Implement radio button for Sky’s fn2

sky/sdk/lib/framework/fn2.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.
 
 library fn;
 
 import 'app.dart';
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
@@ skipping 423 matching lines @@
     super.syncRenderObject(old);
     root.callback = callback;
   }
 
   void _remove() {
     root.callback = null;
     super._remove();
   }
 }
 
+class CustomPaint extends OneChildRenderObjectWrapper {

[abarth-chromium, 2015/06/04 23:58:39]

Can you add a TODO(jackson) to this class about needing a mechanism for marking
the RenderCustomPaint object as needing paint?

[jackson, 2015/06/05 00:23:10]

Done.

+  RenderCustomPaint root;
+  final CustomPaintCallback callback;
+
+  CustomPaint({ this.callback, UINode child, Object key })
+    : super(child: child, key: key);
+
+  RenderCustomPaint createNode() => new RenderCustomPaint(callback: callback);
+
+  void syncRenderObject(CustomPaint old) {
+    super.syncRenderObject(old);
+    root.callback = callback;
+  }
+}

[abarth-chromium, 2015/06/04 23:58:39]

You probably want to implement a _remove similar to  SizeObserver to null out
the callback when this object is removed from the component hierarchy.

[jackson, 2015/06/05 00:23:10]

Done.

 
 final List<UINode> _emptyList = new List<UINode>();
 
 abstract class MultiChildRenderObjectWrapper extends RenderObjectWrapper {
 
   // In MultiChildRenderObjectWrapper subclasses, slots are RenderObject nodes
   // to use as the "insert before" sibling in ContainerRenderObjectMixin.add() calls
 
   final List<UINode> children;
 
@@ skipping 541 matching lines @@
     assert(root.parent is RenderView);
   }
 }
 
 class Text extends Component {
   Text(this.data) : super(key: '*text*');
   final String data;
   bool get interchangeable => true;
   UINode build() => new Paragraph(text: data);
 }