Directionality widget (#11739)

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

@@ -55,6 +55,56 @@ export 'package:flutter/rendering.dart' show
   WrapAlignment,
   WrapCrossAlignment;
 
+// BIDIRECTIONAL TEXT SUPPORT
+
+/// A widget that determines the ambient directionality of text and
+/// text-direction-sensitive render objects.
+///
+/// For example, [Padding] depends on the [Directionality] to resolve
+/// [EdgeInsetsDirectional] objects into absolute [EdgeInsets] objects.
+class Directionality extends InheritedWidget {
+  /// Creates a widget that determines the directionality of text and
+  /// text-direction-sensitive render objects.
+  ///
+  /// The [textDirection] and [child] arguments must not be null.
+  const Directionality({
+    Key key,
+    @required this.textDirection,
+    @required Widget child
+  }) : assert(textDirection != null),
+       assert(child != null),
+       super(key: key, child: child);
+
+  /// The text direction for this subtree.
+  final TextDirection textDirection;
+
+  /// The text direction from the closest instance of this class that encloses
+  /// the given context.
+  ///
+  /// If there is no [Directionality] ancestor widget in the tree at the given
+  /// context, then this will return null.
+  ///
+  /// Typical usage is as follows:
+  ///
+  /// ```dart
+  /// TextDirection textDirection = Directionality.of(context);
+  /// ```
+  static TextDirection of(BuildContext context) {
+    final Directionality widget = context.inheritFromWidgetOfExactType(Directionality);
+    return widget?.textDirection;
+  }
+
+  @override
+  bool updateShouldNotify(Directionality old) => textDirection != old.textDirection;
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder description) {
+    super.debugFillProperties(description);
+    description.add(new EnumProperty<TextDirection>('textDirection', textDirection));
+  }
+}
+
+
 // PAINTING NODES
 
 /// A widget that makes its child partially transparent.

packages/flutter/test/widgets/directionality_test.dart

@@ -0,0 +1,69 @@
+// 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:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('Directionality', (WidgetTester tester) async {
+    final List<TextDirection> log = <TextDirection>[];
+    final Widget inner = new Builder(
+      builder: (BuildContext context) {
+        log.add(Directionality.of(context));
+        return new Placeholder();
+      }
+    );
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: inner,
+      ),
+    );
+    expect(log, <TextDirection>[TextDirection.ltr]);
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: inner,
+      ),
+    );
+    expect(log, <TextDirection>[TextDirection.ltr]);
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.rtl,
+        child: inner,
+      ),
+    );
+    expect(log, <TextDirection>[TextDirection.ltr, TextDirection.rtl]);
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.rtl,
+        child: inner,
+      ),
+    );
+    expect(log, <TextDirection>[TextDirection.ltr, TextDirection.rtl]);
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: inner,
+      ),
+    );
+    expect(log, <TextDirection>[TextDirection.ltr, TextDirection.rtl, TextDirection.ltr]);
+  });
+
+  testWidgets('Directionality default', (WidgetTester tester) async {
+    bool good = false;
+    await tester.pumpWidget(new Builder(
+      builder: (BuildContext context) {
+        expect(Directionality.of(context), isNull);
+        good = true;
+        return new Placeholder();
+      },
+    ));
+    expect(good, isTrue);
+  });
+
+  testWidgets('Directionality can\'t be null', (WidgetTester tester) async {
+    expect(() { new Directionality(textDirection: null, child: new Placeholder()); }, throwsAssertionError);
+  });
+}