Give RenderObject a useful toString().

sky/sdk/lib/framework/rendering/object.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 '../node.dart';
 import '../scheduler.dart' as scheduler;
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 
 class ParentData {
   void detach() {
     detachSiblings();
   }
   void detachSiblings() { } // workaround for lack of inter-class mixins in Dart
   void merge(ParentData other) {
     // override this in subclasses to merge in data from other into this
     assert(other.runtimeType == this.runtimeType);
   }
+  String toString() => '<none>';
 }
 
 const kLayoutDirections = 4;
 
 double clamp({double min: 0.0, double value: 0.0, double max: double.INFINITY}) {
   assert(min != null);
   assert(value != null);
   assert(max != null);
   return math.max(min, math.min(max, value));
 }
@@ skipping 183 matching lines @@
   //   // irregular shape, e.g. if you have a hole.)
   //   // Otherwise:
   //   // For each child that intersects x,y, in z-order starting from the top,
   //   // call hitTest() for that child, passing it /result/, and the coordinates
   //   // converted to the child's coordinate origin, and stop at the first child
   //   // that returns true.
   //   // Then, add yourself to /result/, and return true.
   // }
   // You must not add yourself to /result/ if you return false.
 
+
+  String toString([String prefix = '']) {
+    String header = '${runtimeType}\n';
+    prefix += '  ';
+    String settings = '${debugDescribeSettings(prefix)}';
+    if (settings != '')
+      settings += '\n';
+    return '${header}${settings}${debugDescribeChildren(prefix)}';
+  }
+  String debugDescribeSettings(String prefix) => '${prefix}parentData: ${parentData}';
+  String debugDescribeChildren(String prefix) => '';
+
 }
 
 class HitTestResult {
   final List<RenderObject> path = new List<RenderObject>();
 
   RenderObject get result => path.first;
 
   void add(RenderObject node) {
     path.add(node);
   }
@@ skipping 13 matching lines @@
       adoptChild(_child);
   }
   void attachChildren() {
     if (_child != null)
       _child.attach();
   }
   void detachChildren() {
     if (_child != null)
       _child.detach();
   }
+  String debugDescribeChildren(String prefix) => '${prefix}child: ${child.toString(prefix)}';
 }
 
 
 // GENERIC MIXIN FOR RENDER NODES WITH A LIST OF CHILDREN
 
 abstract class ContainerParentDataMixin<ChildType extends RenderObject> {
   ChildType previousSibling;
   ChildType nextSibling;
   void detachSiblings() {
     if (previousSibling != null) {
@@ skipping 129 matching lines @@
       child = child.parentData.nextSibling;
     }
   }
 
   ChildType get firstChild => _firstChild;
   ChildType get lastChild => _lastChild;
   ChildType childAfter(ChildType child) {
     assert(child.parentData is ParentDataType);
     return child.parentData.nextSibling;
   }
+
+  String debugDescribeChildren(String prefix) {
+    String result = '';
+    int count = 1;
+    ChildType child = _firstChild;
+    while (child != null) {
+      if (result != '')
+        result += '\n';
+      result += '${prefix}child ${count}: ${child.toString(prefix)}';
+      count += 1;
+      child = child.parentData.nextSibling;
+    }
+    return result;
+  }
 }