Make iOS blocks execute synchronously

Question

How can I make a block execute synchronously, or make the function wait for the handler before the return statement, so the data can be passed back from the block?

-(id)performRequest:(id)args
{
__block NSData *data = nil;   

    [xyzclass requestAccessToAccountsWithType:accountType withCompletionHandler:^(BOOL granted, NSError *error) {
        data = [NSData dataWithData:responseData];
    }];

    return data;
}

Answer 1

You can use semaphores in this case.

-(id)performRequest:(id)args
{
    __block NSData *data = nil;   
     dispatch_semaphore_t sem = dispatch_semaphore_create(0);
     [xyzclass requestAccessToAccountsWithType:accountType withCompletionHandler:^(BOOL granted, NSError *error) {
       data = [NSData dataWithData:responseData];
       dispatch_semaphore_signal(sem);
     }];
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    return data;
}

semaphore will block execution of further statements until signal is received, this will make sure that your function does not return prematurely.