Factor a Scrollable base class out of FixedHeightScrollable

sky/framework/components/scrollable.dart

Index: sky/framework/components/scrollable.dart
diff --git a/sky/framework/components/scrollable.dart b/sky/framework/components/scrollable.dart
new file mode 100644
index 0000000000000000000000000000000000000000..9a62a067dd1e8324e69e6407f2627cbda6dbcd43
--- /dev/null
+++ b/sky/framework/components/scrollable.dart
@@ -0,0 +1,88 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import '../animation/fling_curve.dart';
+import '../fn.dart';
+import 'dart:sky' as sky;
+
+abstract class Scrollable extends Component {
+  double minOffset;
+  double maxOffset;
+
+  double get scrollOffset => _scrollOffset;
+
+  double _scrollOffset = 0.0;
+  FlingCurve _flingCurve;
+  int _flingAnimationId;
+
+  Scrollable({
+    Object key,
+    this.minOffset,
+    this.maxOffset
+  }) : super(key: key) {
+    events.listen('gestureflingstart', _handleFlingStart);
+    events.listen('gestureflingcancel', _handleFlingCancel);
+    events.listen('gesturescrollupdate', _handleScrollUpdate);
+    events.listen('wheel', _handleWheel);
+  }
+
+  void didUnmount() {
+    super.didUnmount();
+    _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.GestureEvent event) {
+    scrollBy(-event.dy);
+  }
+
+  void _handleFlingStart(sky.GestureEvent event) {
+    setState(() {
+      _flingCurve = new FlingCurve(-event.velocityY, event.timeStamp);
+      _scheduleFlingUpdate();
+    });
+  }
+
+  void _handleFlingCancel(sky.GestureEvent event) {
+    _stopFling();
+  }
+
+  void _handleWheel(sky.WheelEvent event) {
+    scrollBy(-event.offsetY);
+  }
+}