Add an AlignTransition widget (#12724)

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

@@ -443,6 +443,45 @@ class DecoratedBoxTransition extends AnimatedWidget {
   }
 }
 
+/// Animated version of an [Align] that animates its [Align.alignment] property.
+class AlignTransition extends AnimatedWidget {
+  /// Creates an animated [Align] whose [AlignmentGeometry] animation updates
+  /// the widget.
+  ///
+  /// See also:
+  ///
+  /// * [new Align].
+  const AlignTransition({
+    Key key,
+    @required Animation<AlignmentGeometry> alignment,
+    @required this.child,
+    this.widthFactor,
+    this.heightFactor,
+  }) : super(key: key, listenable: alignment);
+
+  /// The animation that controls the child's alignment.
+  Animation<AlignmentGeometry> get alignment => listenable;
+
+  /// If non-null, the child's width factor, see [Align.widthFactor].
+  final double widthFactor;
+
+  /// If non-null, the child's height factor, see [Align.heightFactor].
+  final double heightFactor;
+
+  /// The widget below this widget in the tree.
+  final Widget child;
+
+  @override
+  Widget build(BuildContext context) {
+    return new Align(
+      alignment: alignment.value,
+      widthFactor: widthFactor,
+      heightFactor: heightFactor,
+      child: child,
+    );
+  }
+}
+
 /// A builder that builds a widget given a child.
 ///
 /// The child should typically be part of the returned widget tree.

packages/flutter/test/widgets/transitions_test.dart

@@ -146,4 +146,49 @@ void main() {
       expect(actualDecoration.boxShadow[0].color, const Color(0x66000000));
     });
   });
+
+  testWidgets('AlignTransition animates', (WidgetTester tester) async {
+    final AnimationController controller = new AnimationController(vsync: const TestVSync());
+    final Animation<Alignment> alignmentTween = new AlignmentTween(
+      begin: const Alignment(-1.0, 0.0),
+      end: const Alignment(1.0, 1.0),
+    ).animate(controller);
+    final Widget widget = new AlignTransition(
+      alignment: alignmentTween,
+      child: const Text('Ready', textDirection: TextDirection.ltr),
+    );
+
+    await tester.pumpWidget(widget);
+
+    final RenderPositionedBox actualPositionedBox = tester.renderObject(find.byType(Align));
+
+    Alignment actualAlignment = actualPositionedBox.alignment;
+    expect(actualAlignment, const Alignment(-1.0, 0.0));
+
+    controller.value = 0.5;
+    await tester.pump();
+    actualAlignment = actualPositionedBox.alignment;
+    expect(actualAlignment, const Alignment(0.0, 0.5));
+  });
+
+  testWidgets('AlignTransition keeps width and height factors', (WidgetTester tester) async {
+    final AnimationController controller = new AnimationController(vsync: const TestVSync());
+    final Animation<Alignment> alignmentTween = new AlignmentTween(
+      begin: const Alignment(-1.0, 0.0),
+      end: const Alignment(1.0, 1.0),
+    ).animate(controller);
+    final Widget widget = new AlignTransition(
+      alignment: alignmentTween,
+      child: const Text('Ready', textDirection: TextDirection.ltr),
+      widthFactor: 0.3,
+      heightFactor: 0.4,
+    );
+
+    await tester.pumpWidget(widget);
+
+    final Align actualAlign = tester.widget(find.byType(Align));
+
+    expect(actualAlign.widthFactor, 0.3);
+    expect(actualAlign.heightFactor, 0.4);
+  });
 }