Adds CustomMultiChildLayout

CustomMultiChildLayout delegates to a MultiChildLayoutDelegate. Widget children are specified by their index in the list of children.

packages/flutter/lib/rendering.dart

@@ -10,6 +10,7 @@ export 'src/rendering/basic_types.dart';
 export 'src/rendering/binding.dart';
 export 'src/rendering/block.dart';
 export 'src/rendering/box.dart';
+export 'src/rendering/custom_layout.dart';
 export 'src/rendering/debug.dart';
 export 'src/rendering/editable_paragraph.dart';
 export 'src/rendering/error.dart';

packages/flutter/lib/src/rendering/custom_layout.dart

@@ -0,0 +1,111 @@
+// 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 'box.dart';
+import 'object.dart';
+
+class _MultiChildParentData extends ContainerBoxParentDataMixin<RenderBox> { }
+
+abstract class MultiChildLayoutDelegate {
+  final List<RenderBox> _indexToChild = <RenderBox>[];
+
+  /// Returns the size of this object given the incomming constraints.
+  Size getSize(BoxConstraints constraints) => constraints.biggest;
+
+  /// Ask the child to update its layout within the limits specified by
+  /// the constraints parameter. The child's size is returned.
+  Size layoutChild(int childIndex, BoxConstraints constraints) {
+    final RenderBox child = _indexToChild[childIndex];
+    child.layout(constraints, parentUsesSize: true);
+    return child.size;
+  }
+
+  /// Specify the child's origin relative to this origin.
+  void positionChild(int childIndex, Point position) {
+    final RenderBox child = _indexToChild[childIndex];
+    final _MultiChildParentData childParentData = child.parentData;
+    childParentData.position = position;
+  }
+
+  void _callPerformLayout(Size size, BoxConstraints constraints, RenderBox firstChild) {
+    RenderBox child = firstChild;
+    while (child != null) {
+      _indexToChild.add(child);
+      final _MultiChildParentData childParentData = child.parentData;
+      child = childParentData.nextSibling;
+    }
+    performLayout(size, constraints, _indexToChild.length);
+    _indexToChild.clear();
+  }
+
+  /// Layout and position all children given this widget's size and the specified
+  /// constraints. This method must apply layoutChild() to each child. It should
+  /// specify the final position of each child with positionChild().
+  void performLayout(Size size, BoxConstraints constraints, int childCount);
+}
+
+class RenderCustomMultiChildLayoutBox extends RenderBox
+  with ContainerRenderObjectMixin<RenderBox, _MultiChildParentData>,
+       RenderBoxContainerDefaultsMixin<RenderBox, _MultiChildParentData> {
+  RenderCustomMultiChildLayoutBox({
+    List<RenderBox> children,
+    MultiChildLayoutDelegate delegate
+  }) : _delegate = delegate {
+    assert(delegate != null);
+    addAll(children);
+  }
+
+  void setupParentData(RenderBox child) {
+    if (child.parentData is! _MultiChildParentData)
+      child.parentData = new _MultiChildParentData();
+  }
+
+  MultiChildLayoutDelegate get delegate => _delegate;
+  MultiChildLayoutDelegate _delegate;
+  void set delegate (MultiChildLayoutDelegate newDelegate) {
+    assert(newDelegate != null);
+    if (_delegate == newDelegate)
+      return;
+    _delegate = newDelegate;
+    markNeedsLayout();
+  }
+
+  Size _getSize(BoxConstraints constraints) {
+    return constraints.constrain(_delegate.getSize(constraints));
+  }
+
+  double getMinIntrinsicWidth(BoxConstraints constraints) {
+    return _getSize(constraints).width;
+  }
+
+  double getMaxIntrinsicWidth(BoxConstraints constraints) {
+    return _getSize(constraints).width;
+  }
+
+  double getMinIntrinsicHeight(BoxConstraints constraints) {
+    return _getSize(constraints).height;
+  }
+
+  double getMaxIntrinsicHeight(BoxConstraints constraints) {
+    return _getSize(constraints).height;
+  }
+
+  bool get sizedByParent => true;
+
+  void performResize() {
+    size = _getSize(constraints);
+  }
+
+  void performLayout() {
+    delegate._callPerformLayout(size, constraints, firstChild);
+  }
+
+  void paint(PaintingContext context, Offset offset) {
+    defaultPaint(context, offset);
+  }
+
+  void hitTestChildren(HitTestResult result, { Point position }) {
+    defaultHitTestChildren(result, position: position);
+  }
+}

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

@@ -282,6 +282,29 @@ class CustomOneChildLayout extends OneChildRenderObjectWidget {
   }
 }
 
+class CustomMultiChildLayout extends MultiChildRenderObjectWidget {
+  CustomMultiChildLayout(List<Widget> children, {
+    Key key,
+    this.delegate,
+    this.token
+  }) : super(key: key, children: children) {
+    assert(delegate != null);
+  }
+
+  final MultiChildLayoutDelegate delegate;
+  final Object token;
+
+  RenderCustomMultiChildLayoutBox createRenderObject() {
+    return new RenderCustomMultiChildLayoutBox(delegate: delegate);
+  }
+
+  void updateRenderObject(RenderCustomMultiChildLayoutBox renderObject, CustomMultiChildLayout oldWidget) {
+    if (oldWidget.token != token)
+      renderObject.markNeedsLayout();
+    renderObject.delegate = delegate;
+  }
+}
+
 class SizedBox extends OneChildRenderObjectWidget {
   SizedBox({ Key key, this.width, this.height, Widget child })
     : super(key: key, child: child);

packages/unit/test/widget/custom_multi_child_layout_test.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+class TestMultiChildLayoutDelegate extends MultiChildLayoutDelegate {
+  BoxConstraints getSizeConstraints;
+
+  Size getSize(BoxConstraints constraints) {
+    getSizeConstraints = constraints;
+    return new Size(200.0, 300.0);
+  }
+
+  Size performLayoutSize;
+  BoxConstraints performLayoutConstraints;
+  int performLayoutChildCount;
+  Size performLayoutSize0;
+  Size performLayoutSize1;
+
+  void performLayout(Size size, BoxConstraints constraints, int childCount) {
+    performLayoutSize = size;
+    performLayoutConstraints = constraints;
+    performLayoutChildCount = childCount;
+    performLayoutSize0 = layoutChild(0, constraints);
+    performLayoutSize1 = layoutChild(1, constraints);
+  }
+}
+
+void main() {
+  test('Control test for CustomMultiChildLayout', () {
+    testWidgets((WidgetTester tester) {
+      TestMultiChildLayoutDelegate delegate = new TestMultiChildLayoutDelegate();
+      tester.pumpWidget(new Center(
+        child: new CustomMultiChildLayout([
+          new Container(width: 150.0, height: 100.0),
+          new Container(width: 100.0, height: 200.0)
+        ],
+          delegate: delegate
+        )
+      ));
+
+      expect(delegate.getSizeConstraints.minWidth, 0.0);
+      expect(delegate.getSizeConstraints.maxWidth, 800.0);
+      expect(delegate.getSizeConstraints.minHeight, 0.0);
+      expect(delegate.getSizeConstraints.maxHeight, 600.0);
+
+      expect(delegate.performLayoutSize.width, 200.0);
+      expect(delegate.performLayoutSize.height, 300.0);
+      expect(delegate.performLayoutConstraints.minWidth, 0.0);
+      expect(delegate.performLayoutConstraints.maxWidth, 800.0);
+      expect(delegate.performLayoutConstraints.minHeight, 0.0);
+      expect(delegate.performLayoutConstraints.maxHeight, 600.0);
+      expect(delegate.performLayoutChildCount, 2);
+      expect(delegate.performLayoutSize0.width, 150.0);
+      expect(delegate.performLayoutSize0.height, 100.0);
+      expect(delegate.performLayoutSize1.width, 100.0);
+      expect(delegate.performLayoutSize1.height, 200.0);
+    });
+  });
+}