Some more tests for AnimatedPositionedDirectional and more debugging checks (#12231)

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

@@ -7,6 +7,7 @@ import 'dart:math' as math;
 import 'package:flutter/foundation.dart';
 
 import 'basic.dart';
+import 'debug.dart';
 import 'framework.dart';
 
 const double _kOffset = 40.0; // distance to bottom of banner, at a 45 degree angle inwards
@@ -294,6 +295,7 @@ class Banner extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    assert((textDirection != null && layoutDirection != null) || debugCheckHasDirectionality(context));
     return new CustomPaint(
       foregroundPainter: new BannerPainter(
         message: message,

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

@@ -3792,11 +3792,10 @@ class RichText extends LeafRenderObjectWidget {
 
   @override
   RenderParagraph createRenderObject(BuildContext context) {
-    final TextDirection direction = textDirection ?? Directionality.of(context);
-    assert(direction != null, 'A RichText was created with no textDirection and no ambient Directionality widget.');
+    assert(textDirection != null || debugCheckHasDirectionality(context));
     return new RenderParagraph(text,
       textAlign: textAlign,
-      textDirection: direction,
+      textDirection: textDirection ?? Directionality.of(context),
       softWrap: softWrap,
       overflow: overflow,
       textScaleFactor: textScaleFactor,
@@ -3806,6 +3805,7 @@ class RichText extends LeafRenderObjectWidget {
 
   @override
   void updateRenderObject(BuildContext context, RenderParagraph renderObject) {
+    assert(textDirection != null || debugCheckHasDirectionality(context));
     renderObject
       ..text = text
       ..textAlign = textAlign

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

@@ -7,6 +7,7 @@ import 'dart:developer' show Timeline; // to disambiguate reference in dartdocs
 
 import 'package:flutter/foundation.dart';
 
+import 'basic.dart';
 import 'framework.dart';
 import 'media_query.dart';
 import 'table.dart';
@@ -202,6 +203,43 @@ bool debugCheckHasMediaQuery(BuildContext context) {
   return true;
 }
 
+/// Asserts that the given context has a [Directionality] ancestor.
+///
+/// Used by various widgets to make sure that they are only used in an
+/// appropriate context.
+///
+/// To invoke this function, use the following pattern, typically in the
+/// relevant Widget's build method:
+///
+/// ```dart
+/// assert(debugCheckHasDirectionality(context));
+/// ```
+///
+/// Does nothing if asserts are disabled. Always returns true.
+bool debugCheckHasDirectionality(BuildContext context) {
+  assert(() {
+    if (context.widget is! Directionality && context.ancestorWidgetOfExactType(Directionality) == null) {
+      final Element element = context;
+      throw new FlutterError(
+        'No Directionality widget found.\n'
+        '${context.widget.runtimeType} widgets require a Directionality widget ancestor.\n'
+        'The specific widget that could not find a Directionality ancestor was:\n'
+        '  ${context.widget}\n'
+        'The ownership chain for the affected widget is:\n'
+        '  ${element.debugGetCreatorChain(10)}\n'
+        'Typically, the Directionality widget is introduced by the MaterialApp '
+        'or WidgetsApp widget at the top of your application widget tree. It '
+        'determines the ambient reading direction and is used, for example, to '
+        'determine how to lay out text, how to interpret "start" and "end" '
+        'values, and to resolve EdgeInsetsDirectional, '
+        'FractionalOffsetDirectional, and other *Directional objects.'
+      );
+    }
+    return true;
+  }());
+  return true;
+}
+
 /// Asserts that the `built` widget is not null.
 ///
 /// Used when the given `widget` calls a builder function to check that the

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

@@ -5,6 +5,7 @@
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
+import 'debug.dart';
 import 'framework.dart';
 import 'icon_data.dart';
 import 'icon_theme.dart';
@@ -84,8 +85,8 @@ class Icon extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    assert(debugCheckHasDirectionality(context));
     final TextDirection textDirection = Directionality.of(context);
-    assert(textDirection != null, 'Icon widgets required an ambient Directionality.');
 
     final IconThemeData iconTheme = IconTheme.of(context);
 

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

@@ -8,6 +8,7 @@ import 'package:vector_math/vector_math_64.dart';
 
 import 'basic.dart';
 import 'container.dart';
+import 'debug.dart';
 import 'framework.dart';
 import 'text.dart';
 import 'ticker_provider.dart';
@@ -486,7 +487,8 @@ class _AnimatedContainerState extends AnimatedWidgetBaseState<AnimatedContainer>
 /// See also:
 ///
 ///  * [AnimatedPositionedDirectional], which adapts to the ambient
-///    [Directionality].
+///    [Directionality] (the same as this widget, but for animating
+///    [PositionedDirectional]).
 class AnimatedPositioned extends ImplicitlyAnimatedWidget {
   /// Creates a widget that animates its position implicitly.
   ///
@@ -545,14 +547,14 @@ class AnimatedPositioned extends ImplicitlyAnimatedWidget {
 
   /// The child's width.
   ///
-  /// Only two out of the three horizontal values (left, right, width) can be
-  /// set. The third must be null.
+  /// Only two out of the three horizontal values ([left], [right], [width]) can
+  /// be set. The third must be null.
   final double width;
 
   /// The child's height.
   ///
-  /// Only two out of the three vertical values (top, bottom, height) can be
-  /// set. The third must be null.
+  /// Only two out of the three vertical values ([top], [bottom], [height]) can
+  /// be set. The third must be null.
   final double height;
 
   @override
@@ -597,7 +599,7 @@ class _AnimatedPositionedState extends AnimatedWidgetBaseState<AnimatedPositione
       right: _right?.evaluate(animation),
       bottom: _bottom?.evaluate(animation),
       width: _width?.evaluate(animation),
-      height: _height?.evaluate(animation)
+      height: _height?.evaluate(animation),
     );
   }
 
@@ -613,21 +615,25 @@ class _AnimatedPositionedState extends AnimatedWidgetBaseState<AnimatedPositione
   }
 }
 
-/// Animated version of [PositionedDirectional] which automatically transitions the child's
-/// position over a given duration whenever the given position changes.
+/// Animated version of [PositionedDirectional] which automatically transitions
+/// the child's position over a given duration whenever the given position
+/// changes.
+///
+/// The ambient [Directionality] is used to determine whether [start] is to the
+/// left or to the right.
 ///
 /// Only works if it's the child of a [Stack].
 ///
 /// See also:
 ///
-///  * [AnimatedPositioned], which specifies the widget's position visually.
+///  * [AnimatedPositioned], which specifies the widget's position visually (the
+///  * same as this widget, but for animating [Positioned]).
 class AnimatedPositionedDirectional extends ImplicitlyAnimatedWidget {
   /// Creates a widget that animates its position implicitly.
   ///
-  /// Only two out of the three horizontal values ([start], [end],
-  /// [width]), and only two out of the three vertical values ([top],
-  /// [bottom], [height]), can be set. In each case, at least one of
-  /// the three must be null.
+  /// Only two out of the three horizontal values ([start], [end], [width]), and
+  /// only two out of the three vertical values ([top], [bottom], [height]), can
+  /// be set. In each case, at least one of the three must be null.
   ///
   /// The [curve] and [duration] arguments must not be null.
   const AnimatedPositionedDirectional({
@@ -662,14 +668,14 @@ class AnimatedPositionedDirectional extends ImplicitlyAnimatedWidget {
 
   /// The child's width.
   ///
-  /// Only two out of the three horizontal values (start, end, width) can be
-  /// set. The third must be null.
+  /// Only two out of the three horizontal values ([start], [end], [width]) can
+  /// be set. The third must be null.
   final double width;
 
   /// The child's height.
   ///
-  /// Only two out of the three vertical values (top, bottom, height) can be
-  /// set. The third must be null.
+  /// Only two out of the three vertical values ([top], [bottom], [height]) can
+  /// be set. The third must be null.
   final double height;
 
   @override
@@ -707,14 +713,16 @@ class _AnimatedPositionedDirectionalState extends AnimatedWidgetBaseState<Animat
 
   @override
   Widget build(BuildContext context) {
-    return new PositionedDirectional(
+    assert(debugCheckHasDirectionality(context));
+    return new Positioned.directional(
+      textDirection: Directionality.of(context),
       child: widget.child,
       start: _start?.evaluate(animation),
       top: _top?.evaluate(animation),
       end: _end?.evaluate(animation),
       bottom: _bottom?.evaluate(animation),
       width: _width?.evaluate(animation),
-      height: _height?.evaluate(animation)
+      height: _height?.evaluate(animation),
     );
   }
 

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

@@ -8,6 +8,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
+import 'debug.dart';
 import 'framework.dart';
 
 /// [NavigationToolbar] is a layout helper to position 3 widgets or groups of
@@ -60,6 +61,7 @@ class NavigationToolbar extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    assert(debugCheckHasDirectionality(context));
     final List<Widget> children = <Widget>[];
 
     if (leading != null)
@@ -72,7 +74,6 @@ class NavigationToolbar extends StatelessWidget {
       children.add(new LayoutId(id: _ToolbarSlot.trailing, child: trailing));
 
     final TextDirection textDirection = Directionality.of(context);
-    assert(textDirection != null);
     return new CustomMultiChildLayout(
       delegate: new _ToolbarLayout(
         centerMiddle: centerMiddle,

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

@@ -10,6 +10,7 @@ import 'package:flutter/physics.dart';
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
+import 'debug.dart';
 import 'framework.dart';
 import 'notification_listener.dart';
 import 'page_storage.dart';
@@ -451,8 +452,8 @@ class _PageViewState extends State<PageView> {
   AxisDirection _getDirection(BuildContext context) {
     switch (widget.scrollDirection) {
       case Axis.horizontal:
+        assert(debugCheckHasDirectionality(context));
         final TextDirection textDirection = Directionality.of(context);
-        assert(textDirection != null);
         final AxisDirection axisDirection = textDirectionToAxisDirection(textDirection);
         return widget.reverse ? flipAxisDirection(axisDirection) : axisDirection;
       case Axis.vertical:

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

@@ -6,6 +6,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
+import 'debug.dart';
 import 'framework.dart';
 import 'primary_scroll_controller.dart';
 import 'scroll_controller.dart';
@@ -180,8 +181,8 @@ abstract class ScrollView extends StatelessWidget {
   AxisDirection getDirection(BuildContext context) {
     switch (scrollDirection) {
       case Axis.horizontal:
+        assert(debugCheckHasDirectionality(context));
         final TextDirection textDirection = Directionality.of(context);
-        assert(textDirection != null);
         final AxisDirection axisDirection = textDirectionToAxisDirection(textDirection);
         return reverse ? flipAxisDirection(axisDirection) : axisDirection;
       case Axis.vertical:

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

@@ -8,6 +8,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
+import 'debug.dart';
 import 'framework.dart';
 import 'primary_scroll_controller.dart';
 import 'scroll_controller.dart';
@@ -117,8 +118,8 @@ class SingleChildScrollView extends StatelessWidget {
   AxisDirection _getDirection(BuildContext context) {
     switch (scrollDirection) {
       case Axis.horizontal:
+        assert(debugCheckHasDirectionality(context));
         final TextDirection textDirection = Directionality.of(context);
-        assert(textDirection != null);
         final AxisDirection axisDirection = textDirectionToAxisDirection(textDirection);
         return reverse ? flipAxisDirection(axisDirection) : axisDirection;
       case Axis.vertical:

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

@@ -202,6 +202,7 @@ class Table extends RenderObjectWidget {
 
   @override
   RenderTable createRenderObject(BuildContext context) {
+    assert(debugCheckHasDirectionality(context));
     return new RenderTable(
       columns: children.isNotEmpty ? children[0].children.length : 0,
       rows: children.length,
@@ -218,6 +219,7 @@ class Table extends RenderObjectWidget {
 
   @override
   void updateRenderObject(BuildContext context, RenderTable renderObject) {
+    assert(debugCheckHasDirectionality(context));
     assert(renderObject.columns == (children.isNotEmpty ? children[0].children.length : 0));
     assert(renderObject.rows == children.length);
     renderObject

packages/flutter/test/widgets/text_test.dart

@@ -34,4 +34,11 @@ void main() {
     expect(text, isNotNull);
     expect(text.textScaleFactor, 3.0);
   });
+
+  testWidgets('Text throws a nice error message if there\'s no Directionality', (WidgetTester tester) async {
+    await tester.pumpWidget(const Text('Hello'));
+    final String message = tester.takeException().toString();
+    expect(message, contains('Directionality'));
+    expect(message, contains(' Text '));
+  });
 }