store: Add and check _disposed on PerAccountStore and UpdateMachine

Author: chrisbobbe

lib/model/store.dart

@@ -454,17 +454,23 @@ class PerAccountStore extends ChangeNotifier with EmojiStore, ChannelStore, Mess
     autocompleteViewManager.reassemble();
   }
 
+  bool _disposed = false;
+
   @override
   void dispose() {
+    assert(!_disposed);
     recentDmConversationsView.dispose();
     unreads.dispose();
     _messages.dispose();
     typingStatus.dispose();
     updateMachine?.dispose();
+    _disposed = true;
     super.dispose();
   }
 
   Future<void> handleEvent(Event event) async {
+    assert(!_disposed);
+
     switch (event) {
       case HeartbeatEvent():
         assert(debugLog("server event: heartbeat"));
@@ -606,6 +612,8 @@ class PerAccountStore extends ChangeNotifier with EmojiStore, ChannelStore, Mess
   }
 
   Future<void> sendMessage({required MessageDestination destination, required String content}) {
+    assert(!_disposed);
+
     // TODO implement outbox; see design at
     //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.23M3881.20Sending.20outbox.20messages.20is.20fraught.20with.20issues/near/1405739
     return _apiSendMessage(connection,
@@ -790,6 +798,8 @@ class UpdateMachine {
   final String queueId;
   int lastEventId;
 
+  bool _disposed = false;
+
   static Future<InitialSnapshot> _registerQueueWithRetry(
       ApiConnection connection) async {
     BackoffMachine? backoffMachine;
@@ -868,8 +878,11 @@ class UpdateMachine {
     BackoffMachine? backoffMachine;
 
     while (true) {
+      if (_disposed) return;
+
       if (_debugLoopSignal != null) {
         await _debugLoopSignal!.future;
+        if (_disposed) return;
         assert(() {
           _debugLoopSignal = Completer();
           return true;
@@ -880,7 +893,10 @@ class UpdateMachine {
       try {
         result = await getEvents(store.connection,
           queueId: queueId, lastEventId: lastEventId);
+        if (_disposed) return;
       } catch (e) {
+        if (_disposed) return;
+
         store.isLoading = true;
         switch (e) {
           case ZulipApiException(code: 'BAD_EVENT_QUEUE_ID'):
@@ -929,6 +945,7 @@ class UpdateMachine {
       final events = result.events;
       for (final event in events) {
         await store.handleEvent(event);
+        if (_disposed) return;
       }
       if (events.isNotEmpty) {
         lastEventId = events.last.id;
@@ -944,6 +961,7 @@ class UpdateMachine {
   // TODO(#322) save acked token, to dedupe updating it on the server
   // TODO(#323) track the addFcmToken/etc request, warn if not succeeding
   Future<void> registerNotificationToken() async {
+    assert(!_disposed);
     if (!debugEnableRegisterNotificationToken) {
       return;
     }
@@ -957,12 +975,14 @@ class UpdateMachine {
     await NotificationService.registerToken(store.connection, token: token);
   }
 
-  /// Cleans up resources.
+  /// Cleans up resources and tells the instance not to make new API requests.
   ///
   /// After this is called, the instance is not in a usable state
   /// and should be abandoned.
   void dispose() { // TODO abort long-poll and close ApiConnection
+    assert(!_disposed);
     NotificationService.instance.token.removeListener(_registerNotificationToken);
+    _disposed = true;
   }
 
   /// In debug mode, controls whether [fetchEmojiData] should