Define LayoutWidgetBuilder in terms of BoxConstraints instead of Size (#4578)

examples/flutter_gallery/lib/demo/pesto_demo.dart

@@ -90,7 +90,8 @@ class _PestoDemoState extends State<PestoDemo> {
         )
       ],
       flexibleSpace: new LayoutBuilder(
-        builder: (BuildContext context, Size size) {
+        builder: (BuildContext context, BoxConstraints constraints) {
+          final Size size = constraints.biggest;
           double appBarHeight = size.height - statusBarHeight;
           double bestHeight = _kLogoImages.keys.lastWhere(
             (double height) => appBarHeight >= height
@@ -293,7 +294,12 @@ class _RecipePageState extends State<_RecipePage> {
     return new Stack(
       children: <Widget>[
         _buildContainer(context),
-        new Padding(
+        new Positioned(
+          top: 0.0,
+          left: 0.0,
+          right: 0.0,
+          child: new Container(
+            height: kToolBarHeight + statusBarHeight,
             padding: new EdgeInsets.only(top: statusBarHeight),
             child: new AppBar(
               backgroundColor: Colors.transparent,
@@ -316,6 +322,7 @@ class _RecipePageState extends State<_RecipePage> {
               ]
             )
           )
+        )
       ]
     );
   }

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

@@ -178,8 +178,9 @@ class AppBar extends StatelessWidget {
     return ((appBarHeight - statusBarHeight) / tabBarHeight).clamp(0.0, 1.0);
   }
 
-  Widget _buildForSize(BuildContext context, Size size) {
-    assert(size.height < double.INFINITY);
+  Widget _buildForSize(BuildContext context, BoxConstraints constraints) {
+    assert(constraints.maxHeight < double.INFINITY);
+    final Size size = constraints.biggest;
     final double statusBarHeight = MediaQuery.of(context).padding.top;
     final ThemeData theme = Theme.of(context);
 
@@ -274,6 +275,8 @@ class AppBar extends StatelessWidget {
           child: appBar
         )
       );
+    } else if (flexibleSpace != null) {
+      appBar = new Positioned(top: 0.0, left: 0.0, right: 0.0, child: appBar);
     }
 
     if (flexibleSpace != null) {

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

@@ -9,14 +9,15 @@ import 'package:flutter/rendering.dart';
 import 'package:meta/meta.dart';
 
 /// The signature of the [LayoutBuilder] builder function.
-typedef Widget LayoutWidgetBuilder(BuildContext context, Size size);
+typedef Widget LayoutWidgetBuilder(BuildContext context, BoxConstraints constraints);
 
 /// Builds a widget tree that can depend on the parent widget's size.
 ///
 /// Similar to the [Builder] widget except that the framework calls the [builder]
-/// function at layout time and provides the parent widget's size. This is useful
-/// when the parent constrains the child's size and doesn't depend on the child's
-/// intrinsic size.
+/// function at layout time and provides the parent widget's constraints. This
+/// is useful when the parent constrains the child's size and doesn't depend on
+/// the child's intrinsic size. The LayoutBuilder's final size will match its
+/// child's size.
 class LayoutBuilder extends RenderObjectWidget {
   /// Creates a widget that defers its building until layout.
   ///
@@ -89,20 +90,16 @@ class _RenderLayoutBuilder extends RenderBox with RenderObjectWithChildMixin<Ren
     return 0.0;
   }
 
-  @override
-  bool get sizedByParent => true;
-
-  @override
-  void performResize() {
-    size = constraints.biggest;
-  }
-
   @override
   void performLayout() {
     if (callback != null)
       invokeLayoutCallback(callback);
-    if (child != null)
-      child.layout(constraints.loosen());
+    if (child != null) {
+      child.layout(constraints, parentUsesSize: true);
+      size = constraints.constrain(child.size);
+    } else {
+      size = constraints.biggest;
+    }
   }
 
   @override
@@ -169,7 +166,7 @@ class _LayoutBuilderElement extends RenderObjectElement {
     owner.lockState(() {
       Widget built;
       try {
-        built = widget.builder(this, constraints.biggest);
+        built = widget.builder(this, constraints);
         debugWidgetBuilderValue(widget, built);
       } catch (e, stack) {
         _debugReportException('building $widget', e, stack);

packages/flutter/test/widget/layout_builder_test.dart

@@ -10,19 +10,20 @@ void main() {
   testWidgets('LayoutBuilder parent size', (WidgetTester tester) async {
     Size layoutBuilderSize;
     Key childKey = new UniqueKey();
+    Key parentKey = new UniqueKey();
 
     await tester.pumpWidget(
       new Center(
-        child: new SizedBox(
-          width: 100.0,
-          height: 200.0,
+        child: new ConstrainedBox(
+          constraints: const BoxConstraints(maxWidth: 100.0, maxHeight: 200.0),
           child: new LayoutBuilder(
-            builder: (BuildContext context, Size size) {
-              layoutBuilderSize = size;
+            key: parentKey,
+            builder: (BuildContext context, BoxConstraints constraints) {
+              layoutBuilderSize = constraints.biggest;
               return new SizedBox(
                 key: childKey,
-                width: size.width / 2.0,
-                height: size.height / 2.0
+                width: layoutBuilderSize.width / 2.0,
+                height: layoutBuilderSize.height / 2.0
               );
             }
           )
@@ -31,21 +32,26 @@ void main() {
     );
 
     expect(layoutBuilderSize, const Size(100.0, 200.0));
-    RenderBox box = tester.renderObject(find.byKey(childKey));
-    expect(box.size, equals(const Size(50.0, 100.0)));
+    RenderBox parentBox = tester.renderObject(find.byKey(parentKey));
+    expect(parentBox.size, equals(const Size(50.0, 100.0)));
+    RenderBox childBox = tester.renderObject(find.byKey(childKey));
+    expect(childBox.size, equals(const Size(50.0, 100.0)));
   });
 
   testWidgets('LayoutBuilder stateful child', (WidgetTester tester) async {
     Size layoutBuilderSize;
     StateSetter setState;
     Key childKey = new UniqueKey();
+    Key parentKey = new UniqueKey();
     double childWidth = 10.0;
     double childHeight = 20.0;
 
     await tester.pumpWidget(
-      new LayoutBuilder(
-        builder: (BuildContext context, Size size) {
-          layoutBuilderSize = size;
+      new Center(
+        child: new LayoutBuilder(
+          key: parentKey,
+          builder: (BuildContext context, BoxConstraints constraints) {
+            layoutBuilderSize = constraints.biggest;
             return new StatefulBuilder(
               builder: (BuildContext context, StateSetter setter) {
                 setState = setter;
@@ -58,19 +64,24 @@ void main() {
             );
           }
         )
+      )
     );
 
     expect(layoutBuilderSize, equals(const Size(800.0, 600.0)));
-    RenderBox box = tester.renderObject(find.byKey(childKey));
-    expect(box.size, equals(const Size(10.0, 20.0)));
+    RenderBox parentBox = tester.renderObject(find.byKey(parentKey));
+    expect(parentBox.size, equals(const Size(10.0, 20.0)));
+    RenderBox childBox = tester.renderObject(find.byKey(childKey));
+    expect(childBox.size, equals(const Size(10.0, 20.0)));
 
     setState(() {
       childWidth = 100.0;
       childHeight = 200.0;
     });
     await tester.pump();
-    box = tester.renderObject(find.byKey(childKey));
-    expect(box.size, equals(const Size(100.0, 200.0)));
+    parentBox = tester.renderObject(find.byKey(parentKey));
+    expect(parentBox.size, equals(const Size(100.0, 200.0)));
+    childBox = tester.renderObject(find.byKey(childKey));
+    expect(childBox.size, equals(const Size(100.0, 200.0)));
   });
 
   testWidgets('LayoutBuilder stateful parent', (WidgetTester tester) async {
@@ -89,12 +100,12 @@ void main() {
               width: childWidth,
               height: childHeight,
               child: new LayoutBuilder(
-                builder: (BuildContext context, Size size) {
-                  layoutBuilderSize = size;
+                builder: (BuildContext context, BoxConstraints constraints) {
+                  layoutBuilderSize = constraints.biggest;
                   return new SizedBox(
                     key: childKey,
-                    width: size.width,
-                    height: size.height
+                    width: layoutBuilderSize.width,
+                    height: layoutBuilderSize.height
                   );
                 }
               )
@@ -117,10 +128,11 @@ void main() {
     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) {
+      builder: (BuildContext context, BoxConstraints constraints) {
         built += 1;
         return new Container();
       }
@@ -143,7 +155,7 @@ void main() {
   testWidgets('LayoutBuilder and Inherited -- do rebuild when using inherited', (WidgetTester tester) async {
     int built = 0;
     Widget target = new LayoutBuilder(
-      builder: (BuildContext context, Size size) {
+      builder: (BuildContext context, BoxConstraints constraints) {
         built += 1;
         MediaQuery.of(context);
         return new Container();