Add constants for FractionalOffsets

Adds some names for common FractionalOffset values.

dev/manual_tests/card_collection.dart

@@ -397,8 +397,8 @@ class CardCollectionState extends State<CardCollection> {
 
   Shader _createShader(Rect bounds) {
     return new LinearGradient(
-        begin: const FractionalOffset(0.0, 0.0),
-        end: const FractionalOffset(0.0, 1.0),
+        begin: FractionalOffset.topLeft,
+        end: FractionalOffset.bottomLeft,
         colors: <Color>[const Color(0x00FFFFFF), const Color(0xFFFFFFFF)],
         stops: <double>[0.1, 0.35]
     )
@@ -441,7 +441,7 @@ class CardCollectionState extends State<CardCollection> {
     if (_snapToCenter) {
       Widget indicator = new IgnorePointer(
         child: new Align(
-          alignment: const FractionalOffset(0.0, 0.5),
+          alignment: FractionalOffset.centerLeft,
           child: new Container(
             height: 1.0,
             decoration: new BoxDecoration(backgroundColor: const Color(0x80FFFFFF))

examples/layers/rendering/hello_world.dart

@@ -13,7 +13,7 @@ void main() {
     // The root of our render tree is a RenderPositionedBox, which centers its
     // child both vertically and horizontally.
     root: new RenderPositionedBox(
-      alignment: const FractionalOffset(0.5, 0.5),
+      alignment: FractionalOffset.center,
       // We use a RenderParagraph to display the text 'Hello, world.' without
       // any explicit styling.
       child: new RenderParagraph(new TextSpan(text: 'Hello, world.'))

examples/layers/rendering/spinning_square.dart

@@ -27,12 +27,12 @@ void main() {
   // spin.
   RenderTransform spin = new RenderTransform(
     transform: new Matrix4.identity(),
-    alignment: const FractionalOffset(0.5, 0.5),
+    alignment: FractionalOffset.center,
     child: square
   );
   // Finally, we center the spinning green square...
   RenderBox root = new RenderPositionedBox(
-    alignment: const FractionalOffset(0.5, 0.5),
+    alignment: FractionalOffset.center,
     child: spin
   );
   // and attach it to the window.

examples/material_gallery/lib/demo/full_screen_dialog_demo.dart

@@ -172,7 +172,7 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
                   border: new Border(bottom: new BorderSide(color: theme.dividerColor))
                 ),
                 child: new Align(
-                  alignment: const FractionalOffset(0.0, 1.0),
+                  alignment: FractionalOffset.bottomLeft,
                   child: new Text('Event name', style: theme.textTheme.display2)
                 )
               ),
@@ -182,7 +182,7 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
                   border: new Border(bottom: new BorderSide(color: theme.dividerColor))
                 ),
                 child: new Align(
-                  alignment: const FractionalOffset(0.0, 1.0),
+                  alignment: FractionalOffset.bottomLeft,
                   child: new Text('Location', style: theme.textTheme.title.copyWith(color: Colors.black54))
                 )
               ),

examples/material_gallery/lib/demo/weather_demo.dart

@@ -285,8 +285,8 @@ class GradientNode extends NodeWithSize {
 
     Rect rect = Point.origin & size;
     Paint gradientPaint = new Paint()..shader = new LinearGradient(
-      begin: const FractionalOffset(0.0, 0.0),
-      end: const FractionalOffset(0.0, 1.0),
+      begin: FractionalOffset.topLeft,
+      end: FractionalOffset.bottomLeft,
       colors: <Color>[colorTop, colorBottom],
       stops: <double>[0.0, 1.0]
     ).createShader(rect);

packages/flutter/lib/src/material/app_bar.dart

@@ -187,7 +187,7 @@ class AppBar extends StatelessWidget {
         padding: new EdgeInsets.only(left: combinedPadding.left, right: combinedPadding.right),
         child: new ClipRect(
           child: new OverflowBox(
-            alignment: const FractionalOffset(0.0, 1.0), // bottom justify
+            alignment: FractionalOffset.bottomLeft, // bottom justify
             minHeight: paddedCollapsedHeight,
             maxHeight: paddedCollapsedHeight,
             child: new Padding(
@@ -203,7 +203,7 @@ class AppBar extends StatelessWidget {
       appBar = new Stack(
         children: <Widget>[
           flexibleSpace(context),
-          new Align(child: appBar, alignment: const FractionalOffset(0.0, 0.0))
+          new Align(child: appBar, alignment: FractionalOffset.topLeft)
         ]
       );
     }

packages/flutter/lib/src/material/drawer.dart

@@ -178,7 +178,7 @@ class DrawerControllerState extends State<DrawerController> {
   Widget build(BuildContext context) {
     if (_controller.status == AnimationStatus.dismissed) {
       return new Align(
-        alignment: const FractionalOffset(0.0, 0.5),
+        alignment: FractionalOffset.centerLeft,
         child: new GestureDetector(
           key: _gestureDetectorKey,
           onHorizontalDragUpdate: _move,
@@ -208,9 +208,9 @@ class DrawerControllerState extends State<DrawerController> {
                 )
               ),
               new Align(
-                alignment: const FractionalOffset(0.0, 0.5),
+                alignment: FractionalOffset.centerLeft,
                 child: new Align(
-                  alignment: const FractionalOffset(1.0, 0.5),
+                  alignment: FractionalOffset.centerRight,
                   widthFactor: _controller.value,
                   child: new RepaintBoundary(
                     child: new Focus(

packages/flutter/lib/src/material/dropdown.dart

@@ -322,7 +322,7 @@ class _DropDownButtonState<T> extends State<DropDownButton<T>> {
               children: config.items,
               key: indexedStackKey,
               index: _selectedIndex,
-              alignment: const FractionalOffset(0.5, 0.0)
+              alignment: FractionalOffset.topCenter
             ),
             new Container(
               child: new Icon(icon: Icons.arrow_drop_down, size: 36.0),

packages/flutter/lib/src/material/flexible_space_bar.dart

@@ -81,7 +81,7 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
             end: new FractionalOffset(0.0, yAlignEnd)
           ).evaluate(scaleAndAlignCurve),
           child: new ScaleTransition(
-            alignment: const FractionalOffset(0.0, 1.0),
+            alignment: FractionalOffset.bottomLeft,
             scale: new Tween<double>(begin: 1.5, end: 1.0).animate(scaleAndAlignCurve),
             child: new Align(
               alignment: new FractionalOffset(0.0, 1.0),

packages/flutter/lib/src/material/popup_menu.dart

@@ -236,7 +236,7 @@ class _PopupMenu<T> extends StatelessWidget {
             type: MaterialType.card,
             elevation: route.elevation,
             child: new Align(
-              alignment: const FractionalOffset(1.0, 0.0),
+              alignment: FractionalOffset.topRight,
               widthFactor: width.evaluate(route.animation),
               heightFactor: height.evaluate(route.animation),
               child: child

packages/flutter/lib/src/material/scaffold.dart

@@ -534,7 +534,7 @@ class ScaffoldState extends State<Scaffold> {
         bottomSheets.add(_currentBottomSheet._widget);
       Widget stack = new Stack(
         children: bottomSheets,
-        alignment: const FractionalOffset(0.5, 1.0) // bottom-aligned, centered
+        alignment: FractionalOffset.bottomCenter
       );
       _addIfNonNull(children, stack, _ScaffoldSlot.bottomSheet);
     }
@@ -661,7 +661,7 @@ class _PersistentBottomSheetState extends State<_PersistentBottomSheet> {
       animation: config.animationController,
       builder: (BuildContext context, Widget child) {
         return new Align(
-          alignment: const FractionalOffset(0.0, 0.0),
+          alignment: FractionalOffset.topLeft,
           heightFactor: config.animationController.value,
           child: child
         );

packages/flutter/lib/src/material/snack_bar.dart

@@ -111,7 +111,7 @@ class SnackBar extends StatelessWidget {
         animation: heightAnimation,
         builder: (BuildContext context, Widget child) {
           return new Align(
-            alignment: const FractionalOffset(0.0, 0.0),
+            alignment: FractionalOffset.topLeft,
             heightFactor: heightAnimation.value,
             child: child
           );

packages/flutter/lib/src/painting/box_painter.dart

@@ -481,8 +481,8 @@ abstract class Gradient {
 /// A 2D linear gradient.
 class LinearGradient extends Gradient {
   const LinearGradient({
-    this.begin: const FractionalOffset(0.0, 0.5),
-    this.end: const FractionalOffset(1.0, 0.5),
+    this.begin: FractionalOffset.centerLeft,
+    this.end: FractionalOffset.centerRight,
     this.colors,
     this.stops,
     this.tileMode: TileMode.clamp
@@ -570,7 +570,7 @@ class LinearGradient extends Gradient {
 /// A 2D radial gradient.
 class RadialGradient extends Gradient {
   const RadialGradient({
-    this.center: const FractionalOffset(0.5, 0.5),
+    this.center: FractionalOffset.center,
     this.radius: 0.5,
     this.colors,
     this.stops,
@@ -856,7 +856,18 @@ class FractionalOffset {
   final double dx;
   final double dy;
 
-  static const FractionalOffset zero = const FractionalOffset(0.0, 0.0);
+  static const FractionalOffset topLeft = const FractionalOffset(0.0, 0.0);
+  static const FractionalOffset topCenter = const FractionalOffset(0.5, 0.0);
+  static const FractionalOffset topRight = const FractionalOffset(1.0, 0.0);
+
+  static const FractionalOffset bottomLeft = const FractionalOffset(0.0, 1.0);
+  static const FractionalOffset bottomCenter = const FractionalOffset(0.5, 1.0);
+  static const FractionalOffset bottomRight = const FractionalOffset(1.0, 1.0);
+
+  static const FractionalOffset centerLeft = const FractionalOffset(0.0, 0.5);
+  static const FractionalOffset centerRight = const FractionalOffset(1.0, 0.5);
+
+  static const FractionalOffset center = const FractionalOffset(0.5, 0.5);
 
   FractionalOffset operator -() {
     return new FractionalOffset(-dx, -dy);

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

@@ -201,7 +201,7 @@ abstract class RenderStackBase extends RenderBox
          RenderBoxContainerDefaultsMixin<RenderBox, StackParentData> {
   RenderStackBase({
     List<RenderBox> children,
-    FractionalOffset alignment: const FractionalOffset(0.0, 0.0)
+    FractionalOffset alignment: FractionalOffset.topLeft
   }) : _alignment = alignment {
     addAll(children);
   }
@@ -440,7 +440,7 @@ abstract class RenderStackBase extends RenderBox
 class RenderStack extends RenderStackBase {
   RenderStack({
     List<RenderBox> children,
-    FractionalOffset alignment: const FractionalOffset(0.0, 0.0)
+    FractionalOffset alignment: FractionalOffset.topLeft
   }) : super(
    children: children,
    alignment: alignment
@@ -460,7 +460,7 @@ class RenderStack extends RenderStackBase {
 class RenderIndexedStack extends RenderStackBase {
   RenderIndexedStack({
     List<RenderBox> children,
-    FractionalOffset alignment: const FractionalOffset(0.0, 0.0),
+    FractionalOffset alignment: FractionalOffset.topLeft,
     int index: 0
   }) : _index = index, super(
    children: children,

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

@@ -191,7 +191,7 @@ class _DismissableState extends State<Dismissable> {
 
   void _updateMoveAnimation() {
     _moveAnimation = new Tween<FractionalOffset>(
-      begin: FractionalOffset.zero,
+      begin: FractionalOffset.topLeft,
       end: _directionIsXAxis ?
              new FractionalOffset(_dragExtent.sign, 0.0) :
              new FractionalOffset(0.0, _dragExtent.sign)

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

@@ -121,7 +121,7 @@ class ScaleTransition extends AnimatedWidget {
   ScaleTransition({
     Key key,
     Animation<double> scale,
-    this.alignment: const FractionalOffset(0.5, 0.5),
+    this.alignment: FractionalOffset.center,
     this.child
   }) : scale = scale, super(key: key, animation: scale);
 
@@ -177,7 +177,7 @@ class RotationTransition extends AnimatedWidget {
     Matrix4 transform = new Matrix4.rotationZ(turnsValue * math.PI * 2.0);
     return new Transform(
       transform: transform,
-      alignment: const FractionalOffset(0.5, 0.5),
+      alignment: FractionalOffset.center,
       child: child
     );
   }

packages/flutter/test/rendering/box_test.dart

@@ -14,7 +14,7 @@ void main() {
       decoration: new BoxDecoration(
         backgroundColor: const Color(0xFF00FF00),
         gradient: new RadialGradient(
-          center: FractionalOffset.zero, radius: 1.8,
+          center: FractionalOffset.topLeft, radius: 1.8,
           colors: <Color>[Colors.yellow[500], Colors.blue[500]]),
         boxShadow: elevationToShadow[3])
     );

packages/flutter/test/widget/shader_mask_test.dart

@@ -10,12 +10,11 @@ import 'package:test/test.dart';
 
 Shader createShader(Rect bounds) {
   return new LinearGradient(
-      begin: const FractionalOffset(0.0, 0.0),
-      end: const FractionalOffset(0.0, 1.0),
-      colors: <Color>[const Color(0x00FFFFFF), const Color(0xFFFFFFFF)],
-      stops: <double>[0.1, 0.35]
-  )
-  .createShader(bounds);
+    begin: FractionalOffset.topLeft,
+    end: FractionalOffset.bottomLeft,
+    colors: <Color>[const Color(0x00FFFFFF), const Color(0xFFFFFFFF)],
+    stops: <double>[0.1, 0.35]
+  ).createShader(bounds);
 }