Include more details in the Red Box of Doom

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

@@ -13,12 +13,14 @@ bool debugCheckHasMaterial(BuildContext context) {
       Element element = context;
       throw new WidgetError(
         'No Material widget found.\n'
-        '${context.widget} widgets require a Material widget ancestor.\n'
+        '${context.widget.runtimeType} widgets require a Material widget ancestor.\n'
         'In material design, most widgets are conceptually "printed" on a sheet of material. In Flutter\'s material library, '
         'that material is represented by the Material widget. It is the Material widget that renders ink splashes, for instance. '
         'Because of this, many material library widgets require that there be a Material widget in the tree above them.\n'
         'To introduce a Material widget, you can either directly include one, or use a widget that contains Material itself, '
         'such as a Card, Dialog, Drawer, or Scaffold.\n'
+        'The specific widget that could not find a Material ancestor was:\n'
+        '  ${context.widget}'
         'The ownership chain for the affected widget is:\n'
         '  ${element.debugGetOwnershipChain(10)}'
       );
@@ -35,7 +37,9 @@ bool debugCheckHasScaffold(BuildContext context) {
       Element element = context;
       throw new WidgetError(
         'No Scaffold widget found.\n'
-        '${context.widget} widgets require a Scaffold widget ancestor.\n'
+        '${context.widget.runtimeType} widgets require a Scaffold widget ancestor.\n'
+        'The specific widget that could not find a Scaffold ancestor was:\n'
+        '  ${context.widget}'
         'The ownership chain for the affected widget is:\n'
         '  ${element.debugGetOwnershipChain(10)}'
       );

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

@@ -53,9 +53,6 @@ bool debugPaintPointersEnabled = false;
 /// The color to use when reporting pointers.
 int debugPaintPointersColorValue = 0x00BBBB;
 
-/// The color to use when painting [RenderErrorBox] objects in checked mode.
-Color debugErrorBoxColor = const Color(0xFFFF0000);
-
 /// Overlay a rotating set of colors when repainting layers in checked mode.
 bool debugRepaintRainbowEnabled = false;
 

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

@@ -2,15 +2,56 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui' as ui show Paragraph, ParagraphBuilder, ParagraphStyle, TextStyle;
+
 import 'box.dart';
-import 'debug.dart';
 import 'object.dart';
 
 const double _kMaxWidth = 100000.0;
 const double _kMaxHeight = 100000.0;
 
-/// A render object used as a placeholder when an error occurs
+/// A render object used as a placeholder when an error occurs.
+///
+/// The box will be painted in the color given by the
+/// [RenderErrorBox.backgroundColor] static property.
+///
+/// A message can be provided. To simplify the class and thus help reduce the
+/// likelihood of this class itself being the source of errors, the message
+/// cannot be changed once the object has been created. If provided, the text
+/// will be painted on top of the background, using the styles given by the
+/// [RenderErrorBox.textStyle] and [RenderErrorBox.paragraphStyle] static
+/// properties.
+///
+/// Again to help simplify the class, this box tries to be 100000.0 pixels wide
+/// and high, to approximate being infinitely high but without using infinities.
 class RenderErrorBox extends RenderBox {
+  /// Constructs a RenderErrorBox render object.
+  ///
+  /// A message can optionally be provided. If a message is provided, an attempt
+  /// will be made to render the message when the box paints.
+  RenderErrorBox([ this.message = '' ]) {
+    try {
+      if (message != '') {
+        // This class is intentionally doing things using the low-level
+        // primitives to avoid depending on any subsystems that may have ended
+        // up in an unstable state -- after all, this class is mainly used when
+        // things have gone wrong.
+        //
+        // Generally, the much better way to draw text in a RenderObject is to
+        // use the TextPainter class. If you're looking for code to crib from,
+        // see the paragraph.dart file and the RenderParagraph class.
+        ui.ParagraphBuilder builder = new ui.ParagraphBuilder();
+        builder.pushStyle(textStyle);
+        builder.addText(message);
+        _paragraph = builder.build(paragraphStyle);
+      }
+    } catch (e) { }
+  }
+
+  /// The message to attempt to display at paint time.
+  final String message;
+
+  ui.Paragraph _paragraph;
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     return constraints.constrainWidth(0.0);
@@ -36,8 +77,36 @@ class RenderErrorBox extends RenderBox {
     size = constraints.constrain(const Size(_kMaxWidth, _kMaxHeight));
   }
 
+  /// The color to use when painting the background of [RenderErrorBox] objects.
+  static Color backgroundColor = const Color(0xF0900000);
+
+  /// The text style to use when painting [RenderErrorBox] objects.
+  static ui.TextStyle textStyle = new ui.TextStyle(
+    color: const Color(0xFFFFFF00),
+    fontFamily: 'monospace',
+    fontSize: 7.0
+  );
+
+  /// The paragraph style to use when painting [RenderErrorBox] objects.
+  static ui.ParagraphStyle paragraphStyle = new ui.ParagraphStyle(
+    lineHeight: 0.25 // TODO(ianh): https://github.com/flutter/flutter/issues/2460 will affect this
+  );
+
   void paint(PaintingContext context, Offset offset) {
-    context.canvas.drawRect(offset & size, new Paint() .. color = debugErrorBoxColor);
+    try {
+      context.canvas.drawRect(offset & size, new Paint() .. color = backgroundColor);
+      if (_paragraph != null) {
+        // See the comment in the RenderErrorBox constructor. This is not the
+        // code you want to be copying and pasting. :-)
+        if (parent is RenderBox) {
+          RenderBox parentBox = parent;
+          _paragraph.maxWidth = parentBox.size.width;
+        } else {
+          _paragraph.maxWidth = size.width;
+        }
+        _paragraph.layout();
+        _paragraph.paint(context.canvas, offset);
+      }
+    } catch (e) { }
   }
-
 }

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

@@ -1016,8 +1016,24 @@ abstract class Element<T extends Widget> implements BuildContext {
   }
 }
 
+/// A widget that renders an exception's message. This widget is used when a
+/// build function fails, to help with determining where the problem lies.
+/// Exceptions are also logged to the console, which you can read using `flutter
+/// logs`. The console will also include additional information such as the
+/// stack trace for the exception.
 class ErrorWidget extends LeafRenderObjectWidget {
-  RenderBox createRenderObject() => new RenderErrorBox();
+  ErrorWidget(
+    Object exception
+  ) : message = _stringify(exception),
+      super(key: new UniqueKey());
+  final String message;
+  static String _stringify(Object exception) {
+    try {
+      return exception.toString();
+    } catch (e) { }
+    return 'Error';
+  }
+  RenderBox createRenderObject() => new RenderErrorBox(message);
 }
 
 typedef void BuildScheduler(BuildableElement element);
@@ -1224,7 +1240,7 @@ abstract class ComponentElement<T extends Widget> extends BuildableElement<T> {
       });
     } catch (e, stack) {
       _debugReportException('building $_widget', e, stack);
-      built = new ErrorWidget();
+      built = new ErrorWidget(e);
     } finally {
       // We delay marking the element as clean until after calling _builder so
       // that attempts to markNeedsBuild() during build() will be ignored.
@@ -1236,7 +1252,7 @@ abstract class ComponentElement<T extends Widget> extends BuildableElement<T> {
       assert(_child != null);
     } catch (e, stack) {
       _debugReportException('building $_widget', e, stack);
-      built = new ErrorWidget();
+      built = new ErrorWidget(e);
       _child = updateChild(null, built, slot);
     }
   }

packages/flutter_test/lib/src/widget_tester.dart

@@ -78,7 +78,7 @@ class WidgetTester extends Instrumentation {
       super(binding: _SteppedWidgetFlutterBinding.ensureInitialized()) {
     timeDilation = 1.0;
     ui.window.onBeginFrame = null;
-    runApp(new ErrorWidget()); // flush out the last build entirely
+    runApp(new Container(key: new UniqueKey())); // flush out the last build entirely
   }
 
   final FakeAsync async;