrainedBox(additionalConstraints: const BoxConstraints.tightFor(width: 20)));
+    final RenderBox parent = RenderConstrainedBox(additionalConstraints: const BoxConstraints.tightFor(width: 20));
+    final AnimationController opacityAnimation = AnimationController(value: 1, vsync: FakeTickerProvider());
+    final RenderSliverAnimatedOpacity opacity = RenderSliverAnimatedOpacity(opacity: opacityAnimation, sliver: sliver);
+    parent.adoptChild(opacity);
+
+    // Make it listen to the animation.
+    opacity.attach(PipelineOwner());
+
+    expect(opacity.paintsChild(sliver), true);
+
+    opacityAnimation.value = 0;
+
+    expect(opacity.paintsChild(sliver), false);
+  });
+
   test('RenderCustomClip extenders respect clipBehavior when asked to describeApproximateClip', () {
     final RenderBox child = RenderConstrainedBox(additionalConstraints: const BoxConstraints.tightFor(width: 200, height: 200));
     final RenderClipRect renderClipRect = RenderClipRect(clipBehavior: Clip.none, child: child);

packages/flutter/test/rendering/sliver_fixed_extent_layout_test.dart

@@ -110,6 +110,44 @@ void main() {
       expect(actual, 0);
     });
   });
+
+  test('Implements paintsChild correctly', () {
+    final List<RenderBox> children = <RenderBox>[
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+    ];
+    final TestRenderSliverBoxChildManager childManager = TestRenderSliverBoxChildManager(
+      children: children,
+    );
+    final RenderViewport root = RenderViewport(
+      crossAxisDirection: AxisDirection.right,
+      offset: ViewportOffset.zero(),
+      cacheExtent: 0,
+      children: <RenderSliver>[
+        childManager.createRenderSliverFillViewport(),
+      ],
+    );
+    layout(root);
+    expect(children.first.parent, isA<RenderSliverMultiBoxAdaptor>());
+
+    final RenderSliverMultiBoxAdaptor parent = children.first.parent! as RenderSliverMultiBoxAdaptor;
+    expect(parent.paintsChild(children[0]), true);
+    expect(parent.paintsChild(children[1]), false);
+    expect(parent.paintsChild(children[2]), false);
+
+    root.offset = ViewportOffset.fixed(600);
+    pumpFrame();
+    expect(parent.paintsChild(children[0]), false);
+    expect(parent.paintsChild(children[1]), true);
+    expect(parent.paintsChild(children[2]), false);
+
+    root.offset = ViewportOffset.fixed(1200);
+    pumpFrame();
+    expect(parent.paintsChild(children[0]), false);
+    expect(parent.paintsChild(children[1]), false);
+    expect(parent.paintsChild(children[2]), true);
+  });
 }
 
 int testGetMaxChildIndexForScrollOffset(double scrollOffset, double itemExtent) {