msglist [nfc]: refactor common code out of scroll and metrics change functions

Author: sirpengi

lib/widgets/message_list.dart

@@ -151,15 +151,23 @@ class _MessageListState extends State<MessageList> {
     });
   }
 
-  void _scrollChanged() {
-    final scrollPosition = scrollController.position;
-    if (scrollPosition.extentBefore == 0) {
+  void _adjustButtonVisibility(ScrollMetrics scrollMetrics) {
+    if (scrollMetrics.extentBefore == 0) {
       _scrollToBottomVisibleValue.value = false;
     } else {
       _scrollToBottomVisibleValue.value = true;
     }
   }
 
+  void _scrollChanged() {
+    _adjustButtonVisibility(scrollController.position);
+  }
+
+  bool _metricsChanged(ScrollMetricsNotification scrollMetricsNotification) {
+    _adjustButtonVisibility(scrollMetricsNotification.metrics);
+    return true;
+  }
+
   @override
   Widget build(BuildContext context) {
     assert(model != null);
@@ -180,15 +188,7 @@ class _MessageListState extends State<MessageList> {
             child: ConstrainedBox(
               constraints: const BoxConstraints(maxWidth: 760),
               child: NotificationListener<ScrollMetricsNotification>(
-                onNotification: (scrollMetricsNotification) {
-                  final metrics = scrollMetricsNotification.metrics;
-                  if (metrics.extentBefore == 0) {
-                    _scrollToBottomVisibleValue.value = false;
-                  } else {
-                    _scrollToBottomVisibleValue.value = true;
-                  }
-                  return true;
-                },
+                onNotification: _metricsChanged,
                 child: Stack(
                   children: <Widget>[
                     _buildListView(context),