Improve the debugging information for Widget

If the build stack got long before, it would get cropped.

packages/flutter/lib/rendering.dart

@@ -11,6 +11,7 @@ export 'package:sky/src/rendering/auto_layout.dart';
 export 'package:sky/src/rendering/block.dart';
 export 'package:sky/src/rendering/box.dart';
 export 'package:sky/src/rendering/debug.dart';
+export 'package:sky/src/rendering/error.dart';
 export 'package:sky/src/rendering/flex.dart';
 export 'package:sky/src/rendering/grid.dart';
 export 'package:sky/src/rendering/hit_test.dart';

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

@@ -21,22 +21,28 @@ bool _initInDebugBuild() {
 bool debugPaintSizeEnabled = false;
 
 /// The color to use when painting RenderObject bounds.
-const sky.Color debugPaintSizeColor = const sky.Color(0xFF00FFFF);
+sky.Color debugPaintSizeColor = const sky.Color(0xFF00FFFF);
 
 /// Causes each RenderBox to paint a line at each of its baselines.
 bool debugPaintBaselinesEnabled = false;
 
 /// The color to use when painting alphabetic baselines.
-const sky.Color debugPaintAlphabeticBaselineColor = const sky.Color(0xFF00FF00);
+sky.Color debugPaintAlphabeticBaselineColor = const sky.Color(0xFF00FF00);
 
 /// The color ot use when painting ideographic baselines.
-const sky.Color debugPaintIdeographicBaselineColor = const sky.Color(0xFFFFD000);
+sky.Color debugPaintIdeographicBaselineColor = const sky.Color(0xFFFFD000);
 
 /// Causes each Layer to paint a box around its bounds.
 bool debugPaintLayerBordersEnabled = false;
 
 /// The color to use when painting Layer borders.
-const sky.Color debugPaintLayerBordersColor = const sky.Color(0xFFFF9800);
+sky.Color debugPaintLayerBordersColor = const sky.Color(0xFFFF9800);
 
 /// Causes RenderObjects to paint warnings when painting outside their bounds.
 bool debugPaintBoundsEnabled = false;
+
+/// The color to use when painting RenderError boxes in checked mode.
+sky.Color debugErrorBoxColor = const sky.Color(0xFFFF0000);
+
+/// How many lines of debugging output to include when an exception is reported.
+int debugRenderObjectDumpMaxLength = 10;

packages/flutter/lib/src/rendering/error.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 'package:sky/src/rendering/box.dart';
+import 'package:sky/src/rendering/debug.dart';
+import 'package:sky/src/rendering/object.dart';
+
+const double _kMaxWidth = 100000.0;
+const double _kMaxHeight = 100000.0;
+
+class RenderErrorBox extends RenderBox {
+
+  double getMinIntrinsicWidth(BoxConstraints constraints) {
+    return constraints.constrainWidth(0.0);
+  }
+
+  double getMaxIntrinsicWidth(BoxConstraints constraints) {
+    return constraints.constrainWidth(_kMaxWidth);
+  }
+
+  double getMinIntrinsicHeight(BoxConstraints constraints) {
+    return constraints.constrainHeight(0.0);
+  }
+
+  double getMaxIntrinsicHeight(BoxConstraints constraints) {
+    return constraints.constrainHeight(_kMaxHeight);
+  }
+
+  bool get sizedByParent => true;
+
+  void performResize() {
+    size = constraints.constrain(const Size(_kMaxWidth, _kMaxHeight));
+  }
+
+  void paint(PaintingContext context, Offset offset) {
+    context.canvas.drawRect(offset & size, new Paint() .. color = debugErrorBoxColor);
+  }
+
+}

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

@@ -439,21 +439,16 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   void visitChildren(RenderObjectVisitor visitor) { }
 
   dynamic debugExceptionContext = '';
-  static dynamic _debugLastException;
-  bool _debugReportException(dynamic exception, String method) {
-    if (!inDebugBuild) {
-      print('Uncaught exception in ${method}():\n$exception');
-      return false;
-    }
-    if (!identical(exception, _debugLastException)) {
-      print('-- EXCEPTION --');
-      print('An exception was raised during ${method}().');
-      'The following RenderObject was being processed when the exception was fired:\n${this}'.split('\n').forEach(print);
-      if (debugExceptionContext != '')
-        'The RenderObject had the following exception context:\n${debugExceptionContext}'.split('\n').forEach(print);
-      _debugLastException = exception;
-    }
-    return true;
+  void _debugReportException(String method, dynamic exception, StackTrace stack) {
+    print('-- EXCEPTION --');
+    print('The following exception was raised during ${method}():');
+    print('$exception');
+    print('Stack trace:');
+    print('$stack');
+    'The following RenderObject was being processed when the exception was fired (limited to $debugRenderObjectDumpMaxLength lines):\n${this}'
+      .split('\n').take(debugRenderObjectDumpMaxLength+1).forEach(print);
+    if (debugExceptionContext != '')
+      'The RenderObject had the following exception context:\n${debugExceptionContext}'.split('\n').forEach(print);
   }
 
   static bool _debugDoingLayout = false;
@@ -598,30 +593,29 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     }
   }
   void _layoutWithoutResize() {
+    assert(_relayoutSubtreeRoot == this);
+    RenderObject debugPreviousActiveLayout;
+    assert(!_debugMutationsLocked);
+    assert(!_doingThisLayoutWithCallback);
+    assert(_debugCanParentUseSize != null);
+    assert(() {
+      _debugMutationsLocked = true;
+      _debugDoingThisLayout = true;
+      debugPreviousActiveLayout = _debugActiveLayout;
+      _debugActiveLayout = this;
+      return true;
+    });
     try {
-      assert(_relayoutSubtreeRoot == this);
-      RenderObject debugPreviousActiveLayout;
-      assert(!_debugMutationsLocked);
-      assert(!_doingThisLayoutWithCallback);
-      assert(_debugCanParentUseSize != null);
-      assert(() {
-        _debugMutationsLocked = true;
-        _debugDoingThisLayout = true;
-        debugPreviousActiveLayout = _debugActiveLayout;
-        _debugActiveLayout = this;
-        return true;
-      });
       performLayout();
-      assert(() {
-        _debugActiveLayout = debugPreviousActiveLayout;
-        _debugDoingThisLayout = false;
-        _debugMutationsLocked = false;
-        return true;
-      });
-    } catch (e) {
-      if (_debugReportException(e, 'layoutWithoutResize'))
-        rethrow;
+    } catch (e, stack) {
+      _debugReportException('performLayout', e, stack);
     }
+    assert(() {
+      _debugActiveLayout = debugPreviousActiveLayout;
+      _debugDoingThisLayout = false;
+      _debugMutationsLocked = false;
+      return true;
+    });
     _needsLayout = false;
     markNeedsPaint();
   }
@@ -669,7 +663,12 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     });
     if (sizedByParent) {
       assert(() { _debugDoingThisResize = true; return true; });
-      performResize();
+      try {
+        performResize();
+        assert(debugDoesMeetConstraints());
+      } catch (e, stack) {
+        _debugReportException('performResize', e, stack);
+      }
       assert(() { _debugDoingThisResize = false; return true; });
     }
     RenderObject debugPreviousActiveLayout;
@@ -681,17 +680,16 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     });
     try {
       performLayout();
-      assert(() {
-        _debugActiveLayout = debugPreviousActiveLayout;
-        _debugDoingThisLayout = false;
-        _debugMutationsLocked = false;
-        return true;
-      });
       assert(debugDoesMeetConstraints());
-    } catch (e) {
-      if (_debugReportException(e, 'layout'))
-        rethrow;
+    } catch (e, stack) {
+      _debugReportException('performLayout', e, stack);
     }
+    assert(() {
+      _debugActiveLayout = debugPreviousActiveLayout;
+      _debugDoingThisLayout = false;
+      _debugMutationsLocked = false;
+      return true;
+    });
     _needsLayout = false;
     markNeedsPaint();
     assert(parent == this.parent); // TODO(ianh): Remove this once the analyzer is cleverer
@@ -938,13 +936,8 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     _layer.removeAllChildren();
     PaintingContext context = new PaintingContext.withLayer(_layer, paintBounds);
     _layer = context._containerLayer;
-    try {
-      _paintWithContext(context, Offset.zero);
-      context.endRecording();
-    } catch (e) {
-      if (_debugReportException(e, '_repaint'))
-        rethrow;
-    }
+    _paintWithContext(context, Offset.zero);
+    context.endRecording();
   }
   void _paintWithContext(PaintingContext context, Offset offset) {
     assert(!_debugDoingThisPaint);
@@ -964,9 +957,13 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       return true;
     });
     _needsPaint = false;
-    paint(context, offset);
-    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
+    try {
+      paint(context, offset);
+      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
+    } catch (e, stack) {
+      _debugReportException('paint', e, stack);
+    }
     assert(() {
       if (debugPaintBoundsEnabled)
         context.canvas.restore();

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

@@ -9,6 +9,7 @@ import 'dart:sky' as sky;
 import 'package:sky/animation.dart';
 import 'package:sky/mojo/activity.dart';
 import 'package:sky/src/rendering/box.dart';
+import 'package:sky/src/rendering/error.dart';
 import 'package:sky/src/rendering/hit_test.dart';
 import 'package:sky/src/rendering/object.dart';
 import 'package:sky/src/rendering/sky_binding.dart';
@@ -742,7 +743,7 @@ abstract class Component extends Widget {
         super._withKey(key);
 
   bool _debugIsBuilding = false;
-  static String _debugLastComponent;
+  static Queue<Component> _debugComponentBuildTree = new Queue<Component>();
 
   bool _dirty = true;
 
@@ -816,37 +817,48 @@ abstract class Component extends Widget {
     Widget oldChild;
     if (old == null) {
       oldChild = _child;
+      _child = null;
     } else {
-      assert(_child == null);
       oldChild = old._child;
+      assert(_child == null);
     }
 
     String _debugPreviousComponent;
     assert(() {
       _debugIsBuilding = true;
-      _debugPreviousComponent = _debugLastComponent;
-      if (_debugLastComponent != null)
-        _debugLastComponent = "$_debugPreviousComponent -> ${this.toStringName()}";
-      else
-        _debugLastComponent = "Build chain: ${this.toStringName()}";
+      _debugComponentBuildTree.add(this);
       return true;
     });
 
     int lastOrder = _currentOrder;
     _currentOrder = _order;
-    _child = build();
+    try {
+      _child = build();
+      assert(_child != null);
+    } catch (e, stack) {
+      _debugReportException("building ${this.toStringName()}", e, stack);
+    }
     _currentOrder = lastOrder;
-    assert(_child != null);
     assert(() { _debugChildTaken = false; return true; });
-    _child = syncChild(_child, oldChild, slot);
-    assert(!_debugChildTaken); // we shouldn't be able to lose our child when we're syncing it!
-    assert(_child != null);
-    assert(_child.parent == this);
-
+    try {
+      // even if build() failed (i.e. _child == null), we still call syncChild(), to remove the oldChild
+      _child = syncChild(_child, oldChild, slot);
+      assert(!_debugChildTaken); // we shouldn't be able to lose our child when we're syncing it!
+      assert(_child == null || _child.parent == this);
+    } catch (e, stack) {
+      _debugReportException('syncing build output of ${this.toStringName()}\n  old child: ${oldChild?.toStringName()}\n  new child: ${_child?.toStringName()}', e, stack);
+      _child = null;
+    }
     assert(() {
+      if (_child == null) {
+        try {
+          _child = new ErrorWidget()..setParent(this).._sync(null, slot);
+        } catch (e) {
+          print('(application is now in an unstable state - ignore any subsequent exceptions)');
+        }
+      }
       _debugIsBuilding = false;
-      _debugLastComponent = _debugPreviousComponent;
-      return true;
+      return identical(_debugComponentBuildTree.removeLast(), this);
     });
 
     _dirty = false;
@@ -1094,7 +1106,7 @@ abstract class RenderObjectWrapper extends Widget {
       assert(_renderObject != null);
     }
     assert(() {
-      _renderObject.debugExceptionContext = Component._debugLastComponent;
+      _renderObject.debugExceptionContext = Component._debugComponentBuildTree.fold('  Widget build stack:', (String s, Component c) => s + '\n    ${c.toStringName()}');
       return true;
     });
     assert(_renderObject == renderObject); // in case a subclass reintroduces it
@@ -1599,3 +1611,20 @@ class RenderBoxToWidgetAdapter extends AbstractWidgetRoot {
 
   Widget build() => builder();
 }
+
+class ErrorWidget extends LeafRenderObjectWrapper {
+  RenderBox createNode() => new RenderErrorBox();
+}
+
+void _debugReportException(String context, dynamic exception, StackTrace stack) {
+  print('------------------------------------------------------------------------');
+  print('Exception caught while $context');
+  print('$exception');
+  print('Stack trace:');
+  '$stack'.split('\n').forEach(print);
+  print('Build stack:');
+  Component._debugComponentBuildTree.forEach((Component c) { print('  ${c.toStringName()}'); });
+  print('Current application widget tree:');
+  debugDumpApp();
+  print('------------------------------------------------------------------------');
+}