swift - 使用默认参数值声明协议(protocol)函数

Question

我希望这个函数在协议(protocol)中:

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

但是当我写这样的协议(protocol)时:

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

我遇到了一个错误:

Default argument not permitted in a protocol method

我知道，我可以这样定义签名:

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

但是，我将无法调用缺少“完成”字的函数:

slideToRight(currentViewController viewController: vc)

Answer 1

不幸的是，协议(protocol)中不允许使用可选参数，但您可以通过创建协议(protocol)扩展来解决此问题:

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))
    }
}