Merge pull request #1826 from Hixie/size-obs-1

SizeObserver crusade: snap alignment

examples/widgets/card_collection.dart

@@ -28,6 +28,8 @@ class CardCollectionState extends State<CardCollection> {
   // TODO(hansmuller): need a local image asset
   static const _sunshineURL = "http://www.walltor.com/images/wallpaper/good-morning-sunshine-58540.jpg";
 
+  static const kCardMargins = 8.0;
+
   final TextStyle backgroundTextStyle =
     Typography.white.title.copyWith(textAlign: TextAlign.center);
 
@@ -41,7 +43,6 @@ class CardCollectionState extends State<CardCollection> {
   bool _sunshine = false;
   bool _varyFontSizes = false;
   InvalidatorCallback _invalidator;
-  Size _cardCollectionSize = new Size(200.0, 200.0);
 
   void _initVariableSizedCardModels() {
     List<double> cardHeights = <double>[
@@ -78,15 +79,14 @@ class CardCollectionState extends State<CardCollection> {
 
   double _variableSizeToSnapOffset(double scrollOffset) {
     double cumulativeHeight = 0.0;
-    double  margins = 8.0;
     List<double> cumulativeHeights = _cardModels.map((CardModel card) {
-      cumulativeHeight += card.height + margins;
+      cumulativeHeight += card.height + kCardMargins;
       return cumulativeHeight;
     })
     .toList();
 
     double offsetForIndex(int i) {
-      return 12.0 + (margins + _cardModels[i].height) / 2.0 + ((i == 0) ? 0.0 : cumulativeHeights[i - 1]);
+      return (kCardMargins + _cardModels[i].height) / 2.0 + ((i == 0) ? 0.0 : cumulativeHeights[i - 1]);
     }
 
     for (int i = 0; i <  cumulativeHeights.length; i++) {
@@ -99,11 +99,14 @@ class CardCollectionState extends State<CardCollection> {
   double _fixedSizeToSnapOffset(double scrollOffset) {
     double cardHeight = _cardModels[0].height;
     int cardIndex = (scrollOffset.clamp(0.0, cardHeight * (_cardModels.length - 1)) / cardHeight).floor();
-    return 12.0 + cardIndex * cardHeight + cardHeight * 0.5;
+    return cardIndex * cardHeight + cardHeight * 0.5;
   }
 
-  double _toSnapOffset(double scrollOffset) {
-    return _fixedSizeCards ? _fixedSizeToSnapOffset(scrollOffset) : _variableSizeToSnapOffset(scrollOffset);
+  double _toSnapOffset(double scrollOffset, Size containerSize) {
+    double halfHeight = containerSize.height / 2.0;
+    scrollOffset += halfHeight;
+    double result = _fixedSizeCards ? _fixedSizeToSnapOffset(scrollOffset) : _variableSizeToSnapOffset(scrollOffset);
+    return result - halfHeight;
   }
 
   void dismissCard(CardModel card) {
@@ -300,7 +303,7 @@ class CardCollectionState extends State<CardCollection> {
         color: Theme.of(context).primarySwatch[cardModel.color],
         child: new Container(
           height: cardModel.height,
-          padding: const EdgeDims.all(8.0),
+          padding: const EdgeDims.all(kCardMargins),
           child: _editable ?
             new Center(
               child: new Input(
@@ -387,12 +390,6 @@ class CardCollectionState extends State<CardCollection> {
     );
   }
 
-  void _updateCardCollectionSize(Size newSize) {
-    setState(() {
-      _cardCollectionSize = newSize;
-    });
-  }
-
   Shader _createShader(Rect bounds) {
     return new LinearGradient(
         begin: bounds.topLeft,
@@ -408,7 +405,6 @@ class CardCollectionState extends State<CardCollection> {
     if (_fixedSizeCards) {
       cardCollection = new ScrollableList (
         snapOffsetCallback: _snapToCenter ? _toSnapOffset : null,
-        snapAlignmentOffset: _cardCollectionSize.height / 2.0,
         itemExtent: _cardModels[0].height,
         children: _cardModels.map((CardModel card) => _buildCard(context, card.value))
       );
@@ -417,7 +413,6 @@ class CardCollectionState extends State<CardCollection> {
         builder: _buildCard,
         token: _cardModels.length,
         snapOffsetCallback: _snapToCenter ? _toSnapOffset : null,
-        snapAlignmentOffset: _cardCollectionSize.height / 2.0,
         onInvalidatorAvailable: (InvalidatorCallback callback) { _invalidator = callback; }
       );
     }
@@ -431,13 +426,10 @@ class CardCollectionState extends State<CardCollection> {
       );
     }
 
-    Widget body = new SizeObserver(
-      onSizeChanged: _updateCardCollectionSize,
-      child: new Container(
-        padding: const EdgeDims.symmetric(vertical: 12.0, horizontal: 8.0),
-        decoration: new BoxDecoration(backgroundColor: Theme.of(context).primarySwatch[50]),
-        child: cardCollection
-      )
+    Widget body = new Container(
+      padding: const EdgeDims.symmetric(vertical: 12.0, horizontal: 8.0),
+      decoration: new BoxDecoration(backgroundColor: Theme.of(context).primarySwatch[50]),
+      child: cardCollection
     );
 
     if (_snapToCenter) {

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

@@ -28,7 +28,6 @@ class PageableList extends Scrollable {
     ScrollListener onScroll,
     ScrollListener onScrollEnd,
     SnapOffsetCallback snapOffsetCallback,
-    double snapAlignmentOffset: 0.0,
     this.itemsWrap: false,
     this.itemsSnapAlignment: ItemsSnapAlignment.adjacentItem,
     this.onPageChanged,
@@ -44,8 +43,7 @@ class PageableList extends Scrollable {
     onScrollStart: onScrollStart,
     onScroll: onScroll,
     onScrollEnd: onScrollEnd,
-    snapOffsetCallback: snapOffsetCallback,
-    snapAlignmentOffset: snapAlignmentOffset
+    snapOffsetCallback: snapOffsetCallback
   );
 
   final bool itemsWrap;

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

@@ -25,7 +25,7 @@ final Tolerance kPixelScrollTolerance = new Tolerance(
 );
 
 typedef void ScrollListener(double scrollOffset);
-typedef double SnapOffsetCallback(double scrollOffset);
+typedef double SnapOffsetCallback(double scrollOffset, Size containerSize);
 
 /// A base class for scrollable widgets.
 ///
@@ -43,12 +43,10 @@ abstract class Scrollable extends StatefulComponent {
     this.onScrollStart,
     this.onScroll,
     this.onScrollEnd,
-    this.snapOffsetCallback,
-    this.snapAlignmentOffset: 0.0
+    this.snapOffsetCallback
   }) : super(key: key) {
     assert(scrollDirection == Axis.vertical || scrollDirection == Axis.horizontal);
     assert(scrollAnchor == ViewportAnchor.start || scrollAnchor == ViewportAnchor.end);
-    assert(snapAlignmentOffset != null);
   }
 
   /// The scroll offset this widget should use when first created.
@@ -68,11 +66,21 @@ abstract class Scrollable extends StatefulComponent {
   /// Called whenever this widget stops scrolling.
   final ScrollListener onScrollEnd;
 
-  /// Called to determine the offset to which scrolling should snap.
+  /// Called to determine the offset to which scrolling should snap,
+  /// when handling a fling.
+  ///
+  /// This callback, if set, will be called with the offset that the
+  /// Scrollable would have scrolled to in the absence of this
+  /// callback, and a Size describing the size of the Scrollable
+  /// itself.
+  ///
+  /// The callback's return value is used as the new scroll offset to
+  /// aim for.
+  ///
+  /// If the callback simply returns its first argument (the offset),
+  /// then it is as if the callback was null.
   final SnapOffsetCallback snapOffsetCallback;
 
-  final double snapAlignmentOffset; // What does this do?
-
   /// The state from the closest instance of this class that encloses the given context.
   static ScrollableState of(BuildContext context) {
     return context.ancestorStateOfType(const TypeMatcher<ScrollableState>());
@@ -294,7 +302,8 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
 
   /// Returns the snapped offset closest to the given scroll offset.
   double snapScrollOffset(double scrollOffset) {
-    return config.snapOffsetCallback == null ? scrollOffset : config.snapOffsetCallback(scrollOffset);
+    RenderBox box = context.findRenderObject();
+    return config.snapOffsetCallback == null ? scrollOffset : config.snapOffsetCallback(scrollOffset, box.size);
   }
 
   /// Whether this scrollable should attempt to snap scroll offsets.
@@ -312,20 +321,20 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
     if (endScrollOffset.isNaN)
       return null;
 
-    final double snappedScrollOffset = snapScrollOffset(endScrollOffset + config.snapAlignmentOffset);
-    final double alignedScrollOffset = snappedScrollOffset - config.snapAlignmentOffset;
-    if (!_scrollOffsetIsInBounds(alignedScrollOffset))
+    final double snappedScrollOffset = snapScrollOffset(endScrollOffset);
+    if (!_scrollOffsetIsInBounds(snappedScrollOffset))
       return null;
 
-    final double snapVelocity = scrollVelocity.abs() * (alignedScrollOffset - scrollOffset).sign;
+    final double snapVelocity = scrollVelocity.abs() * (snappedScrollOffset - scrollOffset).sign;
     final double endVelocity = pixelOffsetToScrollOffset(kPixelScrollTolerance.velocity).abs() * scrollVelocity.sign;
-    Simulation toSnapSimulation =
-      scrollBehavior.createSnapScrollSimulation(scrollOffset, alignedScrollOffset, snapVelocity, endVelocity);
+    Simulation toSnapSimulation = scrollBehavior.createSnapScrollSimulation(
+      scrollOffset, snappedScrollOffset, snapVelocity, endVelocity
+    );
     if (toSnapSimulation == null)
       return null;
 
-    final double scrollOffsetMin = math.min(scrollOffset, alignedScrollOffset);
-    final double scrollOffsetMax = math.max(scrollOffset, alignedScrollOffset);
+    final double scrollOffsetMin = math.min(scrollOffset, snappedScrollOffset);
+    final double scrollOffsetMax = math.max(scrollOffset, snappedScrollOffset);
     return new ClampedSimulation(toSnapSimulation, xMin: scrollOffsetMin, xMax: scrollOffsetMax);
   }
 
@@ -631,7 +640,6 @@ class ScrollableMixedWidgetList extends Scrollable {
     double initialScrollOffset,
     ScrollListener onScroll,
     SnapOffsetCallback snapOffsetCallback,
-    double snapAlignmentOffset: 0.0,
     this.builder,
     this.token,
     this.onInvalidatorAvailable
@@ -639,8 +647,7 @@ class ScrollableMixedWidgetList extends Scrollable {
     key: key,
     initialScrollOffset: initialScrollOffset,
     onScroll: onScroll,
-    snapOffsetCallback: snapOffsetCallback,
-    snapAlignmentOffset: snapAlignmentOffset
+    snapOffsetCallback: snapOffsetCallback
   );
 
   final IndexedBuilder builder;

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

@@ -20,7 +20,6 @@ class ScrollableGrid extends Scrollable {
     double initialScrollOffset,
     ScrollListener onScroll,
     SnapOffsetCallback snapOffsetCallback,
-    double snapAlignmentOffset: 0.0,
     this.delegate,
     this.children
   }) : super(
@@ -31,8 +30,7 @@ class ScrollableGrid extends Scrollable {
     // delegate that places children in column-major order.
     scrollDirection: Axis.vertical,
     onScroll: onScroll,
-    snapOffsetCallback: snapOffsetCallback,
-    snapAlignmentOffset: snapAlignmentOffset
+    snapOffsetCallback: snapOffsetCallback
   );
 
   final GridDelegate delegate;

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

@@ -19,7 +19,6 @@ class ScrollableList extends Scrollable {
     ViewportAnchor scrollAnchor: ViewportAnchor.start,
     ScrollListener onScroll,
     SnapOffsetCallback snapOffsetCallback,
-    double snapAlignmentOffset: 0.0,
     this.itemExtent,
     this.itemsWrap: false,
     this.padding,
@@ -31,8 +30,7 @@ class ScrollableList extends Scrollable {
     scrollDirection: scrollDirection,
     scrollAnchor: scrollAnchor,
     onScroll: onScroll,
-    snapOffsetCallback: snapOffsetCallback,
-    snapAlignmentOffset: snapAlignmentOffset
+    snapOffsetCallback: snapOffsetCallback
   ) {
     assert(itemExtent != null);
   }
@@ -269,7 +267,6 @@ class ScrollableLazyList extends Scrollable {
     ViewportAnchor scrollAnchor: ViewportAnchor.start,
     ScrollListener onScroll,
     SnapOffsetCallback snapOffsetCallback,
-    double snapAlignmentOffset: 0.0,
     this.itemExtent,
     this.itemCount,
     this.itemBuilder,
@@ -281,8 +278,7 @@ class ScrollableLazyList extends Scrollable {
     scrollDirection: scrollDirection,
     scrollAnchor: scrollAnchor,
     onScroll: onScroll,
-    snapOffsetCallback: snapOffsetCallback,
-    snapAlignmentOffset: snapAlignmentOffset
+    snapOffsetCallback: snapOffsetCallback
   ) {
     assert(itemExtent != null);
     assert(itemBuilder != null);

packages/flutter/test/widget/snap_scrolling_test.dart

@@ -20,7 +20,7 @@ Widget buildItem(int item) {
   );
 }
 
-double snapOffsetCallback(double offset) {
+double snapOffsetCallback(double offset, Size size) {
   return (offset / itemExtent).floor() * itemExtent;
 }