LayerLink can temporary allow multiple leaders (#95977)

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

@@ -7,6 +7,7 @@ import 'dart:ui' as ui;
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/painting.dart';
+import 'package:flutter/scheduler.dart';
 import 'package:vector_math/vector_math_64.dart';
 
 import 'debug.dart';
@@ -2110,6 +2111,55 @@ class PhysicalModelLayer extends ContainerLayer {
 class LayerLink {
   LeaderLayer? _leader;
 
+  void _registerLeader(LeaderLayer leader) {
+    assert(_leader != leader);
+    assert((){
+      if (_leader != null) {
+        _debugPreviousLeaders ??= <LeaderLayer>{};
+        _debugPreviousLeaders!.add(_leader!);
+        _debugScheduleLeadersCleanUpCheck();
+      }
+      return true;
+    }());
+    _leader = leader;
+  }
+
+  void _unregisterLeader(LeaderLayer leader) {
+    assert(_leader != null);
+    if (_leader == leader) {
+      _leader = null;
+    } else {
+      assert((){
+        _debugPreviousLeaders!.remove(leader);
+        return true;
+      }());
+    }
+  }
+
+  /// Stores the previous leaders that were replaced by the current [_leader]
+  /// in the current frame.
+  ///
+  /// These leaders need to give up their leaderships of this link by the end of
+  /// the current frame.
+  Set<LeaderLayer>? _debugPreviousLeaders;
+  bool _debugLeaderCheckScheduled = false;
+
+  /// Schedules the check as post frame callback to make sure the
+  /// [_debugPreviousLeaders] is empty.
+  void _debugScheduleLeadersCleanUpCheck() {
+    assert(_debugPreviousLeaders != null);
+    assert(() {
+      if (_debugLeaderCheckScheduled)
+        return true;
+      _debugLeaderCheckScheduled = true;
+      SchedulerBinding.instance!.addPostFrameCallback((Duration timeStamp) {
+        _debugLeaderCheckScheduled = false;
+        assert(_debugPreviousLeaders!.isEmpty);
+      });
+      return true;
+    }());
+  }
+
   int _connectedFollowers = 0;
 
   /// Whether a [LeaderLayer] is currently connected to this link.
@@ -2202,7 +2252,10 @@ class LeaderLayer extends ContainerLayer {
     if (_link == value) {
       return;
     }
-    _link._leader = null;
+    if (attached) {
+      _link._unregisterLeader(this);
+      value._registerLeader(this);
+    }
     _link = value;
   }
 
@@ -2233,16 +2286,14 @@ class LeaderLayer extends ContainerLayer {
   @override
   void attach(Object owner) {
     super.attach(owner);
-    assert(link._leader == null);
     assert(_debugSetLastOffset(null));
-    link._leader = this;
+    _link._registerLeader(this);
   }
 
   @override
   void detach() {
-    assert(link._leader == this);
-    link._leader = null;
     assert(_debugSetLastOffset(null));
+    _link._unregisterLeader(this);
     super.detach();
   }
 

packages/flutter/test/rendering/layers_test.dart

@@ -164,6 +164,18 @@ void main() {
     expect(followerLayer.debugSubtreeNeedsAddToScene, true);
   });
 
+  test('switching layer link of an attached leader layer should not crash', () {
+    final LayerLink link = LayerLink();
+    final LeaderLayer leaderLayer = LeaderLayer(link: link);
+    final RenderView view = RenderView(configuration: const ViewConfiguration(), window: window);
+    leaderLayer.attach(view);
+    final LayerLink link2 = LayerLink();
+    leaderLayer.link = link2;
+    // This should not crash.
+    leaderLayer.detach();
+    expect(leaderLayer.link, link2);
+  });
+
   test('leader layers are always dirty when connected to follower layer', () {
     final ContainerLayer root = ContainerLayer()..attach(Object());
 

packages/flutter/test/rendering/object_test.dart

@@ -153,6 +153,76 @@ void main() {
     expect(renderObject.toStringShort(), contains('DISPOSED'));
   });
 
+  test('Leader layer can switch to a different render object within one frame', () {
+    List<FlutterErrorDetails?>? caughtErrors;
+    renderer.onErrors = () {
+      caughtErrors = renderer.takeAllFlutterErrorDetails().toList();
+    };
+
+    final LayerLink layerLink = LayerLink();
+    // renderObject1 paints the leader layer first.
+    final LeaderLayerRenderObject renderObject1 = LeaderLayerRenderObject();
+    renderObject1.layerLink = layerLink;
+    renderObject1.attach(renderer.pipelineOwner);
+    final OffsetLayer rootLayer1 = OffsetLayer();
+    rootLayer1.attach(renderObject1);
+    renderObject1.scheduleInitialPaint(rootLayer1);
+    renderObject1.layout(const BoxConstraints.tightForFinite());
+
+    final LeaderLayerRenderObject renderObject2 = LeaderLayerRenderObject();
+    final OffsetLayer rootLayer2 = OffsetLayer();
+    rootLayer2.attach(renderObject2);
+    renderObject2.attach(renderer.pipelineOwner);
+    renderObject2.scheduleInitialPaint(rootLayer2);
+    renderObject2.layout(const BoxConstraints.tightForFinite());
+    renderer.pumpCompleteFrame();
+
+    // Swap the layer link to renderObject2 in the same frame
+    renderObject1.layerLink = null;
+    renderObject1.markNeedsPaint();
+    renderObject2.layerLink = layerLink;
+    renderObject2.markNeedsPaint();
+    renderer.pumpCompleteFrame();
+
+    // Swap the layer link to renderObject1 in the same frame
+    renderObject1.layerLink = layerLink;
+    renderObject1.markNeedsPaint();
+    renderObject2.layerLink = null;
+    renderObject2.markNeedsPaint();
+    renderer.pumpCompleteFrame();
+
+    renderer.onErrors = null;
+    expect(caughtErrors, isNull);
+  });
+
+  test('Leader layer append to two render objects does crash', () {
+    List<FlutterErrorDetails?>? caughtErrors;
+    renderer.onErrors = () {
+      caughtErrors = renderer.takeAllFlutterErrorDetails().toList();
+    };
+    final LayerLink layerLink = LayerLink();
+    // renderObject1 paints the leader layer first.
+    final LeaderLayerRenderObject renderObject1 = LeaderLayerRenderObject();
+    renderObject1.layerLink = layerLink;
+    renderObject1.attach(renderer.pipelineOwner);
+    final OffsetLayer rootLayer1 = OffsetLayer();
+    rootLayer1.attach(renderObject1);
+    renderObject1.scheduleInitialPaint(rootLayer1);
+    renderObject1.layout(const BoxConstraints.tightForFinite());
+
+    final LeaderLayerRenderObject renderObject2 = LeaderLayerRenderObject();
+    renderObject2.layerLink = layerLink;
+    final OffsetLayer rootLayer2 = OffsetLayer();
+    rootLayer2.attach(renderObject2);
+    renderObject2.attach(renderer.pipelineOwner);
+    renderObject2.scheduleInitialPaint(rootLayer2);
+    renderObject2.layout(const BoxConstraints.tightForFinite());
+    renderer.pumpCompleteFrame();
+
+    renderer.onErrors = null;
+    expect(caughtErrors!.isNotEmpty, isTrue);
+  });
+
   test('RenderObject.dispose null the layer on repaint boundaries', () {
     final TestRenderObject renderObject = TestRenderObject(allowPaintBounds: true);
     // Force a layer to get set.
@@ -255,6 +325,39 @@ class TestRenderObject extends RenderObject {
   }
 }
 
+class LeaderLayerRenderObject extends RenderObject {
+  LeaderLayerRenderObject();
+
+  LayerLink? layerLink;
+
+  @override
+  bool isRepaintBoundary = true;
+
+  @override
+  void debugAssertDoesMeetConstraints() { }
+
+  @override
+  Rect get paintBounds {
+    return Rect.zero;
+  }
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    if (layerLink != null) {
+      context.pushLayer(LeaderLayer(link: layerLink!), super.paint, offset);
+    }
+  }
+
+  @override
+  void performLayout() { }
+
+  @override
+  void performResize() { }
+
+  @override
+  Rect get semanticBounds => const Rect.fromLTWH(0.0, 0.0, 10.0, 20.0);
+}
+
 class TestThrowingRenderObject extends RenderObject {
   @override
   void performLayout() {

packages/flutter/test/rendering/rendering_tester.dart

@@ -82,6 +82,35 @@ class TestRenderingFlutterBinding extends BindingBase with SchedulerBinding, Ser
 
   EnginePhase phase = EnginePhase.composite;
 
+  /// Pumps a frame and runs its entire life cycle.
+  ///
+  /// This method runs all of the [SchedulerPhase]s in a frame, this is useful
+  /// to test [SchedulerPhase.postFrameCallbacks].
+  void pumpCompleteFrame() {
+    final FlutterExceptionHandler? oldErrorHandler = FlutterError.onError;
+    FlutterError.onError = (FlutterErrorDetails details) {
+      _errors.add(details);
+    };
+    try {
+      renderer.handleBeginFrame(null);
+      renderer.handleDrawFrame();
+    } finally {
+      FlutterError.onError = oldErrorHandler;
+      if (_errors.isNotEmpty) {
+        if (onErrors != null) {
+          onErrors!();
+          if (_errors.isNotEmpty) {
+            _errors.forEach(FlutterError.dumpErrorToConsole);
+            fail('There are more errors than the test inspected using TestRenderingFlutterBinding.takeFlutterErrorDetails.');
+          }
+        } else {
+          _errors.forEach(FlutterError.dumpErrorToConsole);
+          fail('Caught error while rendering frame. See preceding logs for details.');
+        }
+      }
+    }
+  }
+
   @override
   void drawFrame() {
     assert(phase != EnginePhase.build, 'rendering_tester does not support testing the build phase; use flutter_test instead');