Support horizontal scrolling in ScrollableViewport

packages/flutter/lib/rendering/box.dart

@@ -1128,10 +1128,10 @@ class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox
   RenderViewport({
     RenderBox child,
     Offset scrollOffset,
-    ViewportScrollDirection direction: ViewportScrollDirection.vertical
+    ViewportScrollDirection scrollDirection: ViewportScrollDirection.vertical
   }) : _scrollOffset = scrollOffset,
-       _scrollDirection = direction {
-    assert(_offsetIsSane(scrollOffset, direction));
+       _scrollDirection = scrollDirection {
+    assert(_offsetIsSane(scrollOffset, scrollDirection));
     this.child = child;
   }
 

packages/flutter/lib/widgets/basic.dart

@@ -20,7 +20,7 @@ import 'package:sky/widgets/default_text_style.dart';
 import 'package:sky/widgets/framework.dart';
 
 export 'package:sky/base/hit_test.dart' show EventDisposition, combineEventDispositions;
-export 'package:sky/rendering/box.dart' show BackgroundImage, BoxConstraints, BoxDecoration, Border, BorderSide, EdgeDims;
+export 'package:sky/rendering/box.dart' show BackgroundImage, BoxConstraints, BoxDecoration, Border, BorderSide, EdgeDims, ViewportScrollDirection;
 export 'package:sky/rendering/flex.dart' show FlexDirection, FlexJustifyContent, FlexAlignItems;
 export 'package:sky/rendering/object.dart' show Point, Offset, Size, Rect, Color, Paint, Path;
 export 'package:sky/rendering/toggleable.dart' show ValueChanged;
@@ -268,17 +268,23 @@ class Baseline extends OneChildRenderObjectWrapper {
 }
 
 class Viewport extends OneChildRenderObjectWrapper {
-  Viewport({ Key key, this.offset: 0.0, Widget child })
-    : super(key: key, child: child);
+  Viewport({
+    Key key,
+    this.scrollOffset: Offset.zero,
+    this.scrollDirection: ViewportScrollDirection.vertical,
+    Widget child
+  }) : super(key: key, child: child);
 
-  final double offset;
+  final Offset scrollOffset;
+  final ViewportScrollDirection scrollDirection;
 
-  RenderViewport createNode() => new RenderViewport(scrollOffset: new Offset(0.0, offset));
+  RenderViewport createNode() => new RenderViewport(scrollOffset: scrollOffset, scrollDirection: scrollDirection);
   RenderViewport get root => super.root;
 
   void syncRenderObject(Viewport old) {
     super.syncRenderObject(old);
-    root.scrollOffset = new Offset(0.0, offset);
+    root.scrollOffset = scrollOffset;
+    root.scrollDirection = scrollDirection;
   }
 }
 

packages/flutter/lib/widgets/scrollable.dart

@@ -31,18 +31,19 @@ abstract class ScrollClient {
   bool ancestorScrolled(Scrollable ancestor);
 }
 
-enum ScrollDirection { vertical, horizontal }
-
 /// A base class for scrollable widgets that reacts to user input and generates
 /// a scrollOffset.
 abstract class Scrollable extends StatefulComponent {
 
   Scrollable({
     Key key,
-    this.direction: ScrollDirection.vertical
-  }) : super(key: key);
+    this.scrollDirection: ViewportScrollDirection.vertical
+  }) : super(key: key) {
+    assert(scrollDirection == ViewportScrollDirection.vertical ||
+        scrollDirection == ViewportScrollDirection.horizontal);
+  }
 
-  ScrollDirection direction;
+  ViewportScrollDirection scrollDirection;
 
   AnimatedSimulation _toEndAnimation; // See _startToEndAnimation()
   AnimationPerformance _toOffsetAnimation; // Started by scrollTo(offset, duration: d)
@@ -57,12 +58,18 @@ abstract class Scrollable extends StatefulComponent {
   }
 
   void syncFields(Scrollable source) {
-    direction == source.direction;
+    scrollDirection == source.scrollDirection;
   }
 
   double _scrollOffset = 0.0;
   double get scrollOffset => _scrollOffset;
 
+  Offset get scrollOffsetVector {
+    if (scrollDirection == ViewportScrollDirection.horizontal)
+      return new Offset(scrollOffset, 0.0);
+    return new Offset(0.0, scrollOffset);
+  }
+
   ScrollBehavior _scrollBehavior;
   ScrollBehavior createScrollBehavior();
   ScrollBehavior get scrollBehavior {
@@ -188,12 +195,12 @@ abstract class Scrollable extends StatefulComponent {
   }
 
   EventDisposition _handleScrollUpdate(sky.GestureEvent event) {
-    scrollBy(direction == ScrollDirection.horizontal ? event.dx : -event.dy);
+    scrollBy(scrollDirection == ViewportScrollDirection.horizontal ? event.dx : -event.dy);
     return EventDisposition.processed;
   }
 
   EventDisposition _handleFlingStart(sky.GestureEvent event) {
-    double eventVelocity = direction == ScrollDirection.horizontal
+    double eventVelocity = scrollDirection == ViewportScrollDirection.horizontal
       ? -event.velocityX
       : -event.velocityY;
     _startToEndAnimation(velocity: _velocityForFlingGesture(eventVelocity));
@@ -225,7 +232,11 @@ abstract class Scrollable extends StatefulComponent {
 /// A simple scrollable widget that has a single child. Use this component if
 /// you are not worried about offscreen widgets consuming resources.
 class ScrollableViewport extends Scrollable {
-  ScrollableViewport({ Key key, this.child }) : super(key: key);
+  ScrollableViewport({
+    Key key,
+    this.child,
+    ViewportScrollDirection scrollDirection: ViewportScrollDirection.vertical
+  }) : super(key: key, scrollDirection: scrollDirection);
 
   Widget child;
 
@@ -258,7 +269,8 @@ class ScrollableViewport extends Scrollable {
     return new SizeObserver(
       callback: _handleViewportSizeChanged,
       child: new Viewport(
-        offset: scrollOffset,
+        scrollOffset: scrollOffsetVector,
+        scrollDirection: scrollDirection,
         child: new SizeObserver(
           callback: _handleChildSizeChanged,
           child: child
@@ -370,7 +382,7 @@ abstract class FixedHeightScrollable extends Scrollable {
     return new SizeObserver(
       callback: _handleSizeChanged,
       child: new Viewport(
-        offset: offsetY,
+        scrollOffset: new Offset(0.0, offsetY),
         child: new Container(
           padding: padding,
           child: new Block(items)

packages/flutter/lib/widgets/tabs.dart

@@ -396,7 +396,7 @@ class TabBar extends Scrollable {
     this.selectedIndex: 0,
     this.onChanged,
     this.isScrollable: false
-  }) : super(key: key, direction: ScrollDirection.horizontal);
+  }) : super(key: key, scrollDirection: ViewportScrollDirection.horizontal);
 
   Iterable<TabLabel> labels;
   int selectedIndex;