XCode - execute code when block is finished - ios

Question

So I'm trying to log-in/signup users using ACAccountStore. This happens using a view controller that presented modally. It works just fine that way, however, when I dismiss the view controller, the underlying/presenting view controller is still a black window. I assume this happens, because I do not wait for the completion block to finish.
So my question: How do I wait for the completion block to finish before calling [self dismissViewControllerAnimated:YES completion:nil];?
-(void)loginWithTwitter{
ACAccountStore *account = [[ACAccountStore alloc] init];
ACAccountType *accountType = [account accountTypeWithAccountTypeIdentifier:
ACAccountTypeIdentifierTwitter];
[account requestAccessToAccountsWithType:accountType options:nil
completion:^(BOOL granted, NSError *error)
{
if (granted) {
//do something -> call function to handle the data and dismiss the modal controller.
}
else{
//fail and put our error message.
}
}];
}

Answer 1

Completion block is that thing that will be executed after the main process (access to accounts request in this case) is finished. So you can put [self dismissViewControllerAnimated:YES completion:nil] in it.
Another thing: it is bad to have reference to self in block because of retain cycles. You would modify your code to look like this:
ACAccountStore *account = [[ACAccountStore alloc] init];
ACAccountType *accountType = [account accountTypeWithAccountTypeIdentifier:
ACAccountTypeIdentifierTwitter];
__weak UIViewController *weakSelf = self;
[account requestAccessToAccountsWithType:accountType options:nil
completion:^(BOOL granted, NSError *error) {
[weakSelf dismissViewControllerAnimated:YES completion:nil];
if (granted) {
//do something -> call function to handle the data and dismiss the modal controller.
}
else {
//fail and put our error message.
}
}];