Syncing an existing Core Data store with iCloud

Question

I am trying to get iCloud to work with my app, which needs to migrate an existing local store to a ubiquitous store, if the user requests it.

After some nosing around Apple dev forums and elsewhere, I have taken this approach, which is not working consistently. I have actually seen it work, but only after several crashes on Device B (which is populated from iCloud).

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
  if (persistentStoreCoordinator != nil)
    return persistentStoreCoordinator;

  NSURL *legacyStoreUrl = [NSURL fileURLWithPath:[[self applicationDocumentsDirectory] stringByAppendingPathComponent:[self activeStoreFilenameUpgraded:NO]]];
  NSURL *upgradedStoreUrl = [NSURL fileURLWithPath:[[self applicationDocumentsDirectory] stringByAppendingPathComponent:[self activeStoreFilenameUpgraded:YES]]];

persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];

  if ((IOS_VERSION_GREATER_THAN_OR_EQUAL_TO(@"5.0")) && (self.iCloudEnabled)) {
    NSPersistentStoreCoordinator* psc = persistentStoreCoordinator;

    NSFileManager *fileManager = [NSFileManager defaultManager];

    NSDictionary *cloudOptions = nil;
    NSDictionary *localOptions = [NSDictionary dictionaryWithObjectsAndKeys:
                                    [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
                                    [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption,
                                    nil];


    NSURL *cloudURL = [fileManager URLForUbiquityContainerIdentifier:@"<CONTAINER ID>"];
    NSString *coreDataCloudContent = [[cloudURL path] stringByAppendingPathComponent:[NSString stringWithFormat:@"logs%d",[self activeStoreIndex]]];
    if ([coreDataCloudContent length] != 0) {
        // iCloud is available
        cloudURL = [NSURL fileURLWithPath:coreDataCloudContent];

        cloudOptions = [NSDictionary dictionaryWithObjectsAndKeys:
                       [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
                       [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption,
                       @"MyAppStore", NSPersistentStoreUbiquitousContentNameKey,
                       cloudURL, NSPersistentStoreUbiquitousContentURLKey,
                       nil];
    } else {
        // iCloud is not available
    }

    NSError *error = nil;
    [psc lock];
    if(migrateStore) {
        migrateStore = NO;

        NSPersistentStore *srcPS = [psc addPersistentStoreWithType:NSSQLiteStoreType
            configuration:nil
            URL:legacyStoreUrl
            options:localOptions
            error:&error];
        if (![psc migratePersistentStore:srcPS
            toURL:upgradedStoreUrl
            options:cloudOptions
            withType:NSSQLiteStoreType
            error:&error]) {
            NSLog(@"Error migrating data: %@, %@ / %@ / %@", error, [error userInfo], legacyStoreUrl, upgradedStoreUrl);
            abort();
        }
    }
    else {
        if (![psc addPersistentStoreWithType:NSSQLiteStoreType
            configuration:nil
            URL:upgradedStoreUrl
            options:(cloudOptions ? cloudOptions : localOptions)
            error:&error]) {
              NSLog(@"Unresolved iCloud error %@, %@", error, [error userInfo]);
              abort();
        }
    }
    [psc unlock];

    [[NSNotificationCenter defaultCenter] postNotificationName:@"RefetchAllDatabaseData" object:self userInfo:nil];
  } else {
    NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                             [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
                             [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption,
                             nil];

    NSError *error = nil;
    if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:legacyStoreUrl options:options error:&error]) {
        // error
        abort();
    }    
  }

  return persistentStoreCoordinator;
}

Answer 1

iCloud Core Data syncing is terribly broken. A more reliable third-party solution is TICoreDataSync; this fork has support for syncing over iCloud, without relying on iCloud's Core Data sync implementation. It simply uses iCloud to sync files and handles the Core Data object syncing on its own.

I gave up on iCloud Core Data syncing and have been building my app with this library instead; so far it's been working great, with none of the issues I saw with Apple's implementation.