正在学习Leancloud，想用一个工厂方法来封装查询代码，但是不能return block里的值，直接return objectID是空值，求指教，谢谢!

+ (NSString *)getObjectIDbyUsername:(NSString *)_username
{
    __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"];
    }];

    return objectID;
}