Make AnimatedCrossFade's alignment configurable (#9973)

The default is topCenter, which is sensible, but someone might want to use a
bottomCenter alignment, for example.

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

@@ -297,7 +297,7 @@ class RenderStack extends RenderBox
   /// top left corners.
   RenderStack({
     List<RenderBox> children,
-    FractionalOffset alignment: FractionalOffset.center,
+    FractionalOffset alignment: FractionalOffset.topLeft,
     StackFit fit: StackFit.loose,
     Overflow overflow: Overflow.clip
   }) : _alignment = alignment,

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

@@ -51,6 +51,7 @@ class AnimatedCrossFade extends StatefulWidget {
     this.firstCurve: Curves.linear,
     this.secondCurve: Curves.linear,
     this.sizeCurve: Curves.linear,
+    this.alignment: FractionalOffset.topCenter,
     @required this.crossFadeState,
     @required this.duration
   }) : assert(firstCurve != null),
@@ -77,14 +78,25 @@ class AnimatedCrossFade extends StatefulWidget {
   final Duration duration;
 
   /// The fade curve of the first child.
+  ///
+  /// Defaults to [Curves.linear].
   final Curve firstCurve;
 
   /// The fade curve of the second child.
+  ///
+  /// Defaults to [Curves.linear].
   final Curve secondCurve;
 
   /// The curve of the animation between the two children's sizes.
+  ///
+  /// Defaults to [Curves.linear].
   final Curve sizeCurve;
 
+  /// How the children should be aligned while the size is animating.
+  ///
+  /// Defaults to [FractionalOffset.topCenter].
+  final FractionalOffset alignment;
+
   @override
   _AnimatedCrossFadeState createState() => new _AnimatedCrossFadeState();
 }
@@ -152,7 +164,7 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
       children = <Widget>[
         new FadeTransition(
           opacity: _secondAnimation,
-          child: widget.secondChild
+          child: widget.secondChild,
         ),
         new Positioned(
           // TODO(dragostis): Add a way to crop from top right for
@@ -162,15 +174,15 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
           right: 0.0,
           child: new FadeTransition(
             opacity: _firstAnimation,
-            child: widget.firstChild
-          )
-        )
+            child: widget.firstChild,
+          ),
+        ),
       ];
     } else {
       children = <Widget>[
         new FadeTransition(
           opacity: _firstAnimation,
-          child: widget.firstChild
+          child: widget.firstChild,
         ),
         new Positioned(
           // TODO(dragostis): Add a way to crop from top right for
@@ -180,24 +192,24 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
           right: 0.0,
           child: new FadeTransition(
             opacity: _secondAnimation,
-            child: widget.secondChild
-          )
-        )
+            child: widget.secondChild,
+          ),
+        ),
       ];
     }
 
     return new ClipRect(
       child: new AnimatedSize(
         key: new ValueKey<Key>(widget.key),
-        alignment: FractionalOffset.topCenter,
+        alignment: widget.alignment,
         duration: widget.duration,
         curve: widget.sizeCurve,
         vsync: this,
         child: new Stack(
           overflow: Overflow.visible,
-          children: children
-        )
-      )
+          children: children,
+        ),
+      ),
     );
   }
 }

packages/flutter/test/widgets/animated_cross_fade_test.dart

@@ -78,4 +78,57 @@ void main() {
     expect(box.size.width, equals(200.0));
     expect(box.size.height, equals(200.0));
   });
+
+  testWidgets('AnimatedCrossFade alignment', (WidgetTester tester) async {
+    final Key firstKey = new UniqueKey();
+    final Key secondKey = new UniqueKey();
+
+    await tester.pumpWidget(
+      new Center(
+        child: new AnimatedCrossFade(
+          alignment: FractionalOffset.bottomRight,
+          firstChild: new SizedBox(
+            key: firstKey,
+            width: 100.0,
+            height: 100.0
+          ),
+          secondChild: new SizedBox(
+            key: secondKey,
+            width: 200.0,
+            height: 200.0
+          ),
+          duration: const Duration(milliseconds: 200),
+          crossFadeState: CrossFadeState.showFirst
+        )
+      )
+    );
+
+    await tester.pumpWidget(
+      new Center(
+        child: new AnimatedCrossFade(
+          alignment: FractionalOffset.bottomRight,
+          firstChild: new SizedBox(
+            key: firstKey,
+            width: 100.0,
+            height: 100.0
+          ),
+          secondChild: new SizedBox(
+            key: secondKey,
+            width: 200.0,
+            height: 200.0
+          ),
+          duration: const Duration(milliseconds: 200),
+          crossFadeState: CrossFadeState.showSecond
+        )
+      )
+    );
+
+    await tester.pump(const Duration(milliseconds: 100));
+
+    final RenderBox box1 = tester.renderObject(find.byKey(firstKey));
+    final RenderBox box2 = tester.renderObject(find.byKey(secondKey));
+    expect(box1.localToGlobal(Offset.zero), const Offset(275.0, 175.0));
+    expect(box2.localToGlobal(Offset.zero), const Offset(275.0, 175.0));
+  });
+
 }