ios中，用工厂方法如何return block里的值？

Question

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;
}

Answer 1

异步：

+ (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) {
    
}];

同步:

+ (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;
}

Answer 2

1. 把工厂方法的形参加一个block, block 回调objectID

2. dispatch_semaphore_t, 在return前一直wait.

Answer 3

GCD group你值得拥有，假如用swift的话一个@escaping block就搞定了。。