Add Dismissable support for DismissDirection

```
enum DismissDirection {
  vertical,
  horizontal,
  left,
  right,
  up,
  down
}
```

To only enable dismissing to the right create the `Dismissable` with `direction: DismissDirection.right`. By default direction is `DismissDirection.horizontal` (left or right).

Updated the card_collection "Swipe Away" demo with a drawer that can be used to select one of the three X axis dismiss directions. Currently the MixedViewport class doesn't support horizontal scrolling, so the demo doesn't support the X axis dismiss directions.

examples/widgets/card_collection.dart

@@ -23,8 +23,12 @@ class CardCollectionApp extends App {
   final TextStyle backgroundTextStyle =
     typography.white.title.copyWith(textAlign: TextAlign.center);
 
-  MixedViewportLayoutState layoutState = new MixedViewportLayoutState();
-  List<CardModel> cardModels;
+  MixedViewportLayoutState _layoutState = new MixedViewportLayoutState();
+  List<CardModel> _cardModels;
+  DismissDirection _dismissDirection = DismissDirection.horizontal;
+  bool _drawerShowing = false;
+  AnimationStatus _drawerStatus = AnimationStatus.dismissed;
+
 
   void initState() {
     List<double> cardHeights = <double>[
@@ -32,7 +36,7 @@ class CardCollectionApp extends App {
       48.0, 63.0, 82.0, 146.0, 60.0, 55.0, 84.0, 96.0, 50.0,
       48.0, 63.0, 82.0, 146.0, 60.0, 55.0, 84.0, 96.0, 50.0
     ];
-    cardModels = new List.generate(cardHeights.length, (i) {
+    _cardModels = new List.generate(cardHeights.length, (i) {
       Color color = Color.lerp(colors.Red[300], colors.Blue[900], i / cardHeights.length);
       return new CardModel(i, cardHeights[i], color);
     });
@@ -40,20 +44,91 @@ class CardCollectionApp extends App {
   }
 
   void dismissCard(CardModel card) {
-    if (cardModels.contains(card)) {
+    if (_cardModels.contains(card)) {
       setState(() {
-        cardModels.remove(card);
+        _cardModels.remove(card);
       });
     }
   }
 
-  Widget builder(int index) {
-    if (index >= cardModels.length)
+  void _handleOpenDrawer() {
+    setState(() {
+      _drawerShowing = true;
+      _drawerStatus = AnimationStatus.forward;
+    });
+  }
+
+  void _handleDrawerDismissed() {
+    setState(() {
+      _drawerStatus = AnimationStatus.dismissed;
+    });
+  }
+
+  String dismissDirectionText(DismissDirection direction) {
+    String s = direction.toString();
+    return "dismiss ${s.substring(s.indexOf('.') + 1)}";
+  }
+
+  void changeDismissDirection(DismissDirection newDismissDirection) {
+    setState(() {
+      _dismissDirection = newDismissDirection;
+      _drawerStatus = AnimationStatus.dismissed;
+    });
+  }
+
+  Widget buildDrawer() {
+    if (_drawerStatus == AnimationStatus.dismissed)
+      return null;
+
+    Widget buildDrawerItem(DismissDirection direction, String icon) {
+      return new DrawerItem(
+        icon: icon,
+        onPressed: () { changeDismissDirection(direction); },
+        child: new Row([
+          new Flexible(child: new Text(dismissDirectionText(direction))),
+          new Radio(
+            value: direction,
+            onChanged: changeDismissDirection,
+            groupValue: _dismissDirection
+          )
+        ])
+      );
+    }
+
+    return new IconTheme(
+      data: const IconThemeData(color: IconThemeColor.black),
+      child: new Drawer(
+        level: 3,
+        showing: _drawerShowing,
+        onDismissed: _handleDrawerDismissed,
+        children: [
+          new DrawerHeader(child: new Text('Dismiss Direction')),
+          buildDrawerItem(DismissDirection.horizontal, 'action/code'),
+          buildDrawerItem(DismissDirection.left, 'navigation/arrow_back'),
+          buildDrawerItem(DismissDirection.right, 'navigation/arrow_forward')
+        ]
+      )
+    );
+  }
+
+  Widget buildToolBar() {
+    return new ToolBar(
+      left: new IconButton(icon: "navigation/menu", onPressed: _handleOpenDrawer),
+      center: new Text('Swipe Away'),
+      right: [
+        new Text(dismissDirectionText(_dismissDirection))
+      ]
+    );
+  }
+
+  Widget buildCard(int index) {
+    if (index >= _cardModels.length)
       return null;
 
-    CardModel cardModel = cardModels[index];
+    CardModel cardModel = _cardModels[index];
     Widget card = new Dismissable(
-      onResized: () { layoutState.invalidate([index]); },
+      direction: _dismissDirection,
+      onResized: () { _layoutState.invalidate([index]); },
       onDismissed: () { dismissCard(cardModel); },
       child: new Card(
         color: cardModel.color,
@@ -65,8 +140,28 @@ class CardCollectionApp extends App {
       )
     );
 
-    Widget backgroundText =
-      new Text("Swipe in either direction", style: backgroundTextStyle);
+    String backgroundMessage;
+    switch(_dismissDirection) {
+      case DismissDirection.horizontal:
+        backgroundMessage = "Swipe in either direction";
+        break;
+      case DismissDirection.left:
+        backgroundMessage = "Swipe left to dismiss";
+        break;
+      case DismissDirection.right:
+        backgroundMessage = "Swipe right to dismiss";
+        break;
+      default:
+        backgroundMessage = "Unsupported dismissDirection";
+    }
+
+    Widget leftArrowIcon =  new Icon(type: 'navigation/arrow_back', size: 36);
+    if (_dismissDirection == DismissDirection.right)
+      leftArrowIcon = new Opacity(opacity: 0.1, child: leftArrowIcon);
+
+    Widget rightArrowIcon =  new Icon(type: 'navigation/arrow_forward', size: 36);
+    if (_dismissDirection == DismissDirection.left)
+      rightArrowIcon = new Opacity(opacity: 0.1, child: rightArrowIcon);
 
     // The background Widget appears behind the Dismissable card when the card
     // moves to the left or right. The Positioned widget ensures that the
@@ -85,16 +180,20 @@ class CardCollectionApp extends App {
             height: cardModel.height,
             decoration: new BoxDecoration(backgroundColor: Theme.of(this).primaryColor),
             child: new Row([
-              new Icon(type: 'navigation/arrow_back', size: 36),
-              new Flexible(child: backgroundText),
-              new Icon(type: 'navigation/arrow_forward', size: 36)
+              leftArrowIcon,
+              new Flexible(child: new Text(backgroundMessage, style: backgroundTextStyle)),
+              rightArrowIcon
             ])
           )
         )
       )
     );
 
-    return new Stack([background, card], key: cardModel.key);
+    return new IconTheme(
+      key: cardModel.key,
+      data: const IconThemeData(color: IconThemeColor.white),
+      child: new Stack([background, card])
+    );
   }
 
   Widget build() {
@@ -102,26 +201,24 @@ class CardCollectionApp extends App {
       padding: const EdgeDims.symmetric(vertical: 12.0, horizontal: 8.0),
       decoration: new BoxDecoration(backgroundColor: Theme.of(this).primarySwatch[50]),
       child: new ScrollableMixedWidgetList(
-        builder: builder,
-        token: cardModels.length,
-        layoutState: layoutState
+        builder: buildCard,
+        token: _cardModels.length,
+        layoutState: _layoutState
       )
     );
 
-    return new IconTheme(
-      data: const IconThemeData(color: IconThemeColor.white),
-      child: new Theme(
-        data: new ThemeData(
-          brightness: ThemeBrightness.light,
-          primarySwatch: colors.Blue,
-          accentColor: colors.RedAccent[200]
-        ),
-        child: new Title(
-          title: 'Cards',
-          child: new Scaffold(
-            toolbar: new ToolBar(center: new Text('Swipe Away')),
-            body: cardCollection
-          )
+    return new Theme(
+      data: new ThemeData(
+        brightness: ThemeBrightness.light,
+        primarySwatch: colors.Blue,
+        accentColor: colors.RedAccent[200]
+      ),
+      child: new Title(
+        title: 'Cards',
+        child: new Scaffold(
+          toolbar: buildToolBar(),
+          drawer: buildDrawer(),
+          body: cardCollection
         )
       )
     );

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

@@ -20,6 +20,15 @@ const double _kMinFlingVelocityDelta = 400.0;
 const double _kFlingVelocityScale = 1.0 / 300.0;
 const double _kDismissCardThreshold = 0.4;
 
+enum DismissDirection {
+  vertical,
+  horizontal,
+  left,
+  right,
+  up,
+  down
+}
+
 typedef void ResizedCallback();
 typedef void DismissedCallback();
 
@@ -29,41 +38,51 @@ class Dismissable extends StatefulComponent {
     Key key,
     this.child,
     this.onResized,
-    this.onDismissed
-    // TODO(hansmuller): direction
+    this.onDismissed,
+    this.direction: DismissDirection.horizontal
   }) : super(key: key);
 
   Widget child;
   ResizedCallback onResized;
   DismissedCallback onDismissed;
+  DismissDirection direction;
 
   AnimationPerformance _fadePerformance;
   AnimationPerformance _resizePerformance;
 
   Size _size;
-  double _dragX = 0.0;
+  double _dragExtent = 0.0;
   bool _dragUnderway = false;
 
   void initState() {
     _fadePerformance = new AnimationPerformance(duration: _kCardDismissFadeout);
   }
 
-  void _handleFadeCompleted() {
-    if (!_dragUnderway)
-      _startResizePerformance();
-  }
-
   void syncConstructorArguments(Dismissable source) {
     child = source.child;
     onResized = source.onResized;
     onDismissed = source.onDismissed;
+    direction = source.direction;
+  }
+
+  bool get _directionIsYAxis {
+    return
+      direction == DismissDirection.vertical ||
+      direction == DismissDirection.up ||
+      direction == DismissDirection.down;
+  }
+
+  void _handleFadeCompleted() {
+    if (!_dragUnderway)
+      _startResizePerformance();
   }
 
   Point get _activeCardDragEndPoint {
     if (!_isActive)
       return Point.origin;
     assert(_size != null);
-    return new Point(_dragX.sign * _size.width * _kDismissCardThreshold, 0.0);
+    double extent = _directionIsYAxis ? _size.height : _size.width;
+    return new Point(_dragExtent.sign * extent * _kDismissCardThreshold, 0.0);
   }
 
   bool get _isActive {
@@ -104,19 +123,38 @@ class Dismissable extends StatefulComponent {
     if (_fadePerformance.isAnimating)
       return;
     _dragUnderway = true;
-    _dragX = 0.0;
+    _dragExtent = 0.0;
     _fadePerformance.progress = 0.0;
   }
 
   void _handleScrollUpdate(double scrollOffset) {
     if (!_isActive || _fadePerformance.isAnimating)
       return;
-    double oldDragX = _dragX;
-    _dragX -= scrollOffset;
-    if (oldDragX.sign != _dragX.sign)
+
+    double oldDragExtent = _dragExtent;
+    switch(direction) {
+      case DismissDirection.horizontal:
+      case DismissDirection.vertical:
+        _dragExtent -= scrollOffset;
+        break;
+
+      case DismissDirection.up:
+      case DismissDirection.left:
+        if (_dragExtent - scrollOffset < 0)
+          _dragExtent -= scrollOffset;
+        break;
+
+      case DismissDirection.down:
+      case DismissDirection.right:
+        if (_dragExtent - scrollOffset > 0)
+          _dragExtent -= scrollOffset;
+        break;
+    }
+
+    if (oldDragExtent.sign != _dragExtent.sign)
       setState(() {}); // Rebuild to update the new drag endpoint.
     if (!_fadePerformance.isAnimating)
-      _fadePerformance.progress = _dragX.abs() / (_size.width * _kDismissCardThreshold);
+      _fadePerformance.progress = _dragExtent.abs() / (_size.width * _kDismissCardThreshold);
   }
 
   _handleScrollEnd() {
@@ -129,10 +167,33 @@ class Dismissable extends StatefulComponent {
       _fadePerformance.reverse();
   }
 
-  bool _isHorizontalFlingGesture(sky.GestureEvent event) {
-    double vx = event.velocityX.abs();
-    double vy = event.velocityY.abs();
-    return vx - vy > _kMinFlingVelocityDelta && vx > _kMinFlingVelocity;
+  bool _isFlingGesture(sky.GestureEvent event) {
+    double vx = event.velocityX;
+    double vy = event.velocityY;
+    if (_directionIsYAxis) {
+      if (vy.abs() - vx.abs() < _kMinFlingVelocityDelta)
+        return false;
+      switch(direction) {
+        case DismissDirection.vertical:
+          return vy.abs() > _kMinFlingVelocity;
+        case DismissDirection.up:
+          return -vy > _kMinFlingVelocity;
+        default:
+          return vy > _kMinFlingVelocity;
+      }
+    } else {
+      if (vx.abs() - vy.abs() < _kMinFlingVelocityDelta)
+        return false;
+      switch(direction) {
+        case DismissDirection.horizontal:
+          return vx.abs() > _kMinFlingVelocity;
+        case DismissDirection.left:
+          return -vx > _kMinFlingVelocity;
+        default:
+          return vx > _kMinFlingVelocity;
+      }
+    }
+    return false;
   }
 
   EventDisposition _handleFlingStart(sky.GestureEvent event) {
@@ -142,9 +203,10 @@ class Dismissable extends StatefulComponent {
     _dragUnderway = false;
     if (_fadePerformance.isCompleted) { // drag then fling
       _startResizePerformance();
-    } else if (_isHorizontalFlingGesture(event)) {
-      _dragX = event.velocityX.sign;
-      _fadePerformance.fling(velocity: event.velocityX.abs() * _kFlingVelocityScale);
+    } else if (_isFlingGesture(event)) {
+      double velocity = _directionIsYAxis ? event.velocityY : event.velocityX;
+      _dragExtent = velocity.sign;
+      _fadePerformance.fling(velocity: velocity.abs() * _kFlingVelocityScale);
     } else {
       _fadePerformance.reverse();
     }
@@ -160,8 +222,8 @@ class Dismissable extends StatefulComponent {
 
   Widget build() {
     if (_resizePerformance != null) {
-      AnimatedValue<double> dismissHeight = new AnimatedValue<double>(
-        _size.height,
+      AnimatedValue<double> squashAxisExtent = new AnimatedValue<double>(
+        _directionIsYAxis ? _size.width : _size.height,
         end: 0.0,
         curve: ease,
         interval: _kCardDismissResizeInterval
@@ -170,13 +232,18 @@ class Dismissable extends StatefulComponent {
       return new SquashTransition(
         performance: _resizePerformance,
         direction: Direction.forward,
-        height: dismissHeight);
+        width: _directionIsYAxis ? squashAxisExtent : null,
+        height: !_directionIsYAxis ? squashAxisExtent : null
+      );
     }
 
     return new GestureDetector(
-      onHorizontalScrollStart: _handleScrollStart,
-      onHorizontalScrollUpdate: _handleScrollUpdate,
-      onHorizontalScrollEnd: _handleScrollEnd,
+      onHorizontalScrollStart: _directionIsYAxis ? null : _handleScrollStart,
+      onHorizontalScrollUpdate: _directionIsYAxis ? null : _handleScrollUpdate,
+      onHorizontalScrollEnd: _directionIsYAxis ? null : _handleScrollEnd,
+      onVerticalScrollStart: _directionIsYAxis ? _handleScrollStart : null,
+      onVerticalScrollUpdate: _directionIsYAxis ? _handleScrollUpdate : null,
+      onVerticalScrollEnd: _directionIsYAxis ? _handleScrollEnd : null,
       child: new Listener(
         onGestureFlingStart: _handleFlingStart,
         child: new SizeObserver(