Factor ScrollCurve out of Scrollable

sky/framework/animation/scroll_curve.dart

Index: sky/framework/animation/scroll_curve.dart
diff --git a/sky/framework/animation/scroll_curve.dart b/sky/framework/animation/scroll_curve.dart
new file mode 100644
index 0000000000000000000000000000000000000000..07940aee1337040426a9e52e7a36d3f083adc384
--- /dev/null
+++ b/sky/framework/animation/scroll_curve.dart
@@ -0,0 +1,26 @@
+// 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 'dart:math' as math;
+
+abstract class ScrollCurve {
+  // Returns the new scroll offset.
+  double apply(double scrollOffset, double scrollDelta);
+}
+
+class BoundedScrollCurve extends ScrollCurve {
+  double minOffset;
+  double maxOffset;
+
+  BoundedScrollCurve({this.minOffset: 0.0, this.maxOffset});
+
+  double apply(double scrollOffset, double scrollDelta) {
+    double newScrollOffset = scrollOffset + scrollDelta;
+    if (minOffset != null)
+      newScrollOffset = math.max(minOffset, newScrollOffset);
+    if (maxOffset != null)
+      newScrollOffset = math.min(maxOffset, newScrollOffset);
+    return newScrollOffset;
+  }
+}