[Effen] Reduce splashes when scrolling.

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
index dadcf775a67fd2c445a22c267f8231a0acfe1839..a9402fda9e4c80ebc33fbe371365e641a63528cc 100644
--- a/sky/framework/components/scrollable.dart
+++ b/sky/framework/components/scrollable.dart
@@ -17,6 +17,10 @@ double _velocityForFlingGesture(sky.GestureEvent event) {
       -event.velocityY)) / _kMillisecondsPerSecond;
 }
 
+abstract class ScrollNotifiee {
+  bool ancestorScrolled(Scrollable ancestor);
+}
+
 abstract class Scrollable extends Component {
   ScrollBehavior scrollBehavior;
   double get scrollOffset => _scrollOffset;
@@ -43,12 +47,43 @@ abstract class Scrollable extends Component {
     );
   }
 
+  List<ScrollNotifiee> _registeredScrollNotifiees;
+
+  void registerScrollNotifiee(ScrollNotifiee notifiee) {
+    if (_registeredScrollNotifiees == null)
+      _registeredScrollNotifiees = new List<ScrollNotifiee>();
+    setState(() {
+      _registeredScrollNotifiees.add(notifiee);
+    });
+  }
+
+  void unregisterScrollNotifiee(ScrollNotifiee notifiee) {
+    if (_registeredScrollNotifiees == null)
+      return;
+    setState(() {
+      _registeredScrollNotifiees.remove(notifiee);
+    });
+  }
+
   bool scrollTo(double newScrollOffset) {
     if (newScrollOffset == _scrollOffset)
       return false;
     setState(() {
       _scrollOffset = newScrollOffset;
     });
+    if (_registeredScrollNotifiees != null) {
+      var newList = null;
+      _registeredScrollNotifiees.forEach((target) {
+        if (target.ancestorScrolled(this)) {
+          if (newList == null)
+            newList = new List<ScrollNotifiee>();
+          newList.add(target);
+        }
+      });
+      setState(() {
+        _registeredScrollNotifiees = newList;
+      });
+    }
     return true;
   }