Using notifications to keep changes in sync across multiple devices

I don't use notifications. I call a fetch with the last token each time the app enters foreground as described in the code below. If you wish, you could do this and also respond to a notification when your app is active - although that response would apply only when one account has two devices both running the app -overkill IMHO. But then you can clear the notification without needing to worry about it since all active apps will get it.

- (void)applicationWillEnterForeground:(UIApplication *)application {
      [self fetchTheRides];
//  etc..
}


- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo {
    // I don't do this - it's too responsive.  But if you want:
           //  check notification and if appropriate,  
             //      [self fetchTheRides];
             //       mark notification as read
}
   
-(void)fetchTheRides{
    CKRecordZoneID *ridesZoneID=[[CKRecordZoneID alloc] initWithZoneName:@"RidesZone" ownerName:CKOwnerDefaultName];
    CKFetchRecordChangesOperation *getRides=[[CKFetchRecordChangesOperation alloc] initWithRecordZoneID:ridesZoneID previousServerChangeToken:[parameters objectForKey:@"RidesToken"]];
    __weak CKFetchRecordChangesOperation *weakGetRides=getRides;  // because I need to check it inside block
    getRides.qualityOfService=NSQualityOfServiceUserInitiated;
    getRides.recordChangedBlock=^(CKRecord *aRide){
       //handle aRide having changed
    };
    getRides.fetchRecordChangesCompletionBlock=^(CKServerChangeToken *serverChangeToken, NSData *clientChangeTokenData, NSError *operationError){
        if(!weakGetRides.moreComing){  /
            dispatch_async(dispatch_get_main_queue(), ^{
               // all done   
                [[NSNotificationCenter defaultCenter] postNotificationName:@"RidesWereUpdated" object:nil];
            });
        }
     
        if(operationError){
             //handle error
        }else{
            [parameters setObject:serverChangeToken forKey:@"RidesToken"];
            if(weakGetRides.moreComing){
                [self fetchTheRides];
            }else{
                [parameters setValue:[NSNumber numberWithBool:YES] forKey:@"iCloudAvailable"];
                dispatch_async(dispatch_get_main_queue(), ^{
                      [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
                });
            }
        }
    };
    [[[CKContainer defaultContainer] privateCloudDatabase] addOperation:getRides];

Thanks very much for your reply. Unfortunately, I cannot use your approach since I'm using the public database (I need to track changes made by multiple users). The CKFetchRecordChangesOperation will only work with custom zones, which are only available in the private database, if I understand it correctly. Otherwise, your suggestion would be perfect for me.

If you have other suggestions, or believe my understanding is incorrect, please let me know. I'd really appreicate it.

Ok, so divide up the problem into

1) synching between different iCloud Accounts

and

2) synching all devices under the same iCloud Account

Accomplish #1 as you suggest to synch a file in the public database with a file in the private database for an iCloud Account. Do that for each iCloud Account that wants to synch to the file in the public database. You can clear the subscription after you read it. (Note - subscriptions are unique for each iCloud Account so clearing the subscription only ckears it for those devices under that iCloud Account)). Also synch the local files on a specific device when that device updates the file in its private database and reset its token as I describe above.

Use my approach to synch that file, now updated in the private database, among all the other devices under that iCloud Account.

QED.

Thanks. I'll dig in to that approach a bit. It might get tricky since any device that is online will likely get the public DB nofiticaion and I'll have to make sure they both don't update the private database.

I've also toyed with the idea of tracking the notifications on a per-device basis myself (in the private database), and then marking them as read when I know that all the devices have processed them. That way I don't have to have record data in three places (public, private, local).

It still would be nice if CloudKit managed per-device notifications in a better way. That would make all of this easier.

I appreicate your suggestions.

Keep the data only in one file in the public database. When it changes send out a notification to all subscribed devices. When an active device receives a notification it synchs and clears the notification. When a device enters foreground it searches for unread notifications and, if there, synchs and clears. Any device that responds to a notification also changes a private database file - just an inconsequential change will do. When a device enters foreground it checks the private database file for changes. If it detects a change it synchs with the public file but does not update the private file. The only duplication will then be if multiple devices on the same iCloud Account respond to the notification. That will cause the first of those devices to needlessly resynch the next time they enter foreground. No real problem.