title: how to do blocking `dispatch_sync` using `NSOperationQueue`
body: <p>Sometimes I need to do synchronous return. In dispatch_sync it's just:</p>
  <pre><code>__block int foo
  dispatch_sync({
    foo = 3
  });
  return foo;
  </code></pre>
  <p>I am not sure how that translates to NSOperationQueue. I have checked the <code>maxConcurrentOperationCount</code> = 1, but I don't think that's blocking. My understanding is that this only makes the operation queue &quot;serial&quot;, but not &quot;synchronous&quot;.</p>