title: When to use @objc in Swift?
body: <p>In Swift, I see some methods like:</p>
  
  <pre><code>@objc private func doubleTapGestureRecognized(recognizer: UITapGestureRecognizer)
  </code></pre>
  
  <p>I was wondering, when to use @objc? I read some documents, but they are saying when you want it to be callable in Objective-C, you should add @objc flag</p>
  
  <p>However, this is a private function in Swift, what does the @obj do?</p>