title: iOS performSelectorInBackground inside performSelectorInBackground
body: <p>It seems that the function call <b>[self updateUI];</b> blocked by <b>boo</b>.</p>
  
  <p>Is <b>boo</b> run in another background thread or same as <b>foo</b> as the code below?</p>
  
  <p>How can the <b>[self updateUI];</b> not block by boo?</p>
  
  <pre><code>- (void)MainFunction
  {
      [self performSelectorInBackground@selector(foo) withObject:nil];
  }
  
  - (void)foo
  {
      [self performSelectorInBackground@selector(boo) withObject:nil];
  
      //updaate UI in MainThread
      [self updateUI];
  }
  
  - (void)boo
  {
      //function here take long time to run;
  }
  </code></pre>