Add CustomOneChildLayout

A CustomOneChildLayout is a widget that lets you perform a custom layout in the
simplified setting of sizing and positioning a single child.

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

@@ -205,6 +205,71 @@ class RenderPositionedBox extends RenderShiftedBox {
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}alignment: $alignment\n';
 }
 
+/// A delegate for computing the layout of a render object with a single child.
+class OneChildLayoutDelegate {
+  /// Returns the size of this object given the incomming constraints.
+  Size getSize(BoxConstraints constraints) => constraints.biggest;
+
+  /// Returns the box constraints for the child given the incomming constraints.
+  BoxConstraints getConstraintsForChild(BoxConstraints constraints) => constraints;
+
+  /// Returns the position where the child should be placed given the size of this object and the size of the child.
+  Point getPositionForChild(Size size, Size childSize) => Point.origin;
+}
+
+class RenderCustomOneChildLayoutBox extends RenderShiftedBox {
+  RenderCustomOneChildLayoutBox({
+    RenderBox child,
+    OneChildLayoutDelegate delegate
+  }) : _delegate = delegate, super(child) {
+    assert(delegate != null);
+  }
+
+  OneChildLayoutDelegate get delegate => _delegate;
+  OneChildLayoutDelegate _delegate;
+  void set delegate (OneChildLayoutDelegate 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() {
+    if (child != null) {
+      child.layout(delegate.getConstraintsForChild(constraints), parentUsesSize: true);
+      final BoxParentData childParentData = child.parentData;
+      childParentData.position = delegate.getPositionForChild(size, child.size);
+    }
+  }
+}
+
 class RenderBaseline extends RenderShiftedBox {
 
   RenderBaseline({

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

@@ -35,6 +35,7 @@ export 'package:flutter/rendering.dart' show
     LinearGradient,
     Matrix4,
     Offset,
+    OneChildLayoutDelegate,
     Paint,
     Path,
     PlainTextSpan,
@@ -139,13 +140,21 @@ class CustomPaint extends OneChildRenderObjectWidget {
     assert(onPaint != null);
   }
 
+  /// This widget repaints whenver you supply a new onPaint callback.
+  ///
+  /// If you use an anonymous closure for the onPaint callback, you'll trigger
+  /// a repaint every time you build this widget, which might not be what you
+  /// intend. Instead, consider passing a reference to a member function, which
+  /// has a more stable identity.
   final CustomPaintCallback onPaint;
-  final Object token; // set this to be repainted automatically when the token changes
+
+  /// This widget repaints whenever you supply a new token.
+  final Object token;
 
   RenderCustomPaint createRenderObject() => new RenderCustomPaint(onPaint: onPaint);
 
   void updateRenderObject(RenderCustomPaint renderObject, CustomPaint oldWidget) {
-    if (oldWidget != null && oldWidget.token != token)
+    if (oldWidget.token != token)
       renderObject.markNeedsPaint();
     renderObject.onPaint = onPaint;
   }
@@ -243,6 +252,35 @@ class Center extends Align {
     : super(key: key, shrinkWrap: shrinkWrap, child: child);
 }
 
+class CustomOneChildLayout extends OneChildRenderObjectWidget {
+  CustomOneChildLayout({
+    Key key,
+    this.delegate,
+    this.token,
+    Widget child
+  }) : super(key: key, child: child) {
+    assert(delegate != null);
+  }
+
+  /// A long-lived delegate that controls the layout of this widget.
+  ///
+  /// Whenever the delegate changes, we need to recompute the layout of this
+  /// widget, which means you might not want to create a new delegate instance
+  /// every time you build this widget. Instead, consider using a long-lived
+  /// deletate (perhaps held in a component's state) that you re-use every time
+  /// you build this widget.
+  final OneChildLayoutDelegate delegate;
+  final Object token;
+
+  RenderCustomOneChildLayoutBox createRenderObject() => new RenderCustomOneChildLayoutBox(delegate: delegate);
+
+  void updateRenderObject(RenderCustomOneChildLayoutBox renderObject, CustomOneChildLayout 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_one_child_layout_test.dart

+import 'package:flutter/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+class TestOneChildLayoutDelegate extends OneChildLayoutDelegate {
+  BoxConstraints constraintsFromGetSize;
+  BoxConstraints constraintsFromGetConstraintsForChild;
+  Size sizeFromGetPositionForChild;
+  Size childSizeFromGetPositionForChild;
+
+  Size getSize(BoxConstraints constraints) {
+    constraintsFromGetSize = constraints;
+    return new Size(200.0, 300.0);
+  }
+
+  BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
+    constraintsFromGetConstraintsForChild = constraints;
+    return new BoxConstraints(
+      minWidth: 100.0,
+      maxWidth: 150.0,
+      minHeight: 200.0,
+      maxHeight: 400.0
+    );
+  }
+
+  Point getPositionForChild(Size size, Size childSize) {
+    sizeFromGetPositionForChild = size;
+    childSizeFromGetPositionForChild = childSize;
+    return Point.origin;
+  }
+}
+
+void main() {
+  test('Control test for CustomOneChildLayout', () {
+    testWidgets((WidgetTester tester) {
+      TestOneChildLayoutDelegate delegate = new TestOneChildLayoutDelegate();
+      tester.pumpWidget(new Center(
+        child: new CustomOneChildLayout(delegate: delegate, child: new Container())
+      ));
+
+      expect(delegate.constraintsFromGetSize.minWidth, 0.0);
+      expect(delegate.constraintsFromGetSize.maxWidth, 800.0);
+      expect(delegate.constraintsFromGetSize.minHeight, 0.0);
+      expect(delegate.constraintsFromGetSize.maxHeight, 600.0);
+
+      expect(delegate.constraintsFromGetConstraintsForChild.minWidth, 0.0);
+      expect(delegate.constraintsFromGetConstraintsForChild.maxWidth, 800.0);
+      expect(delegate.constraintsFromGetConstraintsForChild.minHeight, 0.0);
+      expect(delegate.constraintsFromGetConstraintsForChild.maxHeight, 600.0);
+
+      expect(delegate.sizeFromGetPositionForChild.width, 200.0);
+      expect(delegate.sizeFromGetPositionForChild.height, 300.0);
+
+      expect(delegate.childSizeFromGetPositionForChild.width, 150.0);
+      expect(delegate.childSizeFromGetPositionForChild.height, 400.0);
+    });
+  });
+}