[Effen] Reduce splashes when scrolling.

sky/framework/components/scrollable.dart

 // 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/generators.dart';
 import '../animation/mechanics.dart';
 import '../animation/scroll_behavior.dart';
 import '../fn.dart';
 import '../theme/view-configuration.dart' as config;
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 
 const double _kMillisecondsPerSecond = 1000.0;
 
 double _velocityForFlingGesture(sky.GestureEvent event) {
   return math.max(-config.kMaxFlingVelocity, math.min(config.kMaxFlingVelocity,
       -event.velocityY)) / _kMillisecondsPerSecond;
 }
 
+abstract class ScrollClient {
+  bool ancestorScrolled(Scrollable ancestor);
+}
+
 abstract class Scrollable extends Component {
   ScrollBehavior scrollBehavior;
   double get scrollOffset => _scrollOffset;
 
   double _scrollOffset = 0.0;
   Simulation _simulation;
 
   Scrollable({Object key, this.scrollBehavior}) : super(key: key) {
     onDidUnmount(_stopSimulation);
   }
 
   UINode buildContent();
 
   UINode build() {
     return new EventListenerNode(
       buildContent(),
       onPointerDown: _handlePointerDown,
       onPointerUp: _handlePointerUpOrCancel,
       onPointerCancel: _handlePointerUpOrCancel,
       onGestureFlingStart: _handleFlingStart,
       onGestureFlingCancel: _handleFlingCancel,
       onGestureScrollUpdate: _handleScrollUpdate,
       onWheel: _handleWheel
     );
   }
 
+  List<ScrollClient> _registeredScrollClients;
+
+  void registerScrollClient(ScrollClient notifiee) {
+    if (_registeredScrollClients == null)
+      _registeredScrollClients = new List<ScrollClient>();
+    setState(() {
+      _registeredScrollClients.add(notifiee);
+    });
+  }
+
+  void unregisterScrollClient(ScrollClient notifiee) {
+    if (_registeredScrollClients == null)
+      return;
+    setState(() {
+      _registeredScrollClients.remove(notifiee);
+    });
+  }
+
   bool scrollTo(double newScrollOffset) {
     if (newScrollOffset == _scrollOffset)
       return false;
     setState(() {
       _scrollOffset = newScrollOffset;
     });
+    if (_registeredScrollClients != null) {
+      var newList = null;
+      _registeredScrollClients.forEach((target) {
+        if (target.ancestorScrolled(this)) {
+          if (newList == null)
+            newList = new List<ScrollClient>();
+          newList.add(target);
+        }
+      });
+      setState(() {
+        _registeredScrollClients = newList;
+      });
+    }
     return true;
   }
 
   bool scrollBy(double scrollDelta) {
     var newScrollOffset = scrollBehavior.applyCurve(_scrollOffset, scrollDelta);
     return scrollTo(newScrollOffset);
   }
 
   void _stopSimulation() {
     if (_simulation == null)
@@ skipping 32 matching lines @@
   }
 
   void _handleFlingCancel(sky.GestureEvent event) {
     _startSimulation(_createParticle());
   }
 
   void _handleWheel(sky.WheelEvent event) {
     scrollBy(-event.offsetY);
   }
 }