Asynchronous:

+ (void)getObjectIDbyUsername:(NSString * )_username compelete:(void(^)(NSString * objectID))compelete{
    
    NSString *username = [NSString stringWithFormat:@"^%@$",_username];
    AVQuery *query = [AVQuery queryWithClassName:@"_User"];
    [query whereKey:@"username" containsString:username];
    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
        if (compelete) {
            compelete(nil);
            compelete = nil;
        }
    }];
}

// use:

[HTBaseContainer getObjectIDbyUsername:@"userName" compelete:^(NSString *objectID) {
    
}];

Sync:

+ (NSString *)getObjectIDbyUsername:(NSString *)_username
{
    dispatch_semaphore_t signal = dispatch_semaphore_create(1);
    
    __block NSString *objectID = NULL;
    NSString *username = [NSString stringWithFormat:@"^%@$",_username];
    AVQuery *query = [AVQuery queryWithClassName:@"_User"];
    [query whereKey:@"username" containsString:username];
    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
        objectID = [objects valueForKey:@"objectId"];
        dispatch_semaphore_signal(signal);
    }];
    
    dispatch_semaphore_wait(signal, DISPATCH_TIME_FOREVER);
    return objectID;
}

1. Add the shape of the factory method to a block, and the block callback objectID

2. dispatch_semaphore_t, wait before return.

You deserve to have GCD group. If you use swift, an @escaping block will do it. .