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    <p>First, if you wanted to coordinate two different threads <code>dispatch_semaphore_t</code> might be more appropriate than <code>dispatch_group_t</code>.</p> <p>Second, and more importantly, you should not take an asynchronous method such as <code>performRequestWithHandler</code>, invoke it from the main queue in a synchronous manner. You <em>never</em> should be blocking the main queue.</p> <p>Fortunately <code>performRequestWithHandler</code> gives us a <code>handler</code> block which we can use to perform actions after the tweet is done. In your comments, you say you simply want to update your HUD after the tweet, so you should do that <code>performRequestWithHandler</code> (dispatching that UI update back to the main queue, because, as <a href="http://developer.apple.com/library/ios/documentation/Social/Reference/SLRequest_Class/Reference/Reference.html#//apple_ref/occ/instm/SLRequest/performRequestWithHandler%3a" rel="nofollow">the documentation</a> says, "handler is not guaranteed to be called on any particular thread"):</p> <pre><code>- (void)postMyTweet { ACAccountStore *accountStore = [[ACAccountStore alloc] init]; ACAccountType *accountType = [accountStore accountTypeWithAccountTypeIdentifier:ACAccountTypeIdentifierTwitter]; [accountStore requestAccessToAccountsWithType:accountType options:nil completion:^(BOOL granted, NSError *error) { if (granted) { NSArray *allAccounts = [accountStore accountsWithAccountType:accountType]; if ([allAccounts count] &gt; 0) { ACAccount *userAccount = [allAccounts objectAtIndex:0]; NSURL *reqURL = [NSURL URLWithString:ENDPOINT_MEDIA_UPLOAD]; NSDictionary *parameter = [NSDictionary dictionaryWithObject:tweetTitle forKey:@"status"]; SLRequest *twitterRequest = [SLRequest requestForServiceType:SLServiceTypeTwitter requestMethod:SLRequestMethodPOST URL:reqURL parameters:parameter]; [twitterRequest addMultipartData:tweetImage withName:PARAM_MEDIA type:CONTENT_TYPE_MULTIPART_FORM_DATA filename:nil]; [twitterRequest setAccount:userAccount]; [twitterRequest performRequestWithHandler:^(NSData *responseData, NSHTTPURLResponse *urlResponse, NSError *error) { if (error) NSLog(@"tweet fail; error = %@", error); else { long result = [urlResponse statusCode]; if (result == 200) NSLog(@"%ld",result); else NSLog(@"Unexpected response: %@", [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding]]); } // Dispatch UI updates back to main queue dispatch_async(dispatch_get_main_queue(), ^{ // do your MBProgressHUD stuff here }); }]; } } else { NSLog(@"Not authorized"); } }]; } </code></pre> <p>You also asked "How can I pass the HTTP response result to the viewcontroller?" You obviously do all of this in <code>performRequestWithHandler</code>, where you have the HTTP response (and the response data).</p> <hr> <p>If you want <code>postTweet</code> to operate synchronously, then best practices would dictate that you don't submit it from the main queue (because, at the risk of sounding like a broken record, you never want to block the main queue). But you could have <code>actuallySendTweet</code> dispatch this tweet from a background queue, e.g.:</p> <pre><code>- (void) actuallySendTweet { dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ PostTweet * pt = [[PostTweet alloc] init]; [pt postTweetSynchronously]; NSLog(@"Done"); dispatch_async(dispatch_get_main_queue(), ^{ // Now do any UI updates you want here. // For example, do your MBProgressHUD update here. }); }); } - (void)postTweetSynchronously { dispatch_semaphore_t semaphore = dispatch_semaphore_create(0); ACAccountStore *accountStore = [[ACAccountStore alloc] init]; ACAccountType *accountType = [accountStore accountTypeWithAccountTypeIdentifier:ACAccountTypeIdentifierTwitter]; [accountStore requestAccessToAccountsWithType:accountType options:nil completion:^(BOOL granted, NSError *error) { if (granted) { NSArray *allAccounts = [accountStore accountsWithAccountType:accountType]; if ([allAccounts count] &gt; 0) { ACAccount *userAccount = [allAccounts objectAtIndex:0]; NSURL *reqURL = [NSURL URLWithString:ENDPOINT_MEDIA_UPLOAD]; NSDictionary *parameter = [NSDictionary dictionaryWithObject:tweetTitle forKey:@"status"]; SLRequest *twitterRequest = [SLRequest requestForServiceType:SLServiceTypeTwitter requestMethod:SLRequestMethodPOST URL:reqURL parameters:parameter]; [twitterRequest addMultipartData:tweetImage withName:PARAM_MEDIA type:CONTENT_TYPE_MULTIPART_FORM_DATA filename:nil]; [twitterRequest setAccount:userAccount]; [twitterRequest performRequestWithHandler:^(NSData *responseData, NSHTTPURLResponse *urlResponse, NSError *error) { // do whatever you want here, perhaps updating some class properties // now that we're done, signal the semaphore dispatch_semaphore_signal(semaphore); }]; } } else { NSLog(@"Not authorized"); dispatch_semaphore_signal(semaphore); // make sure to signal here, too } }]; dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER); } </code></pre>
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    1. POHow to wait then perform an action based on HTTPrequest response iOS
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    1. COIf you refer back to my main Question, you will notice that I am calling (postMyTweet) from viewcontroller.m class. The method which handle posting tweet to twitter is actually in another class. So when I call it, it is doing the job but the NSLog@"Done"; doesn't wait for postMyTweet method to finish and return the http response result. This is my question: how to make it wait and return the response result then I can do any action. Thanks @Rob
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    2. CO@iMubarak The answer to "how to make it wait" is "you can do it using semaphores, but its inadvisable to do that from the main queue." But I've revised answer to show how you could post your tweet synchronously, but I also demonstrate how to make sure you don't do that on the main queue if you want to be careful about it.
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