Use a semaphore to block an asynchronous call

gistfile1.m

__block dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

[self callAsyncBlock:^() {
    // Do some work
    dispatch_semaphore_signal(semaphore);
}];

// Wait until the semaphore has been signaled
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
dispatch_release(semaphore);