Initial commit of Effen reactive framework experiment for Sky

sky/examples/fn/widgets/fixedheightscrollable.dart

+part of widgets;
+
+abstract class FixedHeightScrollable extends Component {
+
+  static Style _style = new Style('''
+    overflow: hidden;
+    position: relative;
+    will-change: transform;'''
+  );
+
+  static Style _scrollAreaStyle = new Style('''
+    position:relative;
+    will-change: transform;'''
+  );
+
+  double itemHeight;
+  double height;
+  double minOffset;
+  double maxOffset;
+
+  double _scrollOffset = 0.0;
+  FlingCurve _flingCurve;
+  int _flingAnimationId;
+
+  FixedHeightScrollable({
+    Object key,
+    this.itemHeight,
+    this.height,
+    this.minOffset,
+    this.maxOffset
+  }) : super(key: key) {}
+
+
+  List<Node> renderItems(int start, int count);
+
+  Node render() {
+    int drawCount = (height / itemHeight).round() + 1;
+    double alignmentDelta = -_scrollOffset % itemHeight;
+    if (alignmentDelta != 0.0) {
+      alignmentDelta -= itemHeight;
+    }
+
+    double drawStart = _scrollOffset + alignmentDelta;
+    int itemNumber = (drawStart / itemHeight).floor();
+
+    var transformStyle =
+        'transform: translateY(${(alignmentDelta).toStringAsFixed(2)}px)';
+
+    var items = renderItems(itemNumber, drawCount);
+
+    return new Container(
+      style: _style,
+      onFlingStart: _handleFlingStart,
+      onFlingCancel: _handleFlingCancel,
+      onScrollUpdate: _handleScrollUpdate,
+      onWheel: _handleWheel,
+      children: [
+        new Container(
+          style: _scrollAreaStyle,
+          inlineStyle: transformStyle,
+          children: items
+        )
+      ]
+    );
+  }
+
+  void willUnmount() {
+    _stopFling();
+  }
+
+  bool _scrollBy(double scrollDelta) {
+    var newScrollOffset = _scrollOffset + scrollDelta;
+    if (minOffset != null && newScrollOffset < minOffset) {
+      newScrollOffset = minOffset;
+    } else if (maxOffset != null && newScrollOffset > maxOffset) {
+      newScrollOffset = maxOffset;
+    }
+    if (newScrollOffset == _scrollOffset) {
+      return false;
+    }
+
+    setState(() {
+      _scrollOffset = newScrollOffset;
+    });
+    return true;
+  }
+
+  void _scheduleFlingUpdate() {
+    _flingAnimationId = sky.window.requestAnimationFrame(_updateFling);
+  }
+
+  void _stopFling() {
+    if (_flingAnimationId == null) {
+      return;
+    }
+
+    sky.window.cancelAnimationFrame(_flingAnimationId);
+    _flingCurve = null;
+    _flingAnimationId = null;
+  }
+
+  void _updateFling(double timeStamp) {
+    double scrollDelta = _flingCurve.update(timeStamp);
+    if (!_scrollBy(scrollDelta))
+      return _stopFling();
+    _scheduleFlingUpdate();
+  }
+
+  void _handleScrollUpdate(sky.Event event) {
+    _scrollBy(-event.dy);
+  }
+
+  void _handleFlingStart(sky.Event event) {
+    setState(() {
+      _flingCurve = new FlingCurve(-event.velocityY, event.timeStamp);
+      _scheduleFlingUpdate();
+    });
+  }
+
+  void _handleFlingCancel(sky.Event event) {
+    _stopFling();
+  }
+
+  void _handleWheel(sky.Event event) {
+    _scrollBy(-event.offsetY);
+  }
+}