Handle stateful widgets in layout builders. (#5752)

Previously, if a StatefulWidget was marked dirty, then removed from the
build, then reinserted using the exact same widget under a widget under
a LayoutBuilder, it wouldn't rebuild.

This fixes that.

It also introduces an assert that's supposed to catch SizeObserver-like
behaviour. Rather than make this patch even bigger, I papered over two
pre-existing bugs which this assert uncovered (and fixed the other
problems it found):

   https://github.com/flutter/flutter/issues/5751
   https://github.com/flutter/flutter/issues/5749

We should fix those before 1.0 though.

dev/benchmarks/microbenchmarks/lib/stocks/build_bench.dart

@@ -30,7 +30,7 @@ Future<Null> main() async {
     watch.start();
     while (watch.elapsed < kBenchmarkTime) {
       appState.markNeedsBuild();
-      buildOwner.buildDirtyElements();
+      buildOwner.buildScope(WidgetsBinding.instance.renderViewElement);
       iterations += 1;
     }
     watch.stop();

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

@@ -5,6 +5,7 @@
 import 'dart:async';
 import 'dart:math' as math;
 
+import 'package:flutter/scheduler.dart';
 import 'package:flutter/widgets.dart';
 import 'package:meta/meta.dart';
 
@@ -249,7 +250,11 @@ class _DropDownMenuRouteLayout<T> extends SingleChildLayoutDelegate {
     if (route.initialLayout) {
       route.initialLayout = false;
       final double scrollOffset = selectedItemOffset - (buttonTop - top);
+      SchedulerBinding.instance.addPostFrameCallback((Duration timeStamp) {
+        // TODO(ianh): Compute and set this during layout instead of being
+        // lagged by one frame. https://github.com/flutter/flutter/issues/5751
         scrollableKey.currentState.scrollTo(scrollOffset);
+      });
     }
 
     return new Offset(buttonRect.left, top);

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

@@ -7,6 +7,7 @@ import 'dart:math' as math;
 
 import 'package:flutter/physics.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/scheduler.dart';
 import 'package:flutter/widgets.dart';
 import 'package:meta/meta.dart';
 
@@ -968,11 +969,18 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
   }
 
   void _layoutChanged(Size tabBarSize, List<double> tabWidths) {
-    setState(() {
+    // This is bad. We should use a LayoutBuilder or CustomMultiChildLayout or some such.
+    // As designed today, tabs are always lagging one frame behind, taking two frames
+    // to handle a layout change.
     _tabBarSize = tabBarSize;
     _tabWidths = tabWidths;
     _indicatorRect = _selection != null ? _tabIndicatorRect(_selection.index) : Rect.zero;
     _updateScrollBehavior();
+    SchedulerBinding.instance.addPostFrameCallback((Duration timeStamp) {
+      setState(() {
+        // the changes were made at layout time
+        // TODO(ianh): remove this setState: https://github.com/flutter/flutter/issues/5749
+      });
     });
   }
 

packages/flutter/lib/src/scheduler/debug.dart

@@ -3,6 +3,18 @@
 // found in the LICENSE file.
 
 /// Print a banner at the beginning of each frame.
+///
+/// Frames triggered by the engine and handler by the scheduler binding will
+/// have a banner saying "Begin Frame" and giving the time stamp of the frame.
+///
+/// Frames triggered eagerly by the widget framework (e.g. when calling
+/// [runApp]) will have a label saying "Begin Warm-Up Frame".
+///
+/// To include a banner at the end of each frame as well, to distinguish
+/// intra-frame output from inter-frame output, set [debugPrintEndFrameBanner]
+/// to true as well.
+///
+/// See [SchedulerBinding.beginFrame].
 bool debugPrintBeginFrameBanner = false;
 
 /// Print a banner at the end of each frame.

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

@@ -10,6 +10,7 @@ import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter/services.dart';
+import 'package:meta/meta.dart';
 
 import 'app.dart';
 import 'framework.dart';
@@ -191,15 +192,41 @@ abstract class WidgetsBinding extends BindingBase implements GestureBinding, Ren
   }
 
   void _handleBuildScheduled() {
-    // If we're in the process of building dirty elements, we're know that any
-    // builds that are scheduled will be run this frame, which means we don't
-    // need to schedule another frame.
-    if (_buildingDirtyElements)
-      return;
+    // If we're in the process of building dirty elements, then changes
+    // should not trigger a new frame.
+    assert(() {
+      if (debugBuildingDirtyElements) {
+        throw new FlutterError(
+          'Build scheduled during frame.\n'
+          'While the widget tree was being built, laid out, and painted, '
+          'a new frame was scheduled to rebuild the widget tree. '
+          'This might be because setState() was called from a layout or '
+          'paint callback. '
+          'If a change is needed to the widget tree, it should be applied '
+          'as the tree is being built. Scheduling a change for the subsequent '
+          'frame instead results in an interface that lags behind by one frame. '
+          'If this was done to make your build dependent on a size measured at '
+          'layout time, consider using a LayoutBuilder, CustomSingleChildLayout, '
+          'or CustomMultiChildLayout. If, on the other hand, the one frame delay '
+          'is the desired effect, for example because this is an '
+          'animation, consider scheduling the frame in a post-frame callback '
+          'using SchedulerBinding.addPostFrameCallback or '
+          'using an AnimationController to trigger the animation.'
+        );
+      }
+      return true;
+    });
     scheduleFrame();
   }
 
-  bool _buildingDirtyElements = false;
+  /// Whether we are currently in a frame. This is used to verify
+  /// that frames are not scheduled redundantly.
+  ///
+  /// This is public so that test frameworks can change it.
+  ///
+  /// This flag is not used in release builds.
+  @protected
+  bool debugBuildingDirtyElements = false;
 
   /// Pump the build and rendering pipeline to generate a frame.
   ///
@@ -260,12 +287,21 @@ abstract class WidgetsBinding extends BindingBase implements GestureBinding, Ren
   // When editing the above, also update rendering/binding.dart's copy.
   @override
   void beginFrame() {
-    assert(!_buildingDirtyElements);
-    _buildingDirtyElements = true;
-    buildOwner.buildDirtyElements();
-    _buildingDirtyElements = false;
+    assert(!debugBuildingDirtyElements);
+    assert(() {
+      debugBuildingDirtyElements = true;
+      return true;
+    });
+    try {
+      buildOwner.buildScope(renderViewElement);
       super.beginFrame();
       buildOwner.finalizeTree();
+    } finally {
+      assert(() {
+        debugBuildingDirtyElements = false;
+        return true;
+      });
+    }
     // TODO(ianh): Following code should not be included in release mode, only profile and debug modes.
     // See https://github.com/dart-lang/sdk/issues/27192
     if (_needToReportFirstFrame) {
@@ -291,7 +327,17 @@ abstract class WidgetsBinding extends BindingBase implements GestureBinding, Ren
       debugShortDescription: '[root]',
       child: app
     ).attachToRenderTree(buildOwner, renderViewElement);
+    assert(() {
+      if (debugPrintBeginFrameBanner)
+        debugPrint('━━━━━━━┫ Begin Warm-Up Frame ┣━━━━━━━');
+      return true;
+    });
     beginFrame();
+    assert(() {
+      if (debugPrintEndFrameBanner)
+        debugPrint('━━━━━━━┫ End of Warm-Up Frame ┣━━━━━━━');
+      return true;
+    });
   }
 
   @override
@@ -364,15 +410,20 @@ class RenderObjectToWidgetAdapter<T extends RenderObject> extends RenderObjectWi
   ///
   /// Used by [runApp] to bootstrap applications.
   RenderObjectToWidgetElement<T> attachToRenderTree(BuildOwner owner, [RenderObjectToWidgetElement<T> element]) {
-    owner.lockState(() {
     if (element == null) {
+      owner.lockState(() {
         element = createElement();
+        assert(element != null);
         element.assignOwner(owner);
+      });
+      owner.buildScope(element, () {
         element.mount(null, null);
+      });
     } else {
+      owner.buildScope(element, () {
         element.update(this);
+      });
     }
-    }, building: true);
     return element;
   }
 

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

@@ -1410,12 +1410,16 @@ class BuildOwner {
   final _InactiveElements _inactiveElements = new _InactiveElements();
 
   final List<BuildableElement> _dirtyElements = <BuildableElement>[];
+  bool _scheduledFlushDirtyElements = false;
 
   /// Adds an element to the dirty elements list so that it will be rebuilt
-  /// when [buildDirtyElements] is called.
+  /// when [WidgetsBinding.beginFrame] calls [buildScope].
   void scheduleBuildFor(BuildableElement element) {
+    assert(element != null);
+    assert(element.owner == this);
+    assert(element._inDirtyList == _dirtyElements.contains(element));
     assert(() {
-      if (_dirtyElements.contains(element)) {
+      if (element._inDirtyList) {
         throw new FlutterError(
           'scheduleBuildFor() called for a widget for which a build was already scheduled.\n'
           'The method was called for the following element:\n'
@@ -1440,9 +1444,12 @@ class BuildOwner {
       }
       return true;
     });
-    if (_dirtyElements.isEmpty && onBuildScheduled != null)
+    if (!_scheduledFlushDirtyElements && onBuildScheduled != null) {
+      _scheduledFlushDirtyElements = true;
       onBuildScheduled();
+    }
     _dirtyElements.add(element);
+    element._inDirtyList = true;
   }
 
   int _debugStateLockLevel = 0;
@@ -1450,24 +1457,15 @@ class BuildOwner {
   bool _debugBuilding = false;
   BuildableElement _debugCurrentBuildTarget;
 
-  /// Establishes a scope in which calls to [State.setState] are forbidden.
-  ///
-  /// This mechanism prevents build functions from transitively requiring other
-  /// build functions to run, potentially causing infinite loops.
-  ///
-  /// If the building argument is true, then this function enables additional
-  /// asserts that check invariants that should apply during building.
+  /// Establishes a scope in which calls to [State.setState] are forbidden, and
+  /// calls the given `callback`.
   ///
-  /// The context argument is used to describe the scope in case an exception is
-  /// caught while invoking the callback.
-  void lockState(void callback(), { bool building: false }) {
-    bool debugPreviouslyBuilding;
+  /// This mechanism is used to ensure that, for instance, [State.dispose] does
+  /// not call [State.setState].
+  void lockState(void callback()) {
+    assert(callback != null);
     assert(_debugStateLockLevel >= 0);
     assert(() {
-      if (building) {
-        debugPreviouslyBuilding = _debugBuilding;
-        _debugBuilding = true;
-      }
       _debugStateLockLevel += 1;
       return true;
     });
@@ -1476,43 +1474,55 @@ class BuildOwner {
     } finally {
       assert(() {
         _debugStateLockLevel -= 1;
-        if (building) {
-          assert(_debugBuilding);
-          _debugBuilding = debugPreviouslyBuilding;
-        }
         return true;
       });
     }
     assert(_debugStateLockLevel >= 0);
   }
 
-  static int _elementSort(BuildableElement a, BuildableElement b) {
-    if (a.depth < b.depth)
-      return -1;
-    if (b.depth < a.depth)
-      return 1;
-    if (b.dirty && !a.dirty)
-      return -1;
-    if (a.dirty && !b.dirty)
-      return 1;
-    return 0;
-  }
-
-  /// Builds all the elements that were marked as dirty using
-  /// [scheduleBuildFor], in depth order. If elements are marked as dirty while
-  /// this runs, they must be deeper than the algorithm has yet reached.
+  /// Establishes a scope for updating the widget tree, and calls the given
+  /// `callback`, if any. Then, builds all the elements that were marked as
+  /// dirty using [scheduleBuildFor], in depth order.
   ///
-  /// This is called by [WidgetsBinding.beginFrame].
-  void buildDirtyElements() {
-    if (_dirtyElements.isEmpty)
+  /// This mechanism prevents build functions from transitively requiring other
+  /// build functions to run, potentially causing infinite loops.
+  ///
+  /// The dirty list is processed after `callback` returns, building all the
+  /// elements that were marked as dirty using [scheduleBuildFor], in depth
+  /// order. If elements are marked as dirty while this method is running, they
+  /// must be deeper than the `context` node, and deeper than any
+  /// previously-built node in this pass.
+  ///
+  /// To flush the current dirty list without performing any other work, this
+  /// function can be called with no callback. This is what the framework does
+  /// each frame, in [WidgetsBinding.beginFrame].
+  ///
+  /// Only one [buildScope] can be active at a time.
+  ///
+  /// A [buildScope] implies a [lockState] scope as well.
+  void buildScope(Element context, [VoidCallback callback]) {
+    if (callback == null && _dirtyElements.isEmpty)
       return;
+    assert(context != null);
+    assert(_debugStateLockLevel >= 0);
+    assert(!_debugBuilding);
+    assert(() {
+      _debugStateLockLevel += 1;
+      _debugBuilding = true;
+      return true;
+    });
     Timeline.startSync('Build');
     try {
-      lockState(() {
+      _scheduledFlushDirtyElements = true;
+      if (callback != null)
+        callback();
       _dirtyElements.sort(_elementSort);
       int dirtyCount = _dirtyElements.length;
       int index = 0;
       while (index < dirtyCount) {
+        assert(_dirtyElements[index] != null);
+        assert(_dirtyElements[index]._inDirtyList);
+        assert(!_dirtyElements[index]._active || _dirtyElements[index]._debugIsInScope(context));
         _dirtyElements[index].rebuild();
         index += 1;
         if (dirtyCount < _dirtyElements.length) {
@@ -1520,12 +1530,35 @@ class BuildOwner {
           dirtyCount = _dirtyElements.length;
         }
       }
-        assert(!_dirtyElements.any((BuildableElement element) => element._active && element.dirty));
-      }, building: true);
     } finally {
+      assert(!_dirtyElements.any((BuildableElement element) => element._active && element.dirty));
+      for (BuildableElement element in _dirtyElements) {
+        assert(element._inDirtyList);
+        element._inDirtyList = false;
+      }
       _dirtyElements.clear();
+      _scheduledFlushDirtyElements = false;
       Timeline.finishSync();
+      assert(_debugBuilding);
+      assert(() {
+        _debugBuilding = false;
+        _debugStateLockLevel -= 1;
+        return true;
+      });
+    }
+    assert(_debugStateLockLevel >= 0);
   }
+
+  static int _elementSort(BuildableElement a, BuildableElement b) {
+    if (a.depth < b.depth)
+      return -1;
+    if (b.depth < a.depth)
+      return 1;
+    if (b.dirty && !a.dirty)
+      return -1;
+    if (a.dirty && !b.dirty)
+      return 1;
+    return 0;
   }
 
   /// Complete the element build pass by unmounting any elements that are no
@@ -1607,6 +1640,15 @@ abstract class Element implements BuildContext {
     });
   }
 
+  bool _debugIsInScope(Element target) {
+    assert(target != null);
+    if (target == this)
+      return true;
+    if (_parent != null)
+      return _parent._debugIsInScope(target);
+    return false;
+  }
+
   RenderObject get renderObject {
     RenderObject result;
     void visit(Element element) {
@@ -1875,6 +1917,7 @@ abstract class Element implements BuildContext {
     });
     assert(_debugLifecycleState == _ElementLifecycle.inactive);
     assert(widget != null);
+    assert(owner != null);
     assert(depth != null);
     assert(!_active);
     _active = true;
@@ -2091,6 +2134,10 @@ abstract class BuildableElement extends Element {
   bool get dirty => _dirty;
   bool _dirty = true;
 
+  // Whether this is in owner._dirtyElements. This is used to know whether we
+  // should be adding the element back into the list when it's reactivated.
+  bool _inDirtyList = false;
+
   // We let widget authors call setState from initState, didUpdateConfig, and
   // build even when state is locked because its convenient and a no-op anyway.
   // This flag ensures that this convenience is only allowed on the element
@@ -2124,15 +2171,7 @@ abstract class BuildableElement extends Element {
           // a current build target when we're building.
           return true;
         }
-        bool foundTarget = false;
-        visitAncestorElements((Element element) {
-          if (element == owner._debugCurrentBuildTarget) {
-            foundTarget = true;
-            return false;
-          }
-          return true;
-        });
-        if (foundTarget)
+        if (_debugIsInScope(owner._debugCurrentBuildTarget))
           return true;
       }
       if (owner._debugStateLocked && (!_debugAllowIgnoredCallsToMarkNeedsBuild || !dirty)) {
@@ -2212,7 +2251,9 @@ abstract class BuildableElement extends Element {
   void activate() {
     final bool shouldRebuild = ((_dependencies != null && _dependencies.length > 0) || _hadUnsatisfiedDependencies);
     super.activate(); // clears _dependencies, and sets active to true
-    if (shouldRebuild) {
+    if (_dirty && !_inDirtyList) {
+      owner.scheduleBuildFor(this);
+    } else if (shouldRebuild) {
       assert(_active); // otherwise markNeedsBuild is a no-op
       markNeedsBuild();
     }
@@ -2904,7 +2945,7 @@ abstract class RootRenderObjectElement extends RenderObjectElement {
   /// The [WidgetsBinding] introduces the primary owner,
   /// [WidgetsBinding.buildOwner], and assigns it to the widget tree in the call
   /// to [runApp]. The binding is responsible for driving the build pipeline by
-  /// calling the build owner's [BuildOwner.buildDirtyElements] method. See
+  /// calling the build owner's [BuildOwner.buildScope] method. See
   /// [WidgetsBinding.beginFrame].
   void assignOwner(BuildOwner owner) {
     _owner = owner;

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

@@ -176,9 +176,9 @@ class _LayoutBuilderElement extends RenderObjectElement {
   }
 
   void _layout(BoxConstraints constraints) {
+    owner.buildScope(this, () {
       Widget built;
       if (widget.builder != null) {
-      owner.lockState(() {
         try {
           built = widget.builder(this, constraints);
           debugWidgetBuilderValue(widget, built);
@@ -186,13 +186,6 @@ class _LayoutBuilderElement extends RenderObjectElement {
           _debugReportException('building $widget', e, stack);
           built = new ErrorWidget(e);
         }
-      });
-    }
-    owner.lockState(() {
-      if (widget.builder == null) {
-        if (_child != null)
-          _child = updateChild(_child, null, null);
-        return;
       }
       try {
         _child = updateChild(_child, built, null);
@@ -202,7 +195,7 @@ class _LayoutBuilderElement extends RenderObjectElement {
         built = new ErrorWidget(e);
         _child = updateChild(null, built, slot);
       }
-    }, building: true);
+    });
   }
 
   @override

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

@@ -629,11 +629,11 @@ class _LazyBlockElement extends RenderObjectElement {
       while (currentLogicalIndex > 0 && currentLogicalOffset > startLogicalOffset) {
         currentLogicalIndex -= 1;
         Element newElement;
-        owner.lockState(() {
+        owner.buildScope(this, () {
           Widget newWidget = _callBuilder(builder, currentLogicalIndex, requireNonNull: true);
           newWidget = new RepaintBoundary.wrap(newWidget, currentLogicalIndex);
           newElement = inflateWidget(newWidget, null);
-        }, building: true);
+        });
         newChildren.add(newElement);
         RenderBox child = block.firstChild;
         assert(child == newChildren.last.renderObject);
@@ -698,14 +698,14 @@ class _LazyBlockElement extends RenderObjectElement {
       if (physicalIndex >= _children.length) {
         assert(physicalIndex == _children.length);
         Element newElement;
-        owner.lockState(() {
+        owner.buildScope(this, () {
           Widget newWidget = _callBuilder(builder, currentLogicalIndex);
           if (newWidget == null)
             return;
           newWidget = new RepaintBoundary.wrap(newWidget, currentLogicalIndex);
           Element previousChild = _children.isEmpty ? null : _children.last;
           newElement = inflateWidget(newWidget, previousChild);
-        }, building: true);
+        });
         if (newElement == null)
           break;
         _children.add(newElement);

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

@@ -373,6 +373,7 @@ class ScrollableState<T extends Scrollable> extends State<T> {
   }
 
   void _handleAnimationStatusChanged(AnimationStatus status) {
+    // this is not called when stop() is called on the controller
     setState(() {
       if (!_controller.isAnimating)
         _simulation = null;
@@ -454,7 +455,8 @@ class ScrollableState<T extends Scrollable> extends State<T> {
   /// If there are no in-progress scrolling physics, this function scrolls to
   /// the given offset instead.
   void didUpdateScrollBehavior(double newScrollOffset) {
-    setState(() { /* The scroll behavior is part of our build state. */ });
+    // This does not call setState, because if anything below actually
+    // changes our build, it will itself independently trigger a frame.
     assert(_controller.isAnimating || _simulation == null);
     if (_numberOfInProgressScrolls > 0) {
       if (_simulation != null) {
@@ -583,16 +585,16 @@ class ScrollableState<T extends Scrollable> extends State<T> {
   }
 
   void _handleDragDown(_) {
+    setState(() {
       _stop();
+    });
   }
 
   void _stop() {
     assert(mounted);
     assert(_controller.isAnimating || _simulation == null);
-    setState(() {
-      _controller.stop();
+    _controller.stop(); // this does not trigger a status notification
     _simulation = null;
-    });
   }
 
   void _handleDragStart(DragStartDetails details) {

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

@@ -5,6 +5,8 @@
 import 'dart:collection';
 import 'dart:math' as math;
 
+import 'package:flutter/foundation.dart';
+import 'package:flutter/scheduler.dart';
 import 'package:flutter/rendering.dart';
 import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
 
@@ -53,8 +55,17 @@ class _SemanticsDebuggerState extends State<SemanticsDebugger> {
   }
 
   void _update() {
+    SchedulerBinding.instance.addPostFrameCallback((Duration timeStamp) {
+      // We want the update to take effect next frame, so to make that
+      // explicit we call setState() in a post-frame callback.
+      if (mounted) {
+        // If we got disposed this frame, we will still get an update,
+        // because the inactive list is flushed after the semantics updates
+        // are transmitted to the semantics clients.
         setState(() {
-      // the generation of the _SemanticsDebuggerListener has changed
+          // The generation of the _SemanticsDebuggerListener has changed.
+        });
+      }
     });
   }
 

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

@@ -175,7 +175,7 @@ abstract class VirtualViewportElement extends RenderObjectElement {
     assert(startOffsetBase != null);
     assert(startOffsetLimit != null);
     _updatePaintOffset();
-    owner.lockState(_materializeChildren, building: true);
+    owner.buildScope(this, _materializeChildren);
   }
 
   void _materializeChildren() {

packages/flutter/test/widget/independent_widget_layout_test.dart

@@ -38,7 +38,7 @@ class OffscreenWidgetTree {
   }
 
   void pumpFrame() {
-    buildOwner.buildDirtyElements();
+    buildOwner.buildScope(root);
     pipelineOwner.flushLayout();
     pipelineOwner.flushCompositingBits();
     pipelineOwner.flushPaint();

packages/flutter/test/widget/layout_builder_and_state_test.dart

+// Copyright 2016 The Chromium 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_test/flutter_test.dart' hide TypeMatcher;
+import 'package:flutter/widgets.dart';
+import 'test_widgets.dart';
+
+class StatefulWrapper extends StatefulWidget {
+  StatefulWrapper({
+    Key key,
+    this.child,
+  }) : super(key: key);
+
+  final Widget child;
+
+  @override
+  StatefulWrapperState createState() => new StatefulWrapperState();
+}
+
+class StatefulWrapperState extends State<StatefulWrapper> {
+
+  void trigger() {
+    setState(() { /* no-op setState */ });
+  }
+
+  bool built = false;
+
+  @override
+  Widget build(BuildContext context) {
+    built = true;
+    return config.child;
+  }
+}
+
+class Wrapper extends StatelessWidget {
+  Wrapper({
+    Key key,
+    this.child,
+  }) : super(key: key);
+
+  final Widget child;
+
+  @override
+  Widget build(BuildContext context) {
+    return child;
+  }
+}
+
+void main() {
+  testWidgets('Calling setState on a widget that moves into a LayoutBuilder in the same frame', (WidgetTester tester) async {
+    StatefulWrapperState statefulWrapper;
+    final Widget inner = new Wrapper(
+      child: new StatefulWrapper(
+        key: new GlobalKey(),
+        child: new Container(),
+      ),
+    );
+    await tester.pumpWidget(new FlipWidget(
+      left: new LayoutBuilder(builder: (BuildContext context, BoxConstraints constraints) {
+        return inner;
+      }),
+      right: inner,
+    ));
+    statefulWrapper = tester.state(find.byType(StatefulWrapper));
+    expect(statefulWrapper.built, true);
+    statefulWrapper.built = false;
+
+    statefulWrapper.trigger();
+    flipStatefulWidget(tester);
+    await tester.pump();
+    expect(statefulWrapper.built, true);
+    statefulWrapper.built = false;
+
+    statefulWrapper.trigger();
+    flipStatefulWidget(tester);
+    await tester.pump();
+    expect(statefulWrapper.built, true);
+    statefulWrapper.built = false;
+
+    statefulWrapper.trigger();
+    flipStatefulWidget(tester);
+    await tester.pump();
+    expect(statefulWrapper.built, true);
+    statefulWrapper.built = false;
+  });
+}

packages/flutter_test/lib/src/binding.dart

@@ -22,9 +22,12 @@ import 'stack_manipulation.dart';
 
 /// Phases that can be reached by [WidgetTester.pumpWidget] and
 /// [TestWidgetsFlutterBinding.pump].
+///
+/// See [WidgetsBinding.beginFrame] for a more detailed description of some of
+/// these phases.
 // TODO(ianh): Merge with near-identical code in the rendering test code.
 enum EnginePhase {
-  /// The build phase in the widgets library. See [BuildOwner.buildDirtyElements].
+  /// The build phase in the widgets library. See [BuildOwner.buildScope].
   build,
 
   /// The layout phase in the rendering library. See [PipelineOwner.flushLayout].
@@ -396,6 +399,9 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
 ///
 /// This binding controls time, allowing tests to verify long
 /// animation sequences without having to execute them in real time.
+///
+/// This class assumes it is always run in checked mode (since tests are always
+/// run in checked mode).
 class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
   @override
   void initInstances() {
@@ -447,7 +453,9 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
   @override
   void beginFrame() {
     assert(inTest);
-    buildOwner.buildDirtyElements();
+    try {
+      debugBuildingDirtyElements = true;
+      buildOwner.buildScope(renderViewElement);
       if (_phase == EnginePhase.build)
         return;
       assert(renderView != null);
@@ -466,7 +474,10 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
       pipelineOwner.flushSemantics();
       if (_phase == EnginePhase.flushSemantics)
         return;
+    } finally {
       buildOwner.finalizeTree();
+      debugBuildingDirtyElements = false;
+    }
   }
 
   @override