title: How to get completion block of NSOperationQueue
body: <p>How to get completion block of NSOperationQueue, here I want to spin activity indicator from start to end of all operation.</p>
  
  <pre><code>NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
  // Set the max number of concurrent operations (threads)
  [operationQueue setMaxConcurrentOperationCount:3];
  [operationQueue addOperations:@[operation, operation1, operation3,...] waitUntilFinished:NO];
  </code></pre>
  
  <p>Thanks.</p>