Merge pull request #781 from Hixie/better-exceptions

Better exception handling for rendering library.

packages/flutter/lib/rendering/flex.dart

@@ -512,7 +512,7 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
 
   String toStringName() {
     String header = super.toStringName();
-    if (_overflow > 0.0)
+    if (_overflow is double && _overflow > 0.0)
       header += ' OVERFLOWING';
     return header;
   }

packages/flutter/lib/rendering/object.dart

@@ -318,6 +318,24 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   // Override in subclasses with children and call the visitor for each child.
   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${debugExceptionContext}'.split('\n').forEach(print);
+      _debugLastException = exception;
+    }
+    return true;
+  }
+
   static bool _debugDoingLayout = false;
   static bool get debugDoingLayout => _debugDoingLayout;
   bool _debugDoingThisResize = false;
@@ -443,10 +461,8 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
         return true;
       });
     } catch (e) {
-      print('Exception raised during layout:\n${e}\nContext:\n${this}');
-      if (inDebugBuild)
+      if (_debugReportException(e, 'layoutWithoutResize'))
         rethrow;
-      return;
     }
     _needsLayout = false;
     markNeedsPaint();
@@ -482,6 +498,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       _debugActiveLayout = this;
       return true;
     });
+    try {
       performLayout();
       assert(() {
         _debugActiveLayout = debugPreviousActiveLayout;
@@ -490,6 +507,10 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
         return true;
       });
       assert(debugDoesMeetConstraints());
+    } catch (e) {
+      if (_debugReportException(e, 'layout'))
+        rethrow;
+    }
     _needsLayout = false;
     markNeedsPaint();
     assert(parent == this.parent); // TODO(ianh): Remove this once the analyzer is cleverer
@@ -666,10 +687,8 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       _paintWithContext(context, Offset.zero);
       context.endRecording();
     } catch (e) {
-      print('Exception raised during _paintLayer:\n${e}\nContext:\n${this}');
-      if (inDebugBuild)
+      if (_debugReportException(e, '_repaint'))
         rethrow;
-      return;
     }
   }
   void _paintWithContext(PaintingContext context, Offset offset) {

packages/flutter/lib/widgets/framework.dart

@@ -6,6 +6,7 @@ import 'dart:async';
 import 'dart:collection';
 import 'dart:sky' as sky;
 
+import 'package:sky/base/debug.dart';
 import 'package:sky/base/hit_test.dart';
 import 'package:sky/base/scheduler.dart' as scheduler;
 import 'package:sky/mojo/activity.dart';
@@ -955,6 +956,13 @@ abstract class RenderObjectWrapper extends Widget {
       _ancestor = old._ancestor;
       assert(_renderObject != null);
     }
+    if (inDebugBuild) {
+      try {
+        throw null;
+      } catch (_, stack) {
+        _renderObject.debugExceptionContext = stack;
+      }
+    }
     assert(_renderObject == renderObject); // in case a subclass reintroduces it
     assert(renderObject != null);
     assert(mounted);