LayoutBuilder widget (#3670)

* LayoutBuilder Widget

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

@@ -2897,9 +2897,11 @@ class Builder extends StatelessWidget {
 
   /// Called to obtain the child widget.
   ///
-  /// This function is invoked whether this widget is included in its parent's
+  /// This function is invoked whenever this widget is included in its parent's
   /// build and the old widget (if any) that it synchronizes with has a distinct
-  /// object identity.
+  /// object identity. Typically the parent's build method will construct
+  /// a new tree of widgets and so a new Builder child will not be [identical]
+  /// to the corresponding old one.
   final WidgetBuilder builder;
 
   @override

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

@@ -4,6 +4,7 @@
 
 import 'dart:collection';
 
+import 'package:flutter/foundation.dart';
 import 'framework.dart';
 import 'table.dart';
 
@@ -99,3 +100,18 @@ bool debugCheckHasTable(BuildContext context) {
   });
   return true;
 }
+
+void debugWidgetBuilderValue(Widget widget, Widget built) {
+  assert(() {
+    if (built == null) {
+      throw new FlutterError(
+        'A build function returned null.\n'
+        'The offending widget is: $widget\n'
+        'Build functions must never return null. '
+        'To return an empty space that causes the building widget to fill available room, return "new Container()". '
+        'To return an empty space that takes as little room as possible, return "new Container(width: 0.0, height: 0.0)".'
+      );
+    }
+    return true;
+  });
+}

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

@@ -1513,18 +1513,7 @@ abstract class ComponentElement extends BuildableElement {
     Widget built;
     try {
       built = _builder(this);
-      assert(() {
-        if (built == null) {
-          throw new FlutterError(
-            'A build function returned null.\n'
-            'The offending widget is: $widget\n'
-            'Build functions must never return null. '
-            'To return an empty space that causes the building widget to fill available room, return "new Container()". '
-            'To return an empty space that takes as little room as possible, return "new Container(width: 0.0, height: 0.0)".'
-          );
-        }
-        return true;
-      });
+      debugWidgetBuilderValue(widget, built);
     } catch (e, stack) {
       _debugReportException('building $_widget', e, stack);
       built = new ErrorWidget(e);

packages/flutter/lib/src/widgets/layout_builder.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 'debug.dart';
+import 'framework.dart';
+
+import 'package:flutter/rendering.dart';
+
+/// The signature of the [LayoutBuilder] builder function.
+typedef Widget LayoutWidgetBuilder(BuildContext context, Size size);
+
+/// 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.
+class LayoutBuilder extends RenderObjectWidget {
+  LayoutBuilder({ Key key, this.builder }) : super(key: key);
+
+  /// Called at layout time to construct the widget tree. The builder must not
+  /// return null.
+  final LayoutWidgetBuilder builder;
+
+  @override
+  _LayoutBuilderElement createElement() => new _LayoutBuilderElement(this);
+
+  @override
+  _RenderLayoutBuilder createRenderObject(BuildContext context) => new _RenderLayoutBuilder();
+}
+
+class _RenderLayoutBuilder extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+  _RenderLayoutBuilder({ LayoutCallback callback }) : _callback = callback;
+
+  LayoutCallback get callback => _callback;
+  LayoutCallback _callback;
+  void set callback(LayoutCallback value) {
+    if (value == _callback)
+      return;
+    _callback = value;
+    markNeedsLayout();
+  }
+
+  double getIntrinsicWidth(BoxConstraints constraints) => constraints.constrainWidth();
+
+  double getIntrinsicHeight(BoxConstraints constraints) => constraints.constrainHeight();
+
+  @override
+  double getMinIntrinsicWidth(BoxConstraints constraints) {
+    assert(constraints.debugAssertIsValid());
+    return getIntrinsicWidth(constraints);
+  }
+
+  @override
+  double getMaxIntrinsicWidth(BoxConstraints constraints) {
+    assert(constraints.debugAssertIsValid());
+    return getIntrinsicWidth(constraints);
+  }
+
+  @override
+  double getMinIntrinsicHeight(BoxConstraints constraints) {
+    assert(constraints.debugAssertIsValid());
+    return getIntrinsicHeight(constraints);
+  }
+
+  @override
+  double getMaxIntrinsicHeight(BoxConstraints constraints) {
+    assert(constraints.debugAssertIsValid());
+    return getIntrinsicHeight(constraints);
+  }
+
+  @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(), parentUsesSize: false);
+  }
+
+  @override
+  bool hitTestChildren(HitTestResult result, { Point position }) {
+    return child?.hitTest(result, position: position) ?? false;
+  }
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    if (child != null)
+      context.paintChild(child, offset);
+  }
+}
+
+class _LayoutBuilderElement extends RenderObjectElement {
+  _LayoutBuilderElement(LayoutBuilder widget) : super(widget);
+
+  @override
+  LayoutBuilder get widget => super.widget;
+
+  @override
+  _RenderLayoutBuilder get renderObject => super.renderObject;
+
+  Element _child;
+
+  @override
+  void visitChildren(ElementVisitor visitor) {
+    if (_child != null)
+      visitor(_child);
+  }
+
+  @override
+  void mount(Element parent, dynamic newSlot) {
+    super.mount(parent, newSlot);    // Creates the renderObject.
+    renderObject.callback = _layout; // The _child will be built during layout.
+  }
+
+  @override
+  void update(LayoutBuilder newWidget) {
+    assert(widget != newWidget);
+    super.update(newWidget);
+    assert(widget == newWidget);
+    renderObject.callback = _layout;
+    renderObject.markNeedsLayout();
+  }
+
+  @override
+  void unmount() {
+    renderObject.callback = null;
+    super.unmount();
+  }
+
+  void _layout(BoxConstraints constraints) {
+    if (widget.builder == null)
+      return;
+    owner.lockState(() {
+      Widget built;
+      try {
+        built = widget.builder(this, constraints.biggest);
+        debugWidgetBuilderValue(widget, built);
+      } catch (e, stack) {
+        _debugReportException('building $widget', e, stack);
+        built = new ErrorWidget(e);
+      }
+
+      try {
+        _child = updateChild(_child, built, null);
+        assert(_child != null);
+      } catch (e, stack) {
+        _debugReportException('building $widget', e, stack);
+        built = new ErrorWidget(e);
+        _child = updateChild(null, built, slot);
+      }
+    }, building: true);
+  }
+
+  @override
+  void insertChildRenderObject(RenderObject child, dynamic slot) {
+    final RenderObjectWithChildMixin<RenderObject> renderObject = this.renderObject;
+    assert(slot == null);
+    renderObject.child = child;
+    assert(renderObject == this.renderObject);
+  }
+
+  @override
+  void moveChildRenderObject(RenderObject child, dynamic slot) {
+    assert(false);
+  }
+
+  @override
+  void removeChildRenderObject(RenderObject child) {
+    final _RenderLayoutBuilder renderObject = this.renderObject;
+    assert(renderObject.child == child);
+    renderObject.child = null;
+    assert(renderObject == this.renderObject);
+  }
+}
+
+void _debugReportException(String context, dynamic exception, StackTrace stack) {
+  FlutterError.reportError(new FlutterErrorDetails(
+    exception: exception,
+    stack: stack,
+    library: 'widgets library',
+    context: context
+  ));
+}

packages/flutter/lib/widgets.dart

@@ -3,7 +3,8 @@
 // found in the LICENSE file.
 
 /// The Flutter widget framework.
-/// 
+///
+///
 /// To use, import `package:flutter/widgets.dart`.
 library widgets;
 
@@ -25,6 +26,7 @@ export 'src/widgets/gesture_detector.dart';
 export 'src/widgets/gridpaper.dart';
 export 'src/widgets/heroes.dart';
 export 'src/widgets/implicit_animations.dart';
+export 'src/widgets/layout_builder.dart';
 export 'src/widgets/lazy_block.dart';
 export 'src/widgets/locale_query.dart';
 export 'src/widgets/media_query.dart';

packages/flutter/test/widget/layout_builder_test.dart

+// Copyright 2015 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';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
+import 'package:test/test.dart';
+
+void main() {
+  testWidgets('LayoutBuilder parent size', (WidgetTester tester) {
+    Size layoutBuilderSize;
+    Key childKey = new UniqueKey();
+
+    tester.pumpWidget(
+      new Center(
+        child: new SizedBox(
+          width: 100.0,
+          height: 200.0,
+          child: new LayoutBuilder(
+            builder: (BuildContext context, Size size) {
+              layoutBuilderSize = size;
+              return new SizedBox(
+                key: childKey,
+                width: size.width / 2.0,
+                height: size.height / 2.0
+              );
+            }
+          )
+        )
+      )
+    );
+
+    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)));
+  });
+
+  testWidgets('LayoutBuilder stateful child', (WidgetTester tester) {
+    Size layoutBuilderSize;
+    StateSetter setState;
+    Key childKey = new UniqueKey();
+    double childWidth = 10.0;
+    double childHeight = 20.0;
+
+    tester.pumpWidget(
+      new LayoutBuilder(
+        builder: (BuildContext context, Size size) {
+          layoutBuilderSize = size;
+          return new StatefulBuilder(
+            builder: (BuildContext context, StateSetter setter) {
+              setState = setter;
+              return new SizedBox(
+                key: childKey,
+                width: childWidth,
+                height: childHeight
+              );
+            }
+          );
+        }
+      )
+    );
+
+    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)));
+
+    setState(() {
+      childWidth = 100.0;
+      childHeight = 200.0;
+    });
+    tester.pump();
+    box = tester.renderObject(find.byKey(childKey));
+    expect(box.size, equals(const Size(100.0, 200.0)));
+  });
+
+  testWidgets('LayoutBuilder stateful parent', (WidgetTester tester) {
+    Size layoutBuilderSize;
+    StateSetter setState;
+    Key childKey = new UniqueKey();
+    double childWidth = 10.0;
+    double childHeight = 20.0;
+
+    tester.pumpWidget(
+      new Center(
+        child: new StatefulBuilder(
+          builder: (BuildContext context, StateSetter setter) {
+            setState = setter;
+            return new SizedBox(
+              width: childWidth,
+              height: childHeight,
+              child: new LayoutBuilder(
+                builder: (BuildContext context, Size size) {
+                  layoutBuilderSize = size;
+                  return new SizedBox(
+                    key: childKey,
+                    width: size.width,
+                    height: size.height
+                  );
+                }
+              )
+            );
+          }
+        )
+      )
+    );
+
+    expect(layoutBuilderSize, equals(const Size(10.0, 20.0)));
+    RenderBox box = tester.renderObject(find.byKey(childKey));
+    expect(box.size, equals(const Size(10.0, 20.0)));
+
+    setState(() {
+      childWidth = 100.0;
+      childHeight = 200.0;
+    });
+    tester.pump();
+    box = tester.renderObject(find.byKey(childKey));
+    expect(box.size, equals(const Size(100.0, 200.0)));
+  });
+}