How to display UIAlertView from block in iOS?

Question

What is the best way to display a UIAlertView from a block?

My code has the following action:

- (IBAction)connectWithTwitterClicked:(id)sender {
    ACAccountStore * account = [[ACAccountStore alloc]init];
    ACAccountType * accountType = [account accountTypeWithAccountTypeIdentifier:ACAccountTypeIdentifierTwitter];

    [account requestAccessToAccountsWithType:accountType options:nil completion:^(BOOL granted, NSError *error) {
        if (granted == YES){
            NSLog(@"Twitter account has been granted");
            NSArray * arrayOfAccounts = [account accountsWithAccountType:accountType];
            if([arrayOfAccounts count] > 0){
                ACAccount * twitterAccount = [arrayOfAccounts lastObject];
                NSLog(@"Found twitter Id of [%@]", twitterAccount.username);
                // continue on to use the twitter Id
            } else {
                // no twitter accounts found, display alert to notify user
            }
        } else{
            NSLog(@"No twitter accounts have been granted");
            // no twitter accounts found, display alert to notify user
        }
    }];
}

I have already tried these solutions:

• Create and show UIAlertView on any of the two commented lines, this leads to the application crash, I believe that this is due to the block being an asynchronous process and does not have access to the UI thread to display a warning
• Created an NSMutableString outside the block, marked it __block, set it to the commented lines, and then display it after. A similar problem here is that the block runs asynchronously, so when the warning is displayed, it does not guarantee NSMutableString is installed.

- ? , .

Answer 1

, , :

- (void)showAlertWithTitle:(NSString *)t
{
    [[[[UIAlertView alloc] initWithTitle:t
                                 message:nil
                                delegate:nil
                       cancelButtonTitle:@"OK"
                       otherButtonTitles:nil
    ] autorelease] show];
}

:

[SomeClass dispatchNastyAsyncBlock:^{
    // ... do stuff, then
    [self performSelectorOnMainThread:@selector(showAlertWithTitle:)
                           withObject:@"Here comes the title"
                        waitUntilDone:YES];
}];

Answer 2

GCD:

[SomeClass dispatchNastyAsyncBlock:^{
    // ... do stuff, then
    dispatch_async(dispatch_get_main_queue(), ^ {
        [[[[UIAlertView alloc] initWithTitle:t
                                     message:nil
                                    delegate:nil
                           cancelButtonTitle:@"OK"
                           otherButtonTitles:nil
        ] autorelease] show];
    });
}];