Prevent looking up inherited widget values from initState() (#5609)

examples/flutter_gallery/lib/demo/animation_demo.dart

@@ -121,9 +121,6 @@ class _PointDemoState extends State<_PointDemo> {
   @override
   void initState() {
     super.initState();
-    final Size screenSize = MediaQuery.of(context).size;
-    _begin = new Point(screenSize.width * 0.5, screenSize.height * 0.2);
-    _end = new Point(screenSize.width * 0.1, screenSize.height * 0.4);
     _animation = new CurvedAnimation(parent: config.controller, curve: Curves.ease);
   }
 
@@ -179,6 +176,10 @@ class _PointDemoState extends State<_PointDemo> {
 
   @override
   Widget build(BuildContext context) {
+    final Size screenSize = MediaQuery.of(context).size;
+    _begin = new Point(screenSize.width * 0.5, screenSize.height * 0.2);
+    _end = new Point(screenSize.width * 0.1, screenSize.height * 0.4);
+
     final MaterialPointArcTween arc = new MaterialPointArcTween(begin: _begin, end: _end);
     return new RawGestureDetector(
       behavior: _dragTarget == null ? HitTestBehavior.deferToChild : HitTestBehavior.opaque,
@@ -281,15 +282,6 @@ class _RectangleDemoState extends State<_RectangleDemo> {
   @override
   void initState() {
     super.initState();
-    final Size screenSize = MediaQuery.of(context).size;
-    _begin = new Rect.fromLTWH(
-      screenSize.width * 0.5, screenSize.height * 0.2,
-      screenSize.width * 0.4, screenSize.height * 0.2
-    );
-    _end = new Rect.fromLTWH(
-      screenSize.width * 0.1, screenSize.height * 0.4,
-      screenSize.width * 0.3, screenSize.height * 0.3
-    );
     _animation = new CurvedAnimation(parent: config.controller, curve: Curves.ease);
   }
 
@@ -344,6 +336,16 @@ class _RectangleDemoState extends State<_RectangleDemo> {
 
   @override
   Widget build(BuildContext context) {
+    final Size screenSize = MediaQuery.of(context).size;
+    _begin = new Rect.fromLTWH(
+      screenSize.width * 0.5, screenSize.height * 0.2,
+      screenSize.width * 0.4, screenSize.height * 0.2
+    );
+    _end = new Rect.fromLTWH(
+      screenSize.width * 0.1, screenSize.height * 0.4,
+      screenSize.width * 0.3, screenSize.height * 0.3
+    );
+
     final MaterialRectArcTween arc = new MaterialRectArcTween(begin: _begin, end: _end);
     return new RawGestureDetector(
       behavior: _dragTarget == null ? HitTestBehavior.deferToChild : HitTestBehavior.opaque,

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

@@ -157,25 +157,12 @@ class RefreshIndicatorState extends State<RefreshIndicator> {
     _sizeFactor = new Tween<double>(begin: 0.0, end: _kDragSizeFactorLimit).animate(_sizeController);
     _scaleFactor = new Tween<double>(begin: 1.0, end: 0.0).animate(_scaleController);
 
-    final ThemeData theme = Theme.of(context);
-
     // The "value" of the circular progress indicator during a drag.
     _value = new Tween<double>(
       begin: 0.0,
       end: 0.75
     )
     .animate(_sizeController);
-
-    // Fully opaque when we've reached config.displacement.
-    _valueColor = new ColorTween(
-      begin: (config.color ?? theme.accentColor).withOpacity(0.0),
-      end: (config.color ?? theme.accentColor).withOpacity(1.0)
-    )
-    .animate(new CurvedAnimation(
-      parent: _sizeController,
-      curve: new Interval(0.0, 1.0 / _kDragSizeFactorLimit)
-    ));
-
   }
 
   @override
@@ -346,8 +333,20 @@ class RefreshIndicatorState extends State<RefreshIndicator> {
 
   @override
   Widget build(BuildContext context) {
+    final ThemeData theme = Theme.of(context);
     final bool showIndeterminateIndicator =
       _mode == _RefreshIndicatorMode.refresh || _mode == _RefreshIndicatorMode.dismiss;
+
+    // Fully opaque when we've reached config.displacement.
+    _valueColor = new ColorTween(
+      begin: (config.color ?? theme.accentColor).withOpacity(0.0),
+      end: (config.color ?? theme.accentColor).withOpacity(1.0)
+    )
+    .animate(new CurvedAnimation(
+      parent: _sizeController,
+      curve: new Interval(0.0, 1.0 / _kDragSizeFactorLimit)
+    ));
+
     return new Listener(
       onPointerDown: _handlePointerDown,
       onPointerMove: _handlePointerMove,

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

@@ -768,19 +768,20 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
   bool _valueIsChanging = false;
   int _lastSelectedIndex = -1;
 
-  void _initSelection(TabBarSelectionState<T> selection) {
+  void _initSelection(TabBarSelectionState<T> newSelection) {
+    if (_selection == newSelection)
+      return;
     _selection?.unregisterAnimationListener(this);
-    _selection = selection;
+    _selection = newSelection;
     _selection?.registerAnimationListener(this);
+    if (_selection != null)
+      _lastSelectedIndex = _selection.index;
   }
 
   @override
   void initState() {
     super.initState();
     scrollBehavior.isScrollable = config.isScrollable;
-    _initSelection(TabBarSelection.of(context));
-    if (_selection != null)
-      _lastSelectedIndex = _selection.index;
   }
 
   @override
@@ -990,8 +991,7 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
   @override
   Widget buildContent(BuildContext context) {
     TabBarSelectionState<T> newSelection = TabBarSelection.of(context);
-    if (_selection != newSelection)
-      _initSelection(newSelection);
+    _initSelection(newSelection);
 
     assert(config.labels.isNotEmpty);
     assert(Material.of(context) != null);
@@ -1097,18 +1097,14 @@ class _TabBarViewState<T> extends PageableListState<TabBarView<T>> implements Ta
   @override
   TargetPlatform get platform => Theme.of(context).platform;
 
-  void _initSelection(TabBarSelectionState<T> selection) {
-    _selection = selection;
-    if (_selection != null) {
-      _selection.registerAnimationListener(this);
+  void _initSelection(TabBarSelectionState<T> newSelection) {
+    if (_selection == newSelection)
+      return;
+    _selection?.unregisterAnimationListener(this);
+    _selection = newSelection;
+    _selection?.registerAnimationListener(this);
+    if (_selection != null)
       _updateItemsAndScrollBehavior();
-    }
-  }
-
-  @override
-  void initState() {
-    super.initState();
-    _initSelection(TabBarSelection.of(context));
   }
 
   @override
@@ -1243,8 +1239,7 @@ class _TabBarViewState<T> extends PageableListState<TabBarView<T>> implements Ta
   @override
   Widget buildContent(BuildContext context) {
     TabBarSelectionState<T> newSelection = TabBarSelection.of(context);
-    if (_selection != newSelection)
-      _initSelection(newSelection);
+    _initSelection(newSelection);
     return new PageViewport(
       itemsWrap: config.itemsWrap,
       mainAxis: config.scrollDirection,

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

@@ -2183,6 +2183,24 @@ class StatefulElement extends ComponentElement {
     _state = null;
   }
 
+  @override
+  InheritedWidget inheritFromWidgetOfExactType(Type targetType) {
+    assert(() {
+      if (state._debugLifecycleState == _StateLifecycle.ready)
+        return true;
+      throw new FlutterError(
+        "inheritFromWidgetOfExactType($targetType) was called before ${_state.runtimeType}.initState() completed.\n"
+        "When an inherited widget changes, for example if the value of Theme.of() changes, "
+        "its dependent widgets are rebuilt. If the dependent widget's reference to "
+        "the inherited widget is in an constructor or an initState() method, "
+        "then the rebuilt dependent widget will not reflect the changes in the "
+        "inherited widget.\n"
+        "Typically references to to inherited widgets should occur in widget build() methods.\n"
+      );
+    });
+    return super.inheritFromWidgetOfExactType(targetType);
+  }
+
   @override
   void dependenciesChanged() {
     super.dependenciesChanged();

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

@@ -187,12 +187,6 @@ class _ImageState extends State<Image> {
   ImageStream _imageStream;
   ImageInfo _imageInfo;
 
-  @override
-  void initState() {
-    super.initState();
-    _resolveImage();
-  }
-
   @override
   void didUpdateConfig(Image oldConfig) {
     if (config.image != oldConfig.image)
@@ -240,6 +234,15 @@ class _ImageState extends State<Image> {
 
   @override
   Widget build(BuildContext context) {
+    // This one-time initialization could have been done in initState() since
+    // changes to the inherited widgets that _resolveImage depends on, notably
+    // DefaultAssetBundle, are handle by the dependenciesChanged() method. We're
+    // doing it here instead to avoid the assert that disallows references to
+    // inherited widgets at initState() time. We've found that assert to be a
+    // reliable source of real bugs, and that it is worth this minor inconvenience.
+    if (_imageStream == null)
+      _resolveImage();
+
     return new RawImage(
       image: _imageInfo?.image,
       width: config.width,

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

@@ -255,12 +255,6 @@ abstract class PageableState<T extends Pageable> extends ScrollableState<T> {
     return null;
   }
 
-  @override
-  void initState() {
-    super.initState();
-    _updateScrollBehavior();
-  }
-
   @override
   void didUpdateConfig(Pageable oldConfig) {
     super.didUpdateConfig(oldConfig);

packages/flutter/test/widget/inherited_test.dart

@@ -29,6 +29,29 @@ class ValueInherited extends InheritedWidget {
   bool updateShouldNotify(ValueInherited oldWidget) => value != oldWidget.value;
 }
 
+class ExpectFail extends StatefulWidget {
+  ExpectFail(this.onError);
+  final VoidCallback onError;
+
+  @override
+  ExpectFailState createState() => new ExpectFailState();
+}
+
+class ExpectFailState extends State<ExpectFail> {
+  @override
+  void initState() {
+    super.initState();
+    try {
+      context.inheritFromWidgetOfExactType(TestInherited); // should fail
+    } catch (e) {
+      config.onError();
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) => new Container();
+}
+
 void main() {
   testWidgets('Inherited notifies dependents', (WidgetTester tester) async {
     List<TestInherited> log = <TestInherited>[];
@@ -437,4 +460,16 @@ void main() {
     );
     expect(buildCount, equals(2));
   });
+
+  testWidgets('initState() dependency on Inherited asserts', (WidgetTester tester) async {
+    // This is a regression test for https://github.com/flutter/flutter/issues/5491
+    bool exceptionCaught = false;
+
+    TestInherited parent = new TestInherited(child: new ExpectFail(() {
+      exceptionCaught = true;
+    }));
+    await tester.pumpWidget(parent);
+
+    expect(exceptionCaught, isTrue);
+  });
 }