Swift: Custom ViewController initializers

Swift 5

If you want to write custom initializer to UIViewController which is initialized with storyBoard.instantiateViewController(withIdentifier: "ViewControllerIdentifier")

You can write custom initializer for only Optional properties.

class MyFooClass: UIViewController {
    var foo: Foo?

    init(with foo: Foo) {
        self.foo = foo
        super.init(nibName: nil, bundle: nil)
    }

    public required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        self.foo = nil
    }
}

Not sure if you managed to fully solve this... but depending on how you want your class's interface to look and whether or not you actually need the coder functionality, you can also use the below:

convenience required init(coder aDecoder: NSCoder)
{
    //set some defaults for leftVC, rightVC, and gap
    self.init(leftVC, rightVC, gap)
}

Since init:leftVC:rightVC:gap is a designated initializer, you can fulfill the requirement of implementing init:coder by making it a convenience initializer that calls your designated initializer.

This could be better than

override init(coder aDecoder: NSCoder)
{
    super.init(coder: aDecoder);
}

because if you need to initialize some properties, then you would need to rewrite them.

For a more generic UIViewController you can use this as of Swift 2.0

init() {
    super.init(nibName: nil, bundle: nil)
}

Solved it! One has to call the designated initializer which in this case is the init with nibName, obviously ...

init(leftVC:UIViewController, rightVC:UIViewController, gap:Int)
{
    self.leftVC = leftVC
    self.rightVC = rightVC
    self.gap = gap

    super.init(nibName: nil, bundle: nil)

    setupScrollView()
    setupViewControllers()
}