title: how to cancel out of operation created with addOperationWithBlock?
body: <p>I'm using NSOperationQueue's addOperationWithBlock.  From within the block, how do I check to see if I'm supposed to cancel the operation?  Or access any NSOperation properties/methods?</p>
  
  <pre><code>[myOperationQueue addOperationWithBlock: ^{
  
    while ( /* long running loop */ )
    {
        // how to determine here if I need to cancel?
        // for that matter, access any NSOperation properties/methods?
  
    }
  
  }];
  </code></pre>
  
  <p>Is the better way to do this to use a NSBlockOperation?</p>