如何在Objective-C iOS中等待完成块完成

我有两个方法的子类：

- (void) aPostRequest:(NSString *) urlStr andJSON:(NSMutableDictionary*) jsonDict completion:(void (^)(NSDictionary *, NSError *))completion
{
    NSError *error = nil;

    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration];
    NSURL *url = [NSURL URLWithString:urlStr];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60.0];

    [request setHTTPMethod:@"POST"];

    [request addValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
    [request addValue:@"application/json" forHTTPHeaderField:@"Accept"];

    // Set up the post data
    NSData *postData = [NSJSONSerialization dataWithJSONObject:jsonDict options:0 error:nil];
    [request setHTTPBody:postData];

    // dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    NSURLSessionDataTask *postDataTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error)
    {
        if (!error)
        {
            // convert the NSData response to a dictionary
            NSDictionary *dictionary = [NSJSONSerialization JSONObjectWithData:data options:0 error:&error];

            if (error)
            {
                // There is a parse error
                completion(nil, error);
            }
            else
            {
                // Success
                completion(dictionary, nil);
            }
        }
        else
        {
            // Error from the session
            completion(nil, error);
        }
        // dispatch_semaphore_signal(semaphore);
    }];
    // dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    [postDataTask resume];
}

这个调用上面方法的方法：

- (NSString *) verifyUnique
{
    // JSON data
    __block NSString *verifyUnique = nil;

    // dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    // URL string
    NSString *url = @"https://the-website-to-handle-this.com";
    NSMutableDictionary *json = [[NSMutableDictionary alloc] init];
    [json setValue:@"testing" forKey:@"name"];

    [self aPostRequest:url andJSON:json completion:^(NSDictionary *response, NSError *error)
     {
         // dispatch_semaphore_signal(semaphore);

         if (!error) {

         } else {  

             verifyUnique = [response objectForKey:@"uniqueness"]; 

             // return verifyUnique;
             // dispatch_semaphore_signal(semaphore);
         }
     }];

    // dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    return verifyUnique;
}

我从另一个视图控制器类调用该函数，然后调用[thisClass verifyUnique];并希望它等待直到返回答案。在视图控制器中调用UI之前，需要先使用活动指示器来处理UI，以便我们等待。我如何等待两个完成块都返回？