Transform Widget alignment is AlignmentGeometry (#12870)

* a => an

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

@@ -1618,13 +1618,15 @@ class RenderTransform extends RenderProxyBox {
   RenderTransform({
     @required Matrix4 transform,
     Offset origin,
-    Alignment alignment,
+    AlignmentGeometry alignment,
+    TextDirection textDirection,
     this.transformHitTests: true,
     RenderBox child
   }) : assert(transform != null),
        super(child) {
     this.transform = transform;
     this.alignment = alignment;
+    this.textDirection = textDirection;
     this.origin = origin;
   }
 
@@ -1646,16 +1648,35 @@ class RenderTransform extends RenderProxyBox {
   ///
   /// This is equivalent to setting an origin based on the size of the box.
   /// If it is specified at the same time as an offset, both are applied.
-  Alignment get alignment => _alignment;
-  Alignment _alignment;
-  set alignment(Alignment value) {
-    assert(value == null || (value.x != null && value.y != null));
+  ///
+  /// An [AlignmentDirectional.start] value is the same as an [Alignment]
+  /// whose [Alignment.x] value is `-1.0` if [textDirection] is
+  /// [TextDirection.ltr], and `1.0` if [textDirection] is [TextDirection.rtl].
+  /// Similarly [AlignmentDirectional.end] is the same as an [Alignment]
+  /// whose [Alignment.x] value is `1.0` if [textDirection] is
+  /// [TextDirection.ltr], and `-1.0` if [textDirection] is [TextDirection.rtl].
+  AlignmentGeometry get alignment => _alignment;
+  AlignmentGeometry _alignment;
+  set alignment(AlignmentGeometry value) {
     if (_alignment == value)
       return;
     _alignment = value;
     markNeedsPaint();
   }
 
+  /// The text direction with which to resolve [alignment].
+  ///
+  /// This may be changed to null, but only after [alignment] has been changed
+  /// to a value that does not depend on the direction.
+  TextDirection get textDirection => _textDirection;
+  TextDirection _textDirection;
+  set textDirection(TextDirection value) {
+    if (_textDirection == value)
+      return;
+    _textDirection = value;
+    markNeedsPaint();
+  }
+
   /// When set to true, hit tests are performed based on the position of the
   /// child as it is painted. When set to false, hit tests are performed
   /// ignoring the transformation.
@@ -1713,18 +1734,19 @@ class RenderTransform extends RenderProxyBox {
   }
 
   Matrix4 get _effectiveTransform {
-    if (_origin == null && _alignment == null)
+    final Alignment resolvedAlignment = alignment?.resolve(textDirection);
+    if (_origin == null && resolvedAlignment == null)
       return _transform;
     final Matrix4 result = new Matrix4.identity();
     if (_origin != null)
       result.translate(_origin.dx, _origin.dy);
     Offset translation;
-    if (_alignment != null) {
-      translation = _alignment.alongSize(size);
+    if (resolvedAlignment != null) {
+      translation = resolvedAlignment.alongSize(size);
       result.translate(translation.dx, translation.dy);
     }
     result.multiply(_transform);
-    if (_alignment != null)
+    if (resolvedAlignment != null)
       result.translate(-translation.dx, -translation.dy);
     if (_origin != null)
       result.translate(-_origin.dx, -_origin.dy);
@@ -1770,6 +1792,7 @@ class RenderTransform extends RenderProxyBox {
     description.add(new TransformProperty('transform matrix', _transform));
     description.add(new DiagnosticsProperty<Offset>('origin', origin));
     description.add(new DiagnosticsProperty<Alignment>('alignment', alignment));
+    description.add(new EnumProperty<TextDirection>('textDirection', textDirection, defaultValue: null));
     description.add(new DiagnosticsProperty<bool>('transformHitTests', transformHitTests));
   }
 }
@@ -1822,7 +1845,7 @@ class RenderFittedBox extends RenderProxyBox {
   /// parent's bounds.  An alignment of (1.0, 0.5) aligns the child to the middle
   /// of the right edge of its parent's bounds.
   ///
-  /// If this is set to a [AlignmentDirectional] object, then
+  /// If this is set to an [AlignmentDirectional] object, then
   /// [textDirection] must not be null.
   AlignmentGeometry get alignment => _alignment;
   AlignmentGeometry _alignment;
@@ -1951,7 +1974,7 @@ class RenderFittedBox extends RenderProxyBox {
 
 /// Applies a translation transformation before painting its child.
 ///
-/// The translation is expressed as a [Offset] scaled to the child's size. For
+/// The translation is expressed as an [Offset] scaled to the child's size. For
 /// example, an [Offset] with a `dx` of 0.25 will result in a horizontal
 /// translation of one quarter the width of the child.
 ///

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

@@ -794,10 +794,17 @@ class Transform extends SingleChildRenderObjectWidget {
   /// The alignment of the origin, relative to the size of the box.
   ///
   /// This is equivalent to setting an origin based on the size of the box.
-  /// If it is specified at the same time as an offset, both are applied.
-  final Alignment alignment;
+  /// If it is specified at the same time as the [origin], both are applied.
+  ///
+  /// An [AlignmentDirectional.start] value is the same as an [Alignment]
+  /// whose [Alignment.x] value is `-1.0` if [textDirection] is
+  /// [TextDirection.ltr], and `1.0` if [textDirection] is [TextDirection.rtl].
+  /// Similarly [AlignmentDirectional.end] is the same as an [Alignment]
+  /// whose [Alignment.x] value is `1.0` if [textDirection] is
+  /// [TextDirection.ltr], and `-1.0` if [textDirection] is [TextDirection.rtl].
+  final AlignmentGeometry alignment;
 
-  /// Whether to apply the translation when performing hit tests.
+  /// Whether to apply the transformation when performing hit tests.
   final bool transformHitTests;
 
   @override
@@ -806,6 +813,7 @@ class Transform extends SingleChildRenderObjectWidget {
       transform: transform,
       origin: origin,
       alignment: alignment,
+      textDirection: Directionality.of(context),
       transformHitTests: transformHitTests
     );
   }
@@ -816,6 +824,7 @@ class Transform extends SingleChildRenderObjectWidget {
       ..transform = transform
       ..origin = origin
       ..alignment = alignment
+      ..textDirection = Directionality.of(context)
       ..transformHitTests = transformHitTests;
   }
 }
@@ -1195,7 +1204,7 @@ class Padding extends SingleChildRenderObjectWidget {
 ///  * [Center], which is the same as [Align] but with the [alignment] always
 ///    set to [Alignment.center].
 ///  * [FractionallySizedBox], which sizes its child based on a fraction of its own
-///    size and positions the child according to a [Alignment] value.
+///    size and positions the child according to an [Alignment] value.
 class Align extends SingleChildRenderObjectWidget {
   /// Creates an alignment widget.
   ///
@@ -1291,9 +1300,9 @@ class Center extends Align {
 ///
 ///  * [SingleChildLayoutDelegate], which controls the layout of the child.
 ///  * [Align], which sizes itself based on its child's size and positions
-///    the child according to a [Alignment] value.
+///    the child according to an [Alignment] value.
 ///  * [FractionallySizedBox], which sizes its child based on a fraction of its own
-///    size and positions the child according to a [Alignment] value.
+///    size and positions the child according to an [Alignment] value.
 ///  * [CustomMultiChildLayout], which uses a delegate to position multiple
 ///    children.
 class CustomSingleChildLayout extends SingleChildRenderObjectWidget {
@@ -1568,7 +1577,7 @@ class ConstrainedBox extends SingleChildRenderObjectWidget {
 /// See also:
 ///
 /// * [Align] (which sizes itself based on its child's size and positions
-///   the child according to a [Alignment] value)
+///   the child according to an [Alignment] value)
 /// * [OverflowBox]
 class FractionallySizedBox extends SingleChildRenderObjectWidget {
   /// Creates a widget that sizes its child to a fraction of the total available space.
@@ -2269,7 +2278,7 @@ class ListBody extends MultiChildRenderObjectWidget {
 /// See also:
 ///
 ///  * [Align], which sizes itself based on its child's size and positions
-///    the child according to a [Alignment] value.
+///    the child according to an [Alignment] value.
 ///  * [CustomSingleChildLayout], which uses a delegate to control the layout of
 ///    a single child.
 ///  * [CustomMultiChildLayout], which uses a delegate to position multiple
@@ -4023,7 +4032,7 @@ class RawImage extends LeafRenderObjectWidget {
   /// How to align the image within its bounds.
   ///
   /// The alignment aligns the given position in the image to the given position
-  /// in the layout bounds. For example, a [Alignment] alignment of (-1.0,
+  /// in the layout bounds. For example, an [Alignment] alignment of (-1.0,
   /// -1.0) aligns the image to the top-left corner of its layout bounds, while a
   /// [Alignment] alignment of (1.0, 1.0) aligns the bottom right of the
   /// image with the bottom right corner of its layout bounds. Similarly, an

packages/flutter/test/widgets/transform_test.dart

@@ -106,6 +106,77 @@ void main() {
     expect(didReceiveTap, isTrue);
   });
 
+  testWidgets('Transform AlignmentDirectional alignment', (WidgetTester tester) async {
+    bool didReceiveTap = false;
+
+    Widget buildFrame(TextDirection textDirection, AlignmentGeometry alignment) {
+      return new Directionality(
+        textDirection: textDirection,
+        child: new Stack(
+          children: <Widget>[
+            new Positioned(
+              top: 100.0,
+              left: 100.0,
+              child: new Container(
+                width: 100.0,
+                height: 100.0,
+                color: const Color(0xFF0000FF),
+              ),
+            ),
+            new Positioned(
+              top: 100.0,
+              left: 100.0,
+              child: new Container(
+                width: 100.0,
+                height: 100.0,
+                child: new Transform(
+                  transform: new Matrix4.diagonal3Values(0.5, 0.5, 1.0),
+                  alignment: alignment,
+                  child: new GestureDetector(
+                    onTap: () {
+                      didReceiveTap = true;
+                    },
+                    child: new Container(
+                      color: const Color(0xFF00FFFF),
+                    ),
+                  ),
+                ),
+              ),
+            ),
+          ],
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(TextDirection.ltr, AlignmentDirectional.centerEnd));
+    didReceiveTap = false;
+    await tester.tapAt(const Offset(110.0, 110.0));
+    expect(didReceiveTap, isFalse);
+    await tester.tapAt(const Offset(190.0, 150.0));
+    expect(didReceiveTap, isTrue);
+
+    await tester.pumpWidget(buildFrame(TextDirection.rtl, AlignmentDirectional.centerStart));
+    didReceiveTap = false;
+    await tester.tapAt(const Offset(110.0, 110.0));
+    expect(didReceiveTap, isFalse);
+    await tester.tapAt(const Offset(190.0, 150.0));
+    expect(didReceiveTap, isTrue);
+
+    await tester.pumpWidget(buildFrame(TextDirection.ltr, AlignmentDirectional.centerStart));
+    didReceiveTap = false;
+    await tester.tapAt(const Offset(190.0, 150.0));
+    expect(didReceiveTap, isFalse);
+    await tester.tapAt(const Offset(110.0, 150.0));
+    expect(didReceiveTap, isTrue);
+
+    await tester.pumpWidget(buildFrame(TextDirection.rtl, AlignmentDirectional.centerEnd));
+    didReceiveTap = false;
+    await tester.tapAt(const Offset(190.0, 150.0));
+    expect(didReceiveTap, isFalse);
+    await tester.tapAt(const Offset(110.0, 150.0));
+    expect(didReceiveTap, isTrue);
+  });
+
   testWidgets('Transform offset + alignment', (WidgetTester tester) async {
     bool didReceiveTap = false;
     await tester.pumpWidget(