title: iOS develop about NSOperationQueue
body: <p>I know the two ways to get a operation queue as follow:</p>
  
  <pre><code>queue1 = [[NSOperationQueue alloc] init];
  queue2 = [[NSOperationQueue mainQueue] retain];
  </code></pre>
  
  <p>But I don't know what differences between them. </p>
  
  <pre><code>[queue1 addOperation:operation1]; 
  [queue2 addOperation:operation2]; 
  </code></pre>
  
  <p>which thread does operation1 run on?main thread? or uncertainty? </p>
  
  <p>I tested.</p>
  
  <pre><code>operation1 --&gt; sometimes mainthread sometimes not. 
  operation2 --&gt; always mainthread. 
  </code></pre>