How to access properties defined during super class initialization

Question

I'm creating a custom view with 2 image views, and I want to be able to animate from one to another, here's my code:

class FlashCard: UIView {

required init?(coder: NSCoder) {
    super.init(coder: coder)

    let firstImageView = UIImageView(frame: self.frame)
    firstImageView.image = UIImage(named: "Lightning")
    addSubview(firstImageView)

    let secondImageView = UIImageView(frame: frame)
    secondImageView.image = UIImage(named: "Tifa")
    addSubview(secondImageView)
    secondImageView.isHidden = true

}

func showBack() {
    UIView.transition(from: firstImageView, to: secondImageView, duration: 1.0, options: UIViewAnimationOptions.transitionCrossDissolve, completion: nil)
}
}

But in my showBack function, I got the error: use of unresolved identifier.

How do I properly access firstImageView and secondImageView ?

Answer 1

You must use two properties:

var firstImageView: UIImageView!
var secondImageView: UIImageView!

at the moment you have the two UIImageView in the local scope of init so you cannot use it in another class method but just in the init scope.

So:

class FlashCard: UIView {

    var firstImageView: UIImageView!
    var secondImageView: UIImageView!

    required init?(coder: NSCoder) {
        super.init(coder: coder)

        firstImageView = UIImageView(frame: self.frame)
        firstImageView.image = UIImage(named: "Lightning")
        addSubview(firstImageView)

        secondImageView = UIImageView(frame: frame)
        secondImageView.image = UIImage(named: "Tifa")
        addSubview(secondImageView)
        secondImageView.isHidden = true
    }

    func showBack() {
        UIView.transition(from: firstImageView, to: secondImageView, duration: 1.0, options: UIViewAnimationOptions.transitionCrossDissolve, completion: nil)
    }
}

Answer 2

There is no need for the image views to be variables, of whatever flavour of optional.

class FlashCard: UIView {

    let firstImageView = UIImageView()
    let secondImageView: UIImageView()

    required init?(coder: NSCoder) {
        super.init(coder: coder)

        firstImageView.frame = frame
        firstImageView.image = UIImage(named: "Lightning")
        addSubview(firstImageView)

        secondImageView.frame = frame
        secondImageView.image = UIImage(named: "Tifa")
        addSubview(secondImageView)
        secondImageView.isHidden = true
    }

    func showBack() {
        UIView.transition(from: firstImageView, to: secondImageView, duration: 1.0, options: UIViewAnimationOptions.transitionCrossDissolve, completion: nil)
    }
}

Properties of a class must be initialised before the super initialiser is called, which is why the other answer used implicitly unwrapped optional variables.