title: UI get blocked when using NSURLSession
body: <p>I'm working on a project which requires a login form, using a webservice for authentication. I have no issue connecting to the server but it seems that NSURLSession blocks my user interface and I really dont know why after a lot of debugging.</p>
  
  <p>For simplicity, here's my code in short:</p>
  
  <pre><code>NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:@"example.com/service"]];
  
  //1
      //_sessionLogin = [NSURLSession sessionWithConfiguration:sessionConfigurationLogin delegate:self delegateQueue:nil];
  //2 //Whether I use 1 or 2, it acts the same way
  
      _sessionLogin = [NSURLSession sharedSession];
  
      NSURLSessionDataTask *sessionDataTaskLogin = [_sessionLogin dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error)
      {
          if(!error)
          {
              NSLog(@"loginWithSuccess");
              UIAlertView *alertError = [[UIAlertView alloc] initWithTitle:@"Login ok" message:@"ok" delegate:self cancelButtonTitle:@"Close" otherButtonTitles: nil];
              [alertError show];
  
      }
  }];
  
  //Begin login request
  [sessionDataTaskLogin resume];
  </code></pre>
  
  <p>_sessionLogin is a NSURLSession</p>
  
  <p>As the connection to my server is fast, the NSLog(@"loginWithSuccess") appears almost right after I pressed the login button but it takes me a while (a long long while) about more than 10 seconds that the UIAlertView is showed. And I can't interact with the UI too.</p>
  
  <p>Thanks in advance for every solution.</p>