Provide some debug-mode time hooks to visually debug painting and layout.

sky/sdk/lib/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 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'dart:sky' show Point, Offset, Size, Rect, Color, Paint, Path;
 
 import 'package:sky/base/debug.dart';
 import 'package:sky/base/hit_test.dart';
@@ skipping 352 matching lines @@
       _paintOnCanvas(canvas, Offset.zero);
     } catch (e) {
       print('Exception raised during _updatePaintingCanvas:\n${e}\nContext:\n${this}');
       if (inDebugBuild)
         rethrow;
       return;
     }
     assert(!_needsLayout); // check that the paint() method didn't mark us dirty again
     assert(!_needsPaint); // check that the paint() method didn't mark us dirty again
     _paintingNode.setBackingDrawable(recorder.endRecordingAsDrawable());
-
     if (canvas._descendentsWithPaintingCanvases != null) {
       for (RenderObject node in canvas._descendentsWithPaintingCanvases) {
         assert(node.attached == attached);
         if (node._needsPaint)
           node._updatePaintingCanvas();
       };
     }
   }
 
   void _paintOnCanvas(PaintingCanvas canvas, Offset offset) {
     _needsPaint = false;
     assert(!_debugDoingThisPaint);
     RenderObject debugLastActivePaint;
     assert(() {
       _debugDoingThisPaint = true;
       debugLastActivePaint = _debugActivePaint;
       _debugActivePaint = this;
+      debugPaint(canvas, offset);
+      if (debugPaintBoundsEnabled) {
+        canvas.save();
+        canvas.clipRect(paintBounds.shift(offset));
+      }
       return true;
     });
     paint(canvas, offset);
     assert(() {
+      if (debugPaintBoundsEnabled)
+        canvas.restore();
       _debugActivePaint = debugLastActivePaint;
       _debugDoingThisPaint = false;
       return true;
     });
     assert(!_needsPaint);
   }
 
   bool get createNewDisplayList => false;
   Rect get paintBounds;
+  void debugPaint(PaintingCanvas canvas, Offset offset) { }
   void paint(PaintingCanvas canvas, Offset offset) { }
 
 
   // EVENTS
 
   void handleEvent(sky.Event event, HitTestEntry entry) {
     // override this if you have a client, to hand it to the client
     // override this if you want to do anything with the event
   }
 
@@ skipping 287 matching lines @@
     int count = 1;
     ChildType child = _firstChild;
     while (child != null) {
       result += '${prefix}child ${count}: ${child.toString(prefix)}';
       count += 1;
       child = child.parentData.nextSibling;
     }
     return result;
   }
 }