title: run a block on main thread immediately after NSBlockOperation on background thread
body: <p>In my project I run an operation on a background thread using <code>NSBlockOperation</code>:</p>
  
  <pre><code>var operationQueue = NSOperationQueue()
  var iop = NSBlockOperation(block: { self.reloadSize() /*calculation...*/ })
  operationQueue.addOperation(iop)
  </code></pre>
  
  <p>Immediately after the calculations in the background thread are completed, I need to call: <code>table.reloadData()</code> on an NSTableView. I would do that in the very same thread, however, due to auto layout issues, the table has to be reloaded on the main thread. How can I accomplish this asynchronous relationship across both threads?</p>