Grand Central Dispatch(GCD)和异步API

我正在使用Twitter API发布tweets。有时这需要一些时间,所以我想在后台执行“Tweet posting”操作。为此,我使用GCD,如下所示:

- (void)myClassMethodToPostTweet {
    dispatch_async(network_queue, ^{
        // … construct the tweet message
        NSString *tweet = @"…";

        // … check if network is available
        [self isConnectedToWeb];

        // … initialize twitter API
        TwitterAPIClass *twitterAPI = [[[TwitterAPIClass alloc] init…] autorelease];
        twitterAPI.delegate = self;
        twitterAPI.APIKey = ...;
        twitterAPI.APISecret = ...;

        // … use twitter API to post the tweet
        [twitterAPI postTweet:tweet];
    });
}

...
/* and when the API reports a successful operation, update the required variables and UI */
...

- (void)twitterAPIDelegateMethodReportingOperationSuccess {
    // … update any variables/records

    // … update UI
    dispatch_async(dispatch_get_main_queue(), ^{
        // … UI updation code
    });
}

问题是,我没有收到委托回调!我错过了什么?