仅在符合 Class AND 协议时才进行 Swift 扩展

Question

I have two protocols with extensions LoginPresenting and LoginDismissing . I want LoginPresenting extension only to apply to UIViewControllers that also implement LoginDismissing . I am trying to do it like so but have so far been unable to work out the syntax.

protocol LoginDismissing : class {
    func loginHasCompleted(withController: UIViewController)
}

extension LoginDismissing where Self:UIViewController {
    func loginHasCompleted(withController controller:UIViewController) {
        //...code ommited
    }
}

protocol LoginPresenting : class {
    func presentLogin()
}

The following is bad code , but I think it explains how I am attempting to make LoginPresenting only apply to UIViewControllers that also implement LoginDismissing.

extension LoginPresenting where Self:UIViewController, LoginDismissing //Syntax error here

    func presentLogin() {
        let lc = LoginViewController()
        let nav = UINavigationController(rootViewController: lc)
        nav.modalPresentationStyle = .fullScreen
        lc.loginDismissingDelegate = self //type LoginDismissing
        self.present(nav, animated: true, completion: nil)
    }
}

Answer 1

You can resolve your syntax error with the use of & instead of a comma:

extension LoginPresenting where Self: UIViewController & LoginDismissing {
    // ...
}

This makes the extension only apply to view controllers that also conform to LoginDismissing .

Answer 2

You should enforce as many requirements as possible on the protocol itself, not the extensions.

protocol LoginPresenting: LoginDismissing & UIViewController {
  func presentLogin()
}

Also, class is old syntax, though not deprecated.

protocol LoginDismissing: AnyObject {