title: What does 'on' mean in Swift?
body: <p>I know I can use '_' to replace it, but I still don't know what the function of 'on' here, a custom argument label?
  <a href="https://i.stack.imgur.com/YcAmj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YcAmj.png" alt="enter image description here"></a></p>