Teach LayoutBuilder about Inherited widgets. (#4140)

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

@@ -1825,7 +1825,6 @@ class InheritedElement extends _ProxyElement {
   /// the build phase.
   void dispatchDependenciesChanged() {
     for (Element dependent in _dependents) {
-      dependent.dependenciesChanged();
       assert(() {
         // check that it really is our descendant
         Element ancestor = dependent._parent;
@@ -1835,6 +1834,7 @@ class InheritedElement extends _ProxyElement {
       });
       // check that it really deepends on us
       assert(dependent._dependencies.contains(this));
+      dependent.dependenciesChanged();
     }
   }
 }

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

@@ -132,6 +132,14 @@ class _LayoutBuilderElement extends RenderObjectElement {
     renderObject.markNeedsLayout();
   }
 
+  @override
+  void performRebuild() {
+    // This gets called if markNeedsBuild() is called on us.
+    // That might happen if, e.g., our builder uses Inherited widgets.
+    renderObject.markNeedsLayout();
+    super.performRebuild(); // calls widget.updateRenderObject
+  }
+
   @override
   void unmount() {
     renderObject.callback = null;

packages/flutter/test/widget/layout_builder_test.dart

@@ -116,4 +116,51 @@ void main() {
     box = tester.renderObject(find.byKey(childKey));
     expect(box.size, equals(const Size(100.0, 200.0)));
   });
+
+  testWidgets('LayoutBuilder and Inherited -- do not rebuild when not using inherited', (WidgetTester tester) async {
+    int built = 0;
+    Widget target = new LayoutBuilder(
+      builder: (BuildContext context, Size size) {
+        built += 1;
+        return new Container();
+      }
+    );
+    expect(built, 0);
+
+    await tester.pumpWidget(new MediaQuery(
+      data: new MediaQueryData(size: const Size(400.0, 300.0)),
+      child: target
+    ));
+    expect(built, 1);
+
+    await tester.pumpWidget(new MediaQuery(
+      data: new MediaQueryData(size: const Size(300.0, 400.0)),
+      child: target
+    ));
+    expect(built, 1);
+  });
+
+  testWidgets('LayoutBuilder and Inherited -- do rebuild when using inherited', (WidgetTester tester) async {
+    int built = 0;
+    Widget target = new LayoutBuilder(
+      builder: (BuildContext context, Size size) {
+        built += 1;
+        MediaQuery.of(context);
+        return new Container();
+      }
+    );
+    expect(built, 0);
+
+    await tester.pumpWidget(new MediaQuery(
+      data: new MediaQueryData(size: const Size(400.0, 300.0)),
+      child: target
+    ));
+    expect(built, 1);
+
+    await tester.pumpWidget(new MediaQuery(
+      data: new MediaQueryData(size: const Size(300.0, 400.0)),
+      child: target
+    ));
+    expect(built, 2);
+  });
 }