title: dispatch async code error - Objective C
body: <p>I am new to Objective C but have been working on Swift for a while now. I assumed Objective c to be logically similar to swift. I have to present an alert controller while I am processing a json data request; so I had to use dispatch async to get it to work in swift. Here's the code I used in Swift:</p>
  
  <pre><code>    func alertMessage(message : String) -&gt; Void {
      let alert = UIAlertController(title: "Alert", message: message, preferredStyle: .Alert)
      let okAction = UIAlertAction(title: "OK", style: .Default, handler: nil)
      alert.addAction(okAction)
      dispatch_async(dispatch_get_main_queue(),{
          self.presentViewController(alert, animated: true, completion: nil)
      })
  }
  </code></pre>
  
  <p>But  I tried to do the same thing in objective C like so</p>
  
  <pre><code>- (void)alertMessage : (NSString*) message {
  UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"Alert" message:message preferredStyle:UIAlertControllerStyleAlert];
  UIAlertAction *okAction = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:NULL];
  [alert addAction:okAction];
  [dispatch_async(dispatch_get_main_queue(), ^(void){
      [self presentViewController:alert animated:true completion:NULL];
  })];
  }
  </code></pre>
  
  <p>I'm getting an error "expected identifier". What am I doing wrong?</p>