message: Handle moved messages from UpdateMessageEvent.

We already handle the case where only a message's content is edited.
This handles the case where messages are moved too, between topics
and/or channels.

This introduces more notifyListener calls, and the listeners can be
notified more than once per UpdateMessage event. This is expected.

If the `generation += 1` line is commented out, the message list
has a race bug where a fetchOlder starts; we reset (because
messages were moved into the narrow); and then the fetch returns
and appends in the wrong spot. These races are detailed in the "fetch
races" test group.

lib/api/model/model.dart

@@ -480,7 +480,7 @@ sealed class Message {
   final int senderId;
   final String senderRealmStr;
   @JsonKey(name: 'subject')
-  final String topic;
+  String topic;
   // final List<string> submessages; // TODO handle
   final int timestamp;
   String get type;
@@ -577,7 +577,7 @@ class StreamMessage extends Message {
   String get type => 'stream';
 
   final String displayRecipient;
-  final int streamId;
+  int streamId;
 
   StreamMessage({
     required super.client,

lib/model/message.dart

@@ -178,22 +178,40 @@ class MessageStoreImpl with MessageStore {
       return;
     }
 
-    if (newStreamId == null
-        && MessageEditState.topicMoveWasResolveOrUnresolve(origTopic, newTopic)) {
-      // The topic was only resolved/unresolved.
-      // No change to the messages' editState.
-      return;
-    }
+    final wasResolveOrUnresolve = (newStreamId == null
+      && MessageEditState.topicMoveWasResolveOrUnresolve(origTopic, newTopic));
 
-    // TODO(#150): Handle message moves.  The views' recipient headers
-    //   may need updating, and consequently showSender too.
-    //   Currently only editState gets updated.
     for (final messageId in event.messageIds) {
       final message = messages[messageId];
       if (message == null) continue;
-      // Do not override the edited marker if the message has also been moved.
-      if (message.editState == MessageEditState.edited) continue;
-      message.editState = MessageEditState.moved;
+
+      if (message is! StreamMessage) {
+        assert(debugLog('Bad UpdateMessageEvent: stream/topic move on a DM')); // TODO(log)
+        continue;
+      }
+
+      if (newStreamId != null) {
+        message.streamId = newStreamId;
+        // TODO update [StreamMessage.displayRecipient]; doesn't usually
+        //   matter, because we only consult it when the stream is unknown
+      }
+
+      message.topic = newTopic;
+
+      if (!wasResolveOrUnresolve
+          && message.editState == MessageEditState.none) {
+        message.editState = MessageEditState.moved;
+      }
+    }
+
+    for (final view in _messageListViews) {
+      view.messagesMoved(
+        origStreamId: origStreamId,
+        newStreamId: newStreamId ?? origStreamId,
+        origTopic: origTopic,
+        newTopic: newTopic,
+        messageIds: event.messageIds,
+      );
     }
   }
 

lib/model/message_list.dart

@@ -65,6 +65,9 @@ class MessageListHistoryStartItem extends MessageListItem {
 ///
 /// This comprises much of the guts of [MessageListView].
 mixin _MessageSequence {
+  /// A sequence number for invalidating stale fetches.
+  int generation = 0;
+
   /// The messages.
   ///
   /// See also [contents] and [items].
@@ -192,6 +195,17 @@ mixin _MessageSequence {
     _reprocessAll();
   }
 
+  /// Reset all [_MessageSequence] data, and cancel any active fetches.
+  void _reset() {
+    generation += 1;
+    messages.clear();
+    _fetched = false;
+    _haveOldest = false;
+    _fetchingOlder = false;
+    contents.clear();
+    items.clear();
+  }
+
   /// Redo all computations from scratch, based on [messages].
   void _recompute() {
     assert(contents.length == messages.length);
@@ -396,12 +410,14 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     assert(!fetched && !haveOldest && !fetchingOlder);
     assert(messages.isEmpty && contents.isEmpty);
     // TODO schedule all this in another isolate
+    final generation = this.generation;
     final result = await getMessages(store.connection,
       narrow: narrow.apiEncode(),
       anchor: AnchorCode.newest,
       numBefore: kMessageListFetchBatchSize,
       numAfter: 0,
     );
+    if (this.generation > generation) return;
     store.reconcileMessages(result.messages);
     for (final message in result.messages) {
       if (_messageVisible(message)) {
@@ -423,6 +439,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     _fetchingOlder = true;
     _updateEndMarkers();
     notifyListeners();
+    final generation = this.generation;
     try {
       final result = await getMessages(store.connection,
         narrow: narrow.apiEncode(),
@@ -431,6 +448,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
         numBefore: kMessageListFetchBatchSize,
         numAfter: 0,
       );
+      if (this.generation > generation) return;
 
       if (result.messages.isNotEmpty
           && result.messages.last.id == messages[0].id) {
@@ -447,9 +465,11 @@ class MessageListView with ChangeNotifier, _MessageSequence {
       _insertAllMessages(0, fetchedMessages);
       _haveOldest = result.foundOldest;
     } finally {
-      _fetchingOlder = false;
-      _updateEndMarkers();
-      notifyListeners();
+      if (this.generation == generation) {
+        _fetchingOlder = false;
+        _updateEndMarkers();
+        notifyListeners();
+      }
     }
   }
 
@@ -485,6 +505,72 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     }
   }
 
+  void _messagesMovedInternally(List<int> messageIds) {
+    for (final messageId in messageIds) {
+      if (_findMessageWithId(messageId) != -1) {
+        _reprocessAll();
+        notifyListeners();
+        return;
+      }
+    }
+  }
+
+  void _messagesMovedIntoNarrow() {
+    // If there are some messages we don't have in [MessageStore], and they
+    // occur later than the messages we have here, then we just have to
+    // re-fetch from scratch.  That's always valid, so just do that always.
+    // TODO in cases where we do have data to do better, do better.
+    _reset();
+    notifyListeners();
+    fetchInitial();
+  }
+
+  void _messagesMovedFromNarrow(List<int> messageIds) {
+    if (_removeMessagesById(messageIds)) {
+      notifyListeners();
+    }
+  }
+
+  void messagesMoved({
+    required int origStreamId,
+    required int newStreamId,
+    required String origTopic,
+    required String newTopic,
+    required List<int> messageIds,
+  }) {
+    switch (narrow) {
+      case DmNarrow():
+        // DMs can't be moved (nor created by moves),
+        // so the messages weren't in this narrow and still aren't.
+        return;
+
+      case CombinedFeedNarrow():
+        // The messages were and remain in this narrow.
+        // TODO(#421): … except they may have become muted or not.
+        //   We'll handle that at the same time as we handle muting itself changing.
+        // Recipient headers, and downstream of those, may change, though.
+        _messagesMovedInternally(messageIds);
+
+      case StreamNarrow(:final streamId):
+        switch ((origStreamId == streamId, newStreamId == streamId)) {
+          case (false, false): return;
+          case (true,  true ): _messagesMovedInternally(messageIds);
+          case (false, true ): _messagesMovedIntoNarrow();
+          case (true,  false): _messagesMovedFromNarrow(messageIds);
+        }
+
+      case TopicNarrow(:final streamId, :final topic):
+        final oldMatch = (origStreamId == streamId && origTopic == topic);
+        final newMatch = (newStreamId == streamId && newTopic == topic);
+        switch ((oldMatch, newMatch)) {
+          case (false, false): return;
+          case (true,  true ): _messagesMovedInternally(messageIds);
+          case (false, true ): _messagesMovedIntoNarrow();
+          case (true,  false): _messagesMovedFromNarrow(messageIds); // TODO handle propagateMode
+        }
+    }
+  }
+
   // Repeal the `@protected` annotation that applies on the base implementation,
   // so we can call this method from [MessageStoreImpl].
   @override