title: ASIHTTP: addOperation when other threads are running
body: <p>I have a project using ASIHTTP to multi download files from a site</p>
  
  <p>when I add a new request:</p>
  
  <pre><code>[networkQueue cancelAllOperations];
  [networkQueue  setDownloadProgressDelegate:a];
  [networkQueue  setDelegate:self];
  [networkQueue   setRequestDidFinishSelector:@selector(requestDone:)];
  
  NSURL *url = [NSURL URLWithString:@"http://www.google.com"];
  ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
  [request setDelegate:self];
  [request startAsynchronous];
  [networkQueue  addOperation:request];
  
  [networkQueue  go];
  </code></pre>
  
  <p>it reported:</p>
  
  <p><strong>* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*</strong> -[ASINetworkQueue addOperation:]: operation is executing and cannot be enqueued'</p>
  
  <p>It looks like I can not add a new request when others are running.</p>
  
  <p>Welcome any comment</p>
  
  <p>Thanks</p>
  
  <p>interdev</p>