Core Data initialization

Looking at Apple documentation, I see that they are recommending removing Core Data initialization code out of AppDelegate. Their approach is below.

What I don't understand are the following

1. The sentence below in the documentation. How is there a callback to app delegate? I don't see one in the code snippets below. Is it something they want us to add.

By initializing a separate controller object with a completion block, you have moved the Core Data stack out of the application delegate, but you still allow a callback to the application delegate so that the user interface can know when to begin requesting data.

2. AppDelegate calls init of DataController and this in turn this calls initializeCoreData. But initializeCoreData sets up the persistent store co-ordinator in a background thread. Which means that if we transition to the application's first view and its view controller requests data from core-data, things are not yet set up. Won't this be an issue? Does this mean they want us to show a different launch screen & register for a callback that tells us that CoreData initialization is done before moving to actual first application view.

AppDelegate code in documentation

-(BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

    [self setDataController:[[DataController alloc] init];
    // Basic User Interface initialization
    return YES;
}

DataController code in documentation

@interface MyDataController : NSObject

@property (strong) NSManagedObjectContext *managedObjectContext;

-(void)initializeCoreData;

@end

@implementation MyDataController

-(id)init {

    self = [super init];
    if (!self) return nil;

    [self initializeCoreData];

    return self;
}

- (void)initializeCoreData {

    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"DataModel" withExtension:@"momd"];
    NSManagedObjectModel *mom = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    NSAssert(mom != nil, @"Error initializing Managed Object Model");

    NSPersistentStoreCoordinator *psc = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:mom];
    NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    [moc setPersistentStoreCoordinator:psc];
    [self setManagedObjectContext:moc];
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSURL *documentsURL = [[fileManager URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
    NSURL *storeURL = [documentsURL URLByAppendingPathComponent:@"DataModel.sqlite"];

    dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {
        NSError *error = nil;
        NSPersistentStoreCoordinator *psc = [[self managedObjectContext] persistentStoreCoordinator];
        NSPersistentStore *store = [psc addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error];
        NSAssert(store != nil, @"Error initializing PSC: %@\n%@", [error localizedDescription], [error userInfo]);
    });
}