msglist: Implement refresh, to be used for unsubscribed channels

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -137,6 +137,17 @@ abstract class MessageListPageState extends State<MessageListPage> {
   /// The narrow for this page's message list.
   Narrow get narrow;
 
+  /// Resets the [MessageListView] model, triggering an initial fetch.
+  ///
+  /// If [anchor] isn't passed, reuses the anchor from the last initial fetch.
+  ///
+  /// Useful when updates won't arrive through the event system,
+  /// as when showing an unsubscribed channel.
+  /// (New-message events aren't sent for unsubscribed channels.)
+  ///
+  /// Does nothing if [MessageList] has not mounted yet.
+  void refresh([Anchor? anchor]);
+
   /// The [ComposeBoxState] for this [MessageListPage]'s compose box,
   /// if this [MessageListPage] offers a compose box and it has mounted,
   /// else null.
@@ -267,6 +278,14 @@ class _MessageListPageState extends State<MessageListPage> implements MessageLis
   @override
   late Narrow narrow;
 
+  @override
+  void refresh([Anchor? anchor]) {
+    // TODO If anchor isn't passed, check if there's some onscreen message
+    //   we can anchor to, before defaulting to model.anchor.
+    //   Update the dartdoc on this method with the new behavior.
+    model?.renarrowAndFetch(narrow, anchor ?? model!.anchor);
+  }
+
   @override
   ComposeBoxState? get composeBoxState => _composeBoxKey.currentState;
   final GlobalKey<ComposeBoxState> _composeBoxKey = GlobalKey();