Declare protocol function with default argument values

Question

I want this function be in protocol:

func slideToRight(currentViewController viewController: UIViewController, completion: ((Bool)->())? = nil) {
    // do some stuff
}

But when I write such protocol:

protocol SomeDelegate { 
    func slideToRight(currentViewController viewController: UIViewController, completion: ((Bool)->())? = nil) 
}

I got an error:

Default argument not permitted in a protocol method

I know, I can define the signature in this way:

protocol SomeDelegate { 
    func slideToRight(currentViewController viewController: UIViewController, completion: ((Bool)->())?) 
}

But then, I won't be able to call the function missing "completion" word:

slideToRight(currentViewController viewController: vc)

Answer 1

Unfortunately optional arguments are not allowed in protocols, but you can work around this by creating an extension of the protocol:

protocol SomeDelegate {
    // with the completion parameter
    func slideToRight(currentViewController viewController: UIViewController, completion: ((Bool)->())?)
}

extension SomeDelegate {
    // without the completion parameter
    func slideToRight(currentViewController viewController: UIViewController) {
        slideToRight(slideToRight(currentViewController: viewController, completion: nil))
    }
}