Factor ScrollCurve out of Scrollable

Intead of hard-coding the notion of bounded scrolling into Scrollable, this CL
factors out a ScrollCurve class that applies the bounds. In the future, we'll
refine this mechanism to implement overflow scrolling.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1005633002

examples/stocks-fn/stocklist.dart

@@ -8,7 +8,7 @@ class Stocklist extends FixedHeightScrollable {
     Object key,
     this.stocks,
     this.query
-  }) : super(key: key, minOffset: 0.0);
+  }) : super(key: key, scrollCurve: new BoundedScrollCurve(minOffset: 0.0));
 
   List<Node> buildItems(int start, int count) {
     return stocks

examples/stocks-fn/stocksapp.dart

@@ -1,6 +1,7 @@
 library stocksapp;
 
 import '../../framework/fn.dart';
+import '../../framework/animation/scroll_curve.dart';
 import '../../framework/components/drawer.dart';
 import '../../framework/components/drawer_header.dart';
 import '../../framework/components/fixed_height_scrollable.dart';

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;
+  }
+}

framework/components/fixed_height_scrollable.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import '../animation/scroll_curve.dart';
 import '../fn.dart';
 import 'dart:sky' as sky;
 import 'scrollable.dart';
@@ -27,10 +28,8 @@ abstract class FixedHeightScrollable extends Scrollable {
 
   FixedHeightScrollable({
     Object key,
-    double minOffset,
-    double maxOffset
-  }) : super(key: key, minOffset: minOffset, maxOffset: maxOffset) {
-  }
+    ScrollCurve scrollCurve
+  }) : super(key: key, scrollCurve: scrollCurve);
 
   void didMount() {
     super.didMount();

framework/components/scrollable.dart

@@ -3,24 +3,19 @@
 // found in the LICENSE file.
 
 import '../animation/fling_curve.dart';
+import '../animation/scroll_curve.dart';
 import '../fn.dart';
 import 'dart:sky' as sky;
 
 abstract class Scrollable extends Component {
-  double minOffset;
-  double maxOffset;
-
+  ScrollCurve scrollCurve;
   double get scrollOffset => _scrollOffset;
 
   double _scrollOffset = 0.0;
   FlingCurve _flingCurve;
   int _flingAnimationId;
 
-  Scrollable({
-    Object key,
-    this.minOffset,
-    this.maxOffset
-  }) : super(key: key) {
+  Scrollable({Object key, this.scrollCurve}) : super(key: key) {
     events.listen('gestureflingstart', _handleFlingStart);
     events.listen('gestureflingcancel', _handleFlingCancel);
     events.listen('gesturescrollupdate', _handleScrollUpdate);
@@ -33,15 +28,9 @@ abstract class Scrollable extends Component {
   }
 
   bool scrollBy(double scrollDelta) {
-    var newScrollOffset = _scrollOffset + scrollDelta;
-    if (minOffset != null && newScrollOffset < minOffset)
-      newScrollOffset = minOffset;
-    else if (maxOffset != null && newScrollOffset > maxOffset)
-      newScrollOffset = maxOffset;
-
+    var newScrollOffset = scrollCurve.apply(_scrollOffset, scrollDelta);
     if (newScrollOffset == _scrollOffset)
       return false;
-
     setState(() {
       _scrollOffset = newScrollOffset;
     });