Synchronize asynchronous task with NSRunloop or diaptch_semaphore

synchronize_with_runloop_or_semaphore.m

//with semaphore
self.sem = disptach_semaphore_create(0);
[someAsyncTask start]; //someAsyncTask needs to call dispatch_semaphore_signal(self.sem) after the its work is done.
disptach_semaphore_wait(self.sem, DISPATCH_TIME_FOREVER);
NSLog(@"task is done!");

//with NSRunloop
self.taskIsDone = NO;
[someAsyncTask start]; //someAsyncTask needs to set self.taskIsDone = YES after the work is done.
while (!self.taskIsDone)
{
  [[NSRunloop currentRunloop] runMode: NSDefaultRunloopMode beforeDate: [NSDate distantFuture]];
}
NSLog(@"task is done!");

The NSRunloop method won't block the calling thread, while the semaphore method could block the calling thread if 
the dispatch_semaphore_signal(self.sem) is assgined to be executed on the calling thread. For example, if the calling 
thread is the main thread, and dispatch_semaphore_signal(self.sem) is setup to be triggered by notification, 
the execution will never happen because the notification center usually uses the main thread to post notification but 
the main thread is blocked by disptach_semaphore_wait.