manage listener with on,didMount

packages/flutter/lib/widgets/variable_height_scrollable.dart

@@ -28,11 +28,20 @@ class VariableHeightScrollable extends Scrollable {
 
   void initState() {
     assert(layoutState != null);
-    layoutState.removeListener(_handleLayoutChanged);
-    layoutState.addListener(_handleLayoutChanged);
     super.initState();
   }
 
+  void didMount() {
+    layoutState.addListener(_handleLayoutChanged);
+    super.didMount();
+  }
+
+  void didUnmount() {
+    layoutState.removeListener(_handleLayoutChanged);
+    super.didUnmount();
+  }
+
+
   void syncFields(VariableHeightScrollable source) {
     builder = source.builder;
     if (token != source.token)
@@ -55,11 +64,6 @@ class VariableHeightScrollable extends Scrollable {
     scrollBehavior.containerSize = newSize.height;
   }
 
-  void didUnmount() {
-    layoutState.removeListener(_handleLayoutChanged);
-    super.didUnmount();
-  }
-
   void _handleLayoutChanged() {
     if (layoutState.didReachLastChild) {
       scrollBehavior.contentsSize = layoutState.contentsSize;