diff --git a/packages/flutter/lib/src/rendering/object.dart b/packages/flutter/lib/src/rendering/object.dart index becd267e4d..e3218cf277 100644 --- a/packages/flutter/lib/src/rendering/object.dart +++ b/packages/flutter/lib/src/rendering/object.dart @@ -987,8 +987,7 @@ class PipelineOwner { try { final List dirtyNodes = _nodesNeedingPaint; _nodesNeedingPaint = []; - // Sort the dirty nodes in reverse order (deepest first). - for (final RenderObject node in dirtyNodes..sort((RenderObject a, RenderObject b) => b.depth - a.depth)) { + for (final RenderObject node in dirtyNodes..sort((RenderObject a, RenderObject b) => a.depth - b.depth)) { assert(node._layerHandle.layer != null); if (node._needsPaint && node.owner == this) { if (node._layerHandle.layer!.attached) { @@ -2365,12 +2364,8 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im return true; }()); // If we still need layout, then that means that we were skipped in the - // layout phase and therefore don't need painting. We might not know that - // yet (that is, our layer might not have been detached yet), because the - // same node that skipped us in layout is above us in the tree (obviously) - // and therefore may not have had a chance to paint yet (since the tree - // paints in reverse order). In particular this will happen if they have - // a different layer, because there's a repaint boundary between us. + // layout phase and therefore don't need painting. This can happen when an + // error occurred prior to our paint and our layer wasn't properly detached. if (_needsLayout) return; if (!kReleaseMode && debugProfilePaintsEnabled) { diff --git a/packages/flutter/test/rendering/paint_order_test.dart b/packages/flutter/test/rendering/paint_order_test.dart new file mode 100644 index 0000000000..8829bb881e --- /dev/null +++ b/packages/flutter/test/rendering/paint_order_test.dart @@ -0,0 +1,84 @@ +// Copyright 2014 The Flutter Authors. All rights reserved. +// Use of this source code is governed by a BSD-style license that can be +// found in the LICENSE file. + +import 'package:flutter/rendering.dart'; +import 'package:flutter_test/flutter_test.dart'; + +import 'rendering_tester.dart'; + +void main() { + TestRenderingFlutterBinding.ensureInitialized(); + + test('paint RenderObjects from top to bottom to avoid duplicated layer tree walks', () { + // Historic Background: The order in which dirty nodes are processed for paint + // in [PipelineOwner.flushPaint] was changed from shallowest first to + // deepest first in August 2015, see https://github.com/flutter/flutter/commit/654fc7346eb79780aadeb9c2883ea9938d5f0bd3#diff-d06e00032e4d722205a2189ffbab26c1d8f5e13652efebc849583d0a1359fec9R612. + // The reasons for this change are lost in history. In February 2022 it + // was determined that deepest first actually caused additional unnecessary + // walks of the layer tree. To avoid those, the processing order was changed + // back to deepest first (which is also the order used by all other flush-methods on + // PipelineOwner). The test below encodes that the framework is not doing + // these unnecessary layer tree walks during paint that would occur during + // deepest first processing. + + late RenderPositionedBox outer, inner; + late TestRenderBox testBox; + + outer = RenderPositionedBox( + child: RenderRepaintBoundary( + child: inner = RenderPositionedBox( + child: testBox = TestRenderBox(), + ), + ), + ); + + // Paint the tree for the first time; Our TestLayer is attached exactly once. + expect((testBox.debugLayer! as TestLayer).attachCount, 0); + expect((testBox.debugLayer! as TestLayer).detachCount, 0); + layout(outer, phase: EnginePhase.paint); + expect((testBox.debugLayer! as TestLayer).attachCount, 1); + expect((testBox.debugLayer! as TestLayer).detachCount, 0); + + // Repaint RenderObjects outside and inside the RepaintBoundary. + outer.markNeedsPaint(); + inner.markNeedsPaint(); + expect((testBox.debugLayer! as TestLayer).attachCount, 1); + expect((testBox.debugLayer! as TestLayer).detachCount, 0); + pumpFrame(phase: EnginePhase.paint); + + // The TestLayer should be detached and reattached exactly once during the + // paint process. More attach/detach would indicate unnecessary + // additional walks of the layer tree. + expect((testBox.debugLayer! as TestLayer).attachCount, 2); + expect((testBox.debugLayer! as TestLayer).detachCount, 1); + }); +} + +class TestRenderBox extends RenderProxyBoxWithHitTestBehavior { + TestRenderBox() { + layer = TestLayer(); + } + + @override + void paint(PaintingContext context, Offset offset) { + context.addLayer(layer!); + } +} + +class TestLayer extends OffsetLayer { + int attachCount = 0; + int detachCount = 0; + + @override + void attach(Object owner) { + super.attach(owner); + attachCount++; + } + + @override + void detach() { + super.detach(); + detachCount++; + } +}