Create one listener that merges the leading and trailing glow controllers and...

Create one listener that merges the leading and trailing glow controllers and use it in each paint (#11311)

If a new listener is created for each paint, then the leading and trailing
controllers will accumulate and invoke a list of all those listeners

packages/flutter/lib/src/widgets/overscroll_indicator.dart

@@ -117,12 +117,14 @@ class GlowingOverscrollIndicator extends StatefulWidget {
 class _GlowingOverscrollIndicatorState extends State<GlowingOverscrollIndicator> with TickerProviderStateMixin {
   _GlowController _leadingController;
   _GlowController _trailingController;
+  Listenable _leadingAndTrailingListener;
 
   @override
   void initState() {
     super.initState();
     _leadingController = new _GlowController(vsync: this, color: widget.color, axis: widget.axis);
     _trailingController = new _GlowController(vsync: this, color: widget.color, axis: widget.axis);
+    _leadingAndTrailingListener = new Listenable.merge(<Listenable>[_leadingController, _trailingController]);
   }
 
   @override
@@ -210,6 +212,7 @@ class _GlowingOverscrollIndicatorState extends State<GlowingOverscrollIndicator>
             leadingController: widget.showLeading ? _leadingController : null,
             trailingController: widget.showTrailing ? _trailingController : null,
             axisDirection: widget.axisDirection,
+            repaint: _leadingAndTrailingListener,
           ),
           child: new RepaintBoundary(
             child: widget.child,
@@ -444,8 +447,9 @@ class _GlowingOverscrollIndicatorPainter extends CustomPainter {
     this.leadingController,
     this.trailingController,
     this.axisDirection,
+    Listenable repaint,
   }) : super(
-    repaint: new Listenable.merge(<Listenable>[leadingController, trailingController])
+    repaint: repaint,
   );
 
   /// The controller for the overscroll glow on the side with negative scroll offsets.