Allow the ErrorWidget to be overridden. (#13578)

Fixes https://github.com/flutter/flutter/issues/10695

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

@@ -140,6 +140,38 @@ class FlutterErrorDetails {
       longMessage = '  <no message available>';
     return longMessage;
   }
+
+  @override
+  String toString() {
+    final StringBuffer buffer = new StringBuffer();
+    if ((library != null && library != '') || (context != null && context != '')) {
+      if (library != null && library != '') {
+        buffer.write('Error caught by $library');
+        if (context != null && context != '')
+          buffer.write(', ');
+      } else {
+        buffer.writeln('Exception ');
+      }
+      if (context != null && context != '')
+        buffer.write('thrown $context');
+      buffer.writeln('.');
+    } else {
+      buffer.write('An error was caught.');
+    }
+    buffer.writeln(exceptionAsString());
+    if (informationCollector != null)
+      informationCollector(buffer);
+    if (stack != null) {
+      Iterable<String> stackLines = stack.toString().trimRight().split('\n');
+      if (stackFilter != null) {
+        stackLines = stackFilter(stackLines);
+      } else {
+        stackLines = FlutterError.defaultStackFilter(stackLines);
+      }
+      buffer.writeAll(stackLines, '\n');
+    }
+    return buffer.toString().trimRight();
+  }
 }
 
 /// Error class used to report Flutter-specific assertion failures and

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

@@ -14,6 +14,7 @@ import 'framework.dart';
 export 'package:flutter/animation.dart';
 export 'package:flutter/foundation.dart' show
   ChangeNotifier,
+  FlutterErrorDetails,
   Listenable,
   TargetPlatform,
   ValueNotifier;

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

@@ -840,13 +840,14 @@ class RenderObjectToWidgetElement<T extends RenderObject> extends RootRenderObje
       _child = updateChild(_child, widget.child, _rootChildSlot);
       assert(_child != null);
     } catch (exception, stack) {
-      FlutterError.reportError(new FlutterErrorDetails(
+      final FlutterErrorDetails details = new FlutterErrorDetails(
         exception: exception,
         stack: stack,
         library: 'widgets library',
         context: 'attaching to the render tree'
-      ));
-      final Widget error = new ErrorWidget(exception);
+      );
+      FlutterError.reportError(details);
+      final Widget error = ErrorWidget.builder(details);
       _child = updateChild(null, error, _rootChildSlot);
     }
   }

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

@@ -3456,6 +3456,20 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
   void performRebuild();
 }
 
+/// Signature for the constructor that is called when an error occurs while
+/// building a widget.
+///
+/// The argument provides information regarding the cause of the error.
+///
+/// See also:
+///
+///  * [ErrorWidget.builder], which can be set to override the default
+///    [ErrorWidget] builder.
+///  * [FlutterError.reportError], which is typically called with the same
+///    [FlutterErrorDetails] object immediately prior to [ErrorWidget.builder]
+///    being called.
+typedef Widget ErrorWidgetBuilder(FlutterErrorDetails details);
+
 /// A widget that renders an exception's message.
 ///
 /// This widget is used when a build method fails, to help with determining
@@ -3467,6 +3481,32 @@ class ErrorWidget extends LeafRenderObjectWidget {
   ErrorWidget(Object exception) : message = _stringify(exception),
       super(key: new UniqueKey());
 
+  /// The configurable factory for [ErrorWidget].
+  ///
+  /// When an error occurs while building a widget, the broken widget is
+  /// replaced by the widget returned by this function. By default, an
+  /// [ErrorWidget] is returned.
+  ///
+  /// The system is typically in an unstable state when this function is called.
+  /// An exception has just been thrown in the middle of build (and possibly
+  /// layout), so surrounding widgets and render objects may be in a rather
+  /// fragile state. The framework itself (especially the [BuildOwner]) may also
+  /// be confused, and additional exceptions are quite likely to be thrown.
+  ///
+  /// Because of this, it is highly recommended that the widget returned from
+  /// this function perform the least amount of work possible. A
+  /// [LeafRenderObjectWidget] is the best choice, especially one that
+  /// corresponds to a [RenderBox] that can handle the most absurd of incoming
+  /// constraints. The default constructor maps to a [RenderErrorBox].
+  ///
+  /// 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) => new ErrorWidget(details.exception);
+
   /// The message to display.
   final String message;
 
@@ -3544,8 +3584,7 @@ abstract class ComponentElement extends Element {
       built = build();
       debugWidgetBuilderValue(widget, built);
     } catch (e, stack) {
-      _debugReportException('building $this', e, stack);
-      built = new ErrorWidget(e);
+      built = ErrorWidget.builder(_debugReportException('building $this', e, stack));
     } finally {
       // We delay marking the element as clean until after calling build() so
       // that attempts to markNeedsBuild() during build() will be ignored.
@@ -3556,8 +3595,7 @@ abstract class ComponentElement extends Element {
       _child = updateChild(_child, built, slot);
       assert(_child != null);
     } catch (e, stack) {
-      _debugReportException('building $this', e, stack);
-      built = new ErrorWidget(e);
+      built = ErrorWidget.builder(_debugReportException('building $this', e, stack));
       _child = updateChild(null, built, slot);
     }
 
@@ -4656,14 +4694,19 @@ class _DebugCreator {
   String toString() => element.debugGetCreatorChain(12);
 }
 
-void _debugReportException(String context, dynamic exception, StackTrace stack, {
+FlutterErrorDetails _debugReportException(
+  String context,
+  dynamic exception,
+  StackTrace stack, {
   InformationCollector informationCollector
 }) {
-  FlutterError.reportError(new FlutterErrorDetails(
+  final FlutterErrorDetails details = new FlutterErrorDetails(
     exception: exception,
     stack: stack,
     library: 'widgets library',
     context: context,
     informationCollector: informationCollector,
-  ));
+  );
+  FlutterError.reportError(details);
+  return details;
 }

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

@@ -111,16 +111,14 @@ class _LayoutBuilderElement extends RenderObjectElement {
           built = widget.builder(this, constraints);
           debugWidgetBuilderValue(widget, built);
         } catch (e, stack) {
-          _debugReportException('building $widget', e, stack);
-          built = new ErrorWidget(e);
+          built = ErrorWidget.builder(_debugReportException('building $widget', e, stack));
         }
       }
       try {
         _child = updateChild(_child, built, null);
         assert(_child != null);
       } catch (e, stack) {
-        _debugReportException('building $widget', e, stack);
-        built = new ErrorWidget(e);
+        built = ErrorWidget.builder(_debugReportException('building $widget', e, stack));
         _child = updateChild(null, built, slot);
       }
     });
@@ -225,11 +223,17 @@ class _RenderLayoutBuilder extends RenderBox with RenderObjectWithChildMixin<Ren
   }
 }
 
-void _debugReportException(String context, dynamic exception, StackTrace stack) {
-  FlutterError.reportError(new FlutterErrorDetails(
+FlutterErrorDetails _debugReportException(
+  String context,
+  dynamic exception,
+  StackTrace stack,
+) {
+  final FlutterErrorDetails details = new FlutterErrorDetails(
     exception: exception,
     stack: stack,
     library: 'widgets library',
     context: context
-  ));
+  );
+  FlutterError.reportError(details);
+  return details;
 }

packages/flutter/test/foundation/assertions_test.dart

@@ -16,7 +16,6 @@ void main() {
     expect(log[1], contains('debugPrintStack'));
   });
 
-
   test('debugPrintStack', () {
     final List<String> log = captureOutput(() {
       final FlutterErrorDetails details = new FlutterErrorDetails(
@@ -42,4 +41,55 @@ void main() {
     expect(joined, contains('\nExample information\n'));
   });
 
+  test('FlutterErrorDetails.toString', () {
+    expect(
+      new FlutterErrorDetails(
+        exception: 'MESSAGE',
+        library: 'LIBRARY',
+        context: 'CONTEXTING',
+        informationCollector: (StringBuffer information) {
+          information.writeln('INFO');
+        },
+      ).toString(),
+      'Error caught by LIBRARY, thrown CONTEXTING.\n'
+      'MESSAGE\n'
+      'INFO',
+    );
+    expect(
+      new FlutterErrorDetails(
+        library: 'LIBRARY',
+        context: 'CONTEXTING',
+        informationCollector: (StringBuffer information) {
+          information.writeln('INFO');
+        },
+      ).toString(),
+      'Error caught by LIBRARY, thrown CONTEXTING.\n'
+      '  null\n'
+      'INFO',
+    );
+    expect(
+      new FlutterErrorDetails(
+        exception: 'MESSAGE',
+        context: 'CONTEXTING',
+        informationCollector: (StringBuffer information) {
+          information.writeln('INFO');
+        },
+      ).toString(),
+      'Error caught by Flutter framework, thrown CONTEXTING.\n'
+      'MESSAGE\n'
+      'INFO',
+    );
+    expect(
+      const FlutterErrorDetails(
+        exception: 'MESSAGE',
+      ).toString(),
+      'Error caught by Flutter framework.\n'
+      'MESSAGE'
+    );
+    expect(
+      const FlutterErrorDetails().toString(),
+      'Error caught by Flutter framework.\n'
+      '  null'
+    );
+  });
 }

packages/flutter/test/widgets/error_widget_builder_test.dart

@@ -0,0 +1,25 @@
+// Copyright 2017 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:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('ErrorWidget.builder', (WidgetTester tester) async {
+    ErrorWidget.builder = (FlutterErrorDetails details) {
+      return const Text('oopsie!', textDirection: TextDirection.ltr);
+    };
+    await tester.pumpWidget(
+      new SizedBox(
+        child: new Builder(
+          builder: (BuildContext context) {
+            throw 'test';
+          },
+        ),
+      ),
+    );
+    expect(tester.takeException().toString(), 'test');
+    expect(find.text('oopsie!'), findsOneWidget);
+  });
+}