Make RotationTransition widget alignment configurable (#22535)

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

@@ -254,6 +254,7 @@ class RotationTransition extends AnimatedWidget {
   const RotationTransition({
     Key key,
     @required Animation<double> turns,
+    this.alignment = Alignment.center,
     this.child,
   }) : super(key: key, listenable: turns);
 
@@ -263,6 +264,13 @@ class RotationTransition extends AnimatedWidget {
   /// rotated v * 2 * pi radians before being painted.
   Animation<double> get turns => listenable;
 
+  /// The alignment of the origin of the coordinate system around which the
+  /// rotation occurs, relative to the size of the box.
+  ///
+  /// For example, to set the origin of the rotation to top right corner, use
+  /// an alignment of (1.0, -1.0) or use [Alignment.topRight]
+  final Alignment alignment;
+
   /// The widget below this widget in the tree.
   ///
   /// {@macro flutter.widgets.child}
@@ -274,7 +282,7 @@ class RotationTransition extends AnimatedWidget {
     final Matrix4 transform = Matrix4.rotationZ(turnsValue * math.pi * 2.0);
     return Transform(
       transform: transform,
-      alignment: Alignment.center,
+      alignment: alignment,
       child: child,
     );
   }

packages/flutter/test/widgets/transitions_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
+
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -253,4 +255,54 @@ void main() {
     await tester.pump();
     expect(actualPositionedBox.widthFactor, 1.0);
   });
+
+  testWidgets('RotationTransition animates', (WidgetTester tester) async {
+    final AnimationController controller =
+        new AnimationController(vsync: const TestVSync());
+    final Widget widget = new RotationTransition(
+      alignment: Alignment.topRight,
+      turns: controller,
+      child: const Text('Rotation', textDirection: TextDirection.ltr),
+    );
+
+    await tester.pumpWidget(widget);
+    Transform actualRotatedBox = tester.widget(find.byType(Transform));
+    Matrix4 actualTurns = actualRotatedBox.transform;
+    expect(actualTurns, equals(Matrix4.rotationZ(0.0)));
+
+    controller.value = 0.5;
+    await tester.pump();
+    actualRotatedBox = tester.widget(find.byType(Transform));
+    actualTurns = actualRotatedBox.transform;
+    expect(actualTurns, Matrix4.rotationZ(math.pi));
+
+    controller.value = 0.75;
+    await tester.pump();
+    actualRotatedBox = tester.widget(find.byType(Transform));
+    actualTurns = actualRotatedBox.transform;
+    expect(actualTurns, Matrix4.rotationZ(math.pi * 1.5));
+  });
+
+  testWidgets('RotationTransition maintains chosen alignment during animation',
+      (WidgetTester tester) async {
+    final AnimationController controller =
+        new AnimationController(vsync: const TestVSync());
+    final Widget widget = new RotationTransition(
+      alignment: Alignment.topRight,
+      turns: controller,
+      child: const Text('Rotation', textDirection: TextDirection.ltr),
+    );
+
+    await tester.pumpWidget(widget);
+    RotationTransition actualRotatedBox =
+        tester.widget(find.byType(RotationTransition));
+    Alignment actualAlignment = actualRotatedBox.alignment;
+    expect(actualAlignment, const Alignment(1.0, -1.0));
+
+    controller.value = 0.5;
+    await tester.pump();
+    actualRotatedBox = tester.widget(find.byType(RotationTransition));
+    actualAlignment = actualRotatedBox.alignment;
+    expect(actualAlignment, const Alignment(1.0, -1.0));
+  });
 }