Try a mildly prettier FlutterError and make it less drastic in release mode. (#44967)

packages/flutter/lib/src/foundation/assertions.dart

@@ -479,6 +479,11 @@ class FlutterErrorDetails extends Diagnosticable {
 
 /// Error class used to report Flutter-specific assertion failures and
 /// contract violations.
+///
+/// See also:
+///
+///  * <https://flutter.dev/docs/testing/errors>, more information about error
+///    handling in Flutter.
 class FlutterError extends Error with DiagnosticableTreeMixin implements AssertionError {
   /// Create an error message from a string.
   ///

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

@@ -10,9 +10,6 @@ import 'object.dart';
 const double _kMaxWidth = 100000.0;
 const double _kMaxHeight = 100000.0;
 
-// Line length to fit small phones without dynamically checking size.
-const String _kLine = '\n\n────────────────────\n\n';
-
 /// A render object used as a placeholder when an error occurs.
 ///
 /// The box will be painted in the color given by the
@@ -25,8 +22,9 @@ const String _kLine = '\n\n─────────────────
 /// [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.
+/// Again to help simplify the class, if the parent has left the constraints
+/// unbounded, this box tries to be 100000.0 pixels wide and high, to
+/// approximate being infinitely high but without using infinities.
 class RenderErrorBox extends RenderBox {
   /// Creates a RenderErrorBox render object.
   ///
@@ -45,13 +43,10 @@ class RenderErrorBox extends RenderBox {
         // see the paragraph.dart file and the RenderParagraph class.
         final ui.ParagraphBuilder builder = ui.ParagraphBuilder(paragraphStyle);
         builder.pushStyle(textStyle);
-        builder.addText(
-          '$message$_kLine$message$_kLine$message$_kLine$message$_kLine$message$_kLine$message$_kLine'
-          '$message$_kLine$message$_kLine$message$_kLine$message$_kLine$message$_kLine$message'
-        );
+        builder.addText(message);
         _paragraph = builder.build();
       }
-    } catch (e) {
+    } catch (error) {
       // Intentionally left empty.
     }
   }
@@ -82,39 +77,87 @@ class RenderErrorBox extends RenderBox {
     size = constraints.constrain(const Size(_kMaxWidth, _kMaxHeight));
   }
 
+  /// The distance to place around the text.
+  ///
+  /// This is intended to ensure that if the [RenderErrorBox] is placed at the top left
+  /// of the screen, under the system's status bar, the error text is still visible in
+  /// the area below the status bar.
+  ///
+  /// The padding is ignored if the error box is smaller than the padding.
+  ///
+  /// See also:
+  ///
+  ///  * [minimumWidth], which controls how wide the box must be before the
+  //     horizontal padding is applied.
+  static EdgeInsets padding = const EdgeInsets.fromLTRB(64.0, 96.0, 64.0, 12.0);
+
+  /// The width below which the horizontal padding is not applied.
+  ///
+  /// If the left and right padding would reduce the available width to less than
+  /// this value, then the text is rendered flush with the left edge.
+  static double minimumWidth = 200.0;
+
   /// The color to use when painting the background of [RenderErrorBox] objects.
-  static Color backgroundColor = const Color(0xF0900000);
+  ///
+  /// Defaults to red in debug mode, a light gray otherwise.
+  static Color backgroundColor = _initBackgroundColor();
+
+  static Color _initBackgroundColor() {
+    Color result = const Color(0xF0C0C0C0);
+    assert(() {
+      result = const Color(0xF0900000);
+      return true;
+    }());
+    return result;
+  }
 
   /// The text style to use when painting [RenderErrorBox] objects.
-  static ui.TextStyle textStyle = ui.TextStyle(
-    color: const Color(0xFFFFFF66),
-    fontFamily: 'monospace',
-    fontSize: 14.0,
-    fontWeight: FontWeight.bold,
-  );
+  ///
+  /// Defaults to a yellow monospace font in debug mode, and a dark gray
+  /// sans-serif font otherwise.
+  static ui.TextStyle textStyle = _initTextStyle();
+
+  static ui.TextStyle _initTextStyle() {
+    ui.TextStyle result = ui.TextStyle(
+      color: const Color(0xFF303030),
+      fontFamily: 'sans-serif',
+      fontSize: 18.0,
+    );
+    assert(() {
+      result = ui.TextStyle(
+        color: const Color(0xFFFFFF66),
+        fontFamily: 'monospace',
+        fontSize: 14.0,
+        fontWeight: FontWeight.bold,
+      );
+      return true;
+    }());
+    return result;
+  }
 
   /// The paragraph style to use when painting [RenderErrorBox] objects.
   static ui.ParagraphStyle paragraphStyle = ui.ParagraphStyle(
-    height: 1.0,
+    textDirection: TextDirection.ltr,
+    textAlign: TextAlign.left,
   );
 
   @override
   void paint(PaintingContext context, Offset offset) {
     try {
       context.canvas.drawRect(offset & size, Paint() .. color = backgroundColor);
-      double width;
       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) {
-          final RenderBox parentBox = parent;
-          width = parentBox.size.width;
-        } else {
-          width = size.width;
+        double width = size.width;
+        double left = 0.0;
+        double top = 0.0;
+        if (width > padding.left + minimumWidth + padding.right) {
+          width -= padding.left + padding.right;
+          left += padding.left;
         }
         _paragraph.layout(ui.ParagraphConstraints(width: width));
-
-        context.canvas.drawParagraph(_paragraph, offset);
+        if (size.height > padding.top + _paragraph.height + padding.bottom) {
+          top += padding.top;
+        }
+        context.canvas.drawParagraph(_paragraph, offset + Offset(left, top));
       }
     } catch (e) {
       // Intentionally left empty.

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

@@ -3810,13 +3810,71 @@ typedef ErrorWidgetBuilder = Widget Function(FlutterErrorDetails details);
 /// 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.
+///
+/// It is possible to override this widget.
+///
+/// {@tool snippet --template=freeform}
+/// ```dart
+/// import 'package:flutter/material.dart';
+///
+/// void main() {
+///   ErrorWidget.builder = (FlutterErrorDetails details) {
+///     bool inDebug = false;
+///     assert(() { inDebug = true; return true; }());
+///     // In debug mode, use the normal error widget which shows
+///     // the error message:
+///     if (inDebug)
+///       return ErrorWidget(details.exception);
+///     // In release builds, show a yellow-on-blue message instead:
+///     return Container(
+///       alignment: Alignment.center,
+///       child: Text(
+///         'Error!',
+///         style: TextStyle(color: Colors.yellow),
+///         textDirection: TextDirection.ltr,
+///       ),
+///     );
+///   };
+///   // Here we would normally runApp() the root widget, but to demonstrate
+///   // the error handling we artificially fail:
+///   return runApp(Builder(
+///     builder: (BuildContext context) {
+///       throw 'oh no, an error';
+///     },
+///   ));
+/// }
+/// ```
+/// {@end-tool}
+///
+/// See also:
+///
+///  * [FlutterError.onError], which can be set to a method that exits the
+///    application if that is preferable to showing an error message.
+///  * <https://flutter.dev/docs/testing/errors>, more information about error
+///    handling in Flutter.
 class ErrorWidget extends LeafRenderObjectWidget {
-  /// Creates a widget that displays the given error message.
+  /// Creates a widget that displays the given exception.
+  ///
+  /// The message will be the stringification of the given exception, unless
+  /// computing that value itself throws an exception, in which case it will
+  /// be the string "Error".
+  ///
+  /// If this object is inspected from an IDE or the devtools, and the original
+  /// exception is a [FlutterError] object, the original exception itself will
+  /// be shown in the inspection output.
   ErrorWidget(Object exception)
     : message = _stringify(exception),
       _flutterError = exception is FlutterError ? exception : null,
       super(key: UniqueKey());
 
+  /// Creates a widget that displays the given error message.
+  ///
+  /// An explicit [FlutterError] can be provided to be reported to inspection
+  /// tools. It need not match the message.
+  ErrorWidget.withDetails({ this.message = '', FlutterError error })
+    : _flutterError = error,
+      super(key: UniqueKey());
+
   /// The configurable factory for [ErrorWidget].
   ///
   /// When an error occurs while building a widget, the broken widget is
@@ -3835,17 +3893,28 @@ class ErrorWidget extends LeafRenderObjectWidget {
   /// corresponds to a [RenderBox] that can handle the most absurd of incoming
   /// constraints. The default constructor maps to a [RenderErrorBox].
   ///
+  /// The default behavior is to show the exception's message in debug mode,
+  /// and to show nothing but a gray background in release builds.
+  ///
   /// See also:
   ///
   ///  * [FlutterError.onError], which is typically called with the same
   ///    [FlutterErrorDetails] object immediately prior to this callback being
   ///    invoked, and which can also be configured to control how errors are
   ///    reported.
-  static ErrorWidgetBuilder builder = (FlutterErrorDetails details) => ErrorWidget(details.exception);
+  ///  * <https://flutter.dev/docs/testing/errors>, more information about error
+  ///    handling in Flutter.
+  static ErrorWidgetBuilder builder = _defaultErrorWidgetBuilder;
 
-  /// The message to display.
-  final String message;
-  final FlutterError _flutterError;
+  static Widget _defaultErrorWidgetBuilder(FlutterErrorDetails details) {
+    String message = '';
+    assert(() {
+      message = _stringify(details.exception) + '\nSee also: https://flutter.dev/docs/testing/errors';
+      return true;
+    }());
+    final Object exception = details.exception;
+    return ErrorWidget.withDetails(message: message, error: exception is FlutterError ? exception : null);
+  }
 
   static String _stringify(Object exception) {
     try {
@@ -3856,6 +3925,10 @@ class ErrorWidget extends LeafRenderObjectWidget {
     return 'Error';
   }
 
+  /// The message to display.
+  final String message;
+  final FlutterError _flutterError;
+
   @override
   RenderBox createRenderObject(BuildContext context) => RenderErrorBox(message);
 

packages/flutter/test/rendering/error_test.dart

@@ -14,9 +14,45 @@ void main() {
 
   testWidgets('test draw error paragraph', (WidgetTester tester) async {
     await tester.pumpWidget(ErrorWidget(Exception(errorMessage)));
+    expect(find.byType(ErrorWidget), paints
+        ..rect(rect: const Rect.fromLTWH(0.0, 0.0, 800.0, 600.0))
+        ..paragraph(offset: const Offset(64.0, 96.0)));
+
+    final Widget _error = Builder(builder: (BuildContext context) => throw 'pillow');
+
+    await tester.pumpWidget(Center(child: SizedBox(width: 100.0, child: _error)));
+    expect(tester.takeException(), 'pillow');
+    expect(find.byType(ErrorWidget), paints
+        ..rect(rect: const Rect.fromLTWH(0.0, 0.0, 100.0, 600.0))
+        ..paragraph(offset: const Offset(0.0, 96.0)));
+
+    await tester.pumpWidget(Center(child: SizedBox(height: 100.0, child: _error)));
+    expect(tester.takeException(), null);
 
+    await tester.pumpWidget(Center(child: SizedBox(key: UniqueKey(), height: 100.0, child: _error)));
+    expect(tester.takeException(), 'pillow');
+    expect(find.byType(ErrorWidget), paints
+        ..rect(rect: const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0))
+        ..paragraph(offset: const Offset(64.0, 0.0)));
+
+    RenderErrorBox.minimumWidth = 800.0;
+    await tester.pumpWidget(Center(child: _error));
+    expect(tester.takeException(), 'pillow');
     expect(find.byType(ErrorWidget), paints
         ..rect(rect: const Rect.fromLTWH(0.0, 0.0, 800.0, 600.0))
-        ..paragraph(offset: Offset.zero));
+        ..paragraph(offset: const Offset(0.0, 96.0)));
+
+    await tester.pumpWidget(Center(child: _error));
+    expect(tester.takeException(), null);
+    expect(find.byType(ErrorWidget), paints
+        ..rect(color: const Color(0xF0900000))
+        ..paragraph());
+
+    RenderErrorBox.backgroundColor = const Color(0xFF112233);
+    await tester.pumpWidget(Center(child: _error));
+    expect(tester.takeException(), null);
+    expect(find.byType(ErrorWidget), paints
+        ..rect(color: const Color(0xFF112233))
+        ..paragraph());
   });
 }

packages/flutter/test/widgets/error_widget_builder_test.dart

@@ -5,6 +5,8 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
+import '../rendering/mock_canvas.dart';
+
 void main() {
   testWidgets('ErrorWidget.builder', (WidgetTester tester) async {
     final ErrorWidgetBuilder oldBuilder = ErrorWidget.builder;
@@ -24,4 +26,23 @@ void main() {
     expect(find.text('oopsie!'), findsOneWidget);
     ErrorWidget.builder = oldBuilder;
   });
+
+  testWidgets('ErrorWidget.builder', (WidgetTester tester) async {
+    final ErrorWidgetBuilder oldBuilder = ErrorWidget.builder;
+    ErrorWidget.builder = (FlutterErrorDetails details) {
+      return ErrorWidget('');
+    };
+    await tester.pumpWidget(
+      SizedBox(
+        child: Builder(
+          builder: (BuildContext context) {
+            throw 'test';
+          },
+        ),
+      ),
+    );
+    expect(tester.takeException().toString(), 'test');
+    expect(find.byType(ErrorWidget), isNot(paints..paragraph()));
+    ErrorWidget.builder = oldBuilder;
+  });
 }