title: wait for other thread to finish in objective c
body: <p>I am using MGTwitterEngine to fetch tweets from twitter. This uses a asynchrounous paradigm to fetch that tweetsin another thread. It returns the fetched results to the main thread.</p>
  
  <p>Because I have some processing todo after the tweets are fetched, I would like to introduce another thread to prevent locking the UI thread.
  I would lik to do it this way:
  UI thread starts a new thread X. thread X starts the asynchronous fetching of tweets with MGTEngine, and <strong>waits</strong> for that to finish. When MGTwitterEngine returns, thread X processes the tweets, and notifies the UI thread that we are ready.</p>
  
  <p>My question is: How to set thread X to wait till MGTwitterEngine is reade?</p>