error: Cannot convert value of type '(_) -> ()' to expected argument type '(() -> Void)?'

Question

我最近从 swift3 迁移到 swift 4.2。现在构建时出现以下错误：

Cannot convert value of type '(_) -> ()' to expected argument type '(() -> Void)?'

错误出现在以下代码行：

topWindow.rootViewController?.present(alert, animated: true, completion: { _ in })

我不太确定如何解决该错误。提供的任何帮助将不胜感激

    let topWindow = UIWindow(frame: UIScreen.main.bounds)

    topWindow.rootViewController = UIViewController()
    topWindow.windowLevel = UIWindow.Level.alert + 1
    //let storyboard = UIStoryboard(name: "Main", bundle: nil)
   // let topViewController = storyboard.instantiateViewController(withIdentifier: identifier) as? BaseViewController
    let alert = UIAlertController(title: "iPanel", message: t_alert, preferredStyle: .alert)
    let yesButton = UIAlertAction(title: get_error(eng_error: "open"), style: .default, handler: {(_ action: UIAlertAction) -> Void in
        print("you pressed Yes, please button")
        //topWindow.isHidden = true
        //trying to fix reject 154 sending user to survey from push when app is in forground
        //take user to controller
        DispatchQueue.main.async {
            let storyboard = UIStoryboard(name: "Main", bundle: nil)
            let vc = storyboard.instantiateViewController(withIdentifier: "ccontroller") as! UINavigationController
            topWindow.rootViewController?.present(vc, animated: true, completion: nil)
        }
    })

    let noButton = UIAlertAction(title: get_error(eng_error: "ignore"), style: .default, handler: {(_ action: UIAlertAction) -> Void in
        print("you pressed No, thanks button")
        topWindow.isHidden = true
    })

    alert.addAction(noButton)
    alert.addAction(yesButton)

    topWindow.makeKeyAndVisible()
    topWindow.rootViewController?.present(alert, animated: true, completion: { _ in })
}

Answer 1

最后一行的编译应该是 nil - 错误是关于它的。

topWindow.rootViewController?.present(alert, animated: true, completion: nil)

Answer 2

您不必填写 completion 参数，因为此参数有 default value 即 nil

topWindow.rootViewController?.present(alert, animated: true)

无论如何，如果你想声明 completion，你不需要 _ in 因为 completion 不带任何参数

topWindow.rootViewController?.present(alert, animated: true, completion: { })

或者只是

topWindow.rootViewController?.present(alert, animated: true) { }

error: Cannot convert value of type '(_) -> ()' to expected argument type '(() -> Void)?'

error: Cannot convert value of type '(_) -> ()' to expected argument type '(() -> Void)?'

swift

swift4.2

xcode10.1