让其他函数调用一个函数，但是谁提供了参数值？

Question

The code is silly simple. There is a button in the view, during run time when user taps the button, the button will move 20 point downwards.

the the viewController has a IBOutlet and a IBAction, both of them are connected to the button in the interface builder

Here comes the code of the viewControleller

class viewController : UIViewController {

    @IBOutlet weak var myButton: UIButton!

    @IBAction func moveMyButton(_ sender: Any) {

        func whatToAnimate() {
            self.myButton.frame.origin.y += 20
        }

        func whatToDoLater(finished: Bool){
            print("finished: \(finished)") //who passed the "true" value to the finished variable
        }

        UIView.animate(withDuration: 0.4, 
                    animations: whatToAnimate, 
                    completion: whatToDoLater)
    }

}

When animate function has been called in the run time, the function whatToDoLater() -> void was passed into the animate method as a argument.

The string "finished: true" prints in the console when user taps the button

Question :

Who passed the Boolean value "true" to the whatToDoLater function's argument finished ? I think I didn't do that. What's the mechanism behind it?

Thanks for your time

Answer 1

The UIView animate method passes the parameter since it is what calls the completion handler.

This pattern is seen with any use of a completion handler. Whatever actually calls the completion handler is also responsible for passing the proper parameter values to the completion handler.