Add a simple_render_tree example

This example shows how to draw a circle using subclasses of RenderNode.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1144193004

examples/raw/simple_render_tree.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 'dart:math';
+import 'dart:sky';
+import 'package:sky/framework/layout2.dart';
+
+class RenderBlueCircle extends RenderBox {
+  void paint(RenderNodeDisplayList canvas) {
+    double radius = min(width, height) * 0.45;
+    Paint paint = new Paint()..setARGB(255, 0, 255, 255);
+    canvas.drawCircle(width / 2, height / 2, radius, paint);
+  }
+}
+
+void main() {
+  RenderView renderView = new RenderView(root: new RenderBlueCircle());
+  renderView.layout(newWidth: view.width, newHeight: view.height);
+  renderView.paintFrame();
+}

sdk/BUILD.gn

@@ -67,6 +67,7 @@ dart_pkg("sdk") {
     "lib/framework/embedder.dart",
     "lib/framework/fn.dart",
     "lib/framework/layout.dart",
+    "lib/framework/layout2.dart",
     "lib/framework/layouts/block.dart",
     "lib/framework/net/fetch.dart",
     "lib/framework/node.dart",

sdk/lib/framework/layout2.dart

@@ -228,7 +228,7 @@ abstract class RenderNode extends AbstractNode {
     var ancestor = this;
     while (ancestor.parent != null)
       ancestor = ancestor.parent;
-    assert(ancestor is Screen);
+    assert(ancestor is RenderView);
     ancestor.paintFrame();
   }
   void paint(RenderNodeDisplayList canvas) { }
@@ -450,11 +450,11 @@ abstract class RenderBox extends RenderNode {
 }
 
 
-// SCREEN LAYOUT MANAGER
+// RENDER VIEW LAYOUT MANAGER
 
-class Screen extends RenderNode {
+class RenderView extends RenderNode {
 
-  Screen({
+  RenderView({
     RenderBox root,
     this.timeForRotation: const Duration(microseconds: 83333)
   }) {
@@ -474,7 +474,7 @@ class Screen extends RenderNode {
   RenderBox _root;
   RenderBox get root => _root;
   void set root (RenderBox value) {
-    assert(root != null);
+    assert(value != null);
     _root = value;
     adoptChild(_root);
     markNeedsLayout();
@@ -547,12 +547,12 @@ class EdgeDims {
 
 class BlockParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> { }
 
-class BlockBox extends RenderBox with ContainerRenderNodeMixin<RenderBox, BlockParentData> {
+class RenderBlock extends RenderBox with ContainerRenderNodeMixin<RenderBox, BlockParentData> {
   // lays out RenderBox children in a vertical stack
   // uses the maximum width provided by the parent
   // sizes itself to the height of its child stack
 
-  BlockBox({
+  RenderBlock({
     EdgeDims padding: const EdgeDims(0.0, 0.0, 0.0, 0.0)
   }) {
     _padding = padding;