Why is animation parameter for UIView.transition(with:duration:options:animations:completion:) optional

Possibly because "NULL" is not the same as "nil"!

At least... they are the same, but they aren't.

• NULL is a generic pointer value
• nil is the absence of a value of a certain type

See https://swiftbydeya.com/swift-the-difference-between-nil-nil-null-nsnull/

I tested the following:

    var animationContainerView: UIView!

    override func viewDidLoad() {
        super.viewDidLoad()
        animationContainerView = UIView(frame: view.bounds)
        animationContainerView.frame = view.bounds
        view.addSubview(animationContainerView)
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        print("APPEAR")
        let newView = UIImageView(image: UIImage(named: "Sa Riera"))
        newView.center = animationContainerView.center
        UIView.transition(with: animationContainerView,
                          duration: 0.33,
                          options: [.curveEaseOut, .transitionFlipFromBottom],
                          animations:  { self.animationContainerView.addSubview(newView)  },
                          completion: { _ in print("COMPLETED")}
        )
    }

As expected, I get the image appear with the transition animation, and logs

• APPEAR
• COMPLETED

I then tested with nil animation:

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        print("APPEAR")
        let newView = UIImageView(image: UIImage(named: "An image"))
        newView.center = animationContainerView.center
        UIView.transition(with: animationContainerView,
                          duration: 0.33,
                          options: [.curveEaseOut, .transitionFlipFromBottom],
                          animations: nil,
                          completion: { _ in print("COMPLETED")}
        )
    }

No crash; I get the expected logs

• APPEAR
• COMPLETED

So, animations: nil works, doing nothing.

I tested with empty closure

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        print("APPEAR")
        let newView = UIImageView(image: UIImage(named: "An image"))
        newView.center = animationContainerView.center
        UIView.transition(with: animationContainerView,
                          duration: 0.33,
                          options: [.curveEaseOut, .transitionFlipFromBottom],
                          animations: { },
                          completion: { _ in print("COMPLETED")}
        )
    }

Same result, nothing but no crash and correct log.

I tested passing a func:

    var newView = UIImageView()

    func transition() {
        self.animationContainerView.addSubview(newView)
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        print("APPEAR")
        newView = UIImageView(image: UIImage(named: "An image"))
        newView.center = animationContainerView.center
        UIView.transition(with: animationContainerView,
                          duration: 0.33,
                          options: [.curveEaseOut, .transitionFlipFromBottom],
                          animations: transition,
                          completion: { _ in print("COMPLETED")}
        )
    }

Works as well, image appears with animation effect.

and finally passing a nil func:

    var transition: (() -> Void)? = nil

No crash there either.

So it seems doc only means that passing nil will result in doing nothing ?

Not clear then why this warning in doc nor why animation parameter is an optional.