错误：无法将类型“（_）->（）”的值转换为预期的参数类型“（（）-> Void）？”

Posted 2023-02-24

【中文标题】错误：无法将类型“（_）->（）”的值转换为预期的参数类型“（（）-> Void）？”

【英文标题】：error: Cannot convert value of type '(_) -> ()' to expected argument type '(() -> Void)?'

【发布时间】：2019-03-12 13:23:46

【问题描述】：

我最近从 swift3 迁移到 swift 4.2。 在构建时，我现在收到以下错误：

无法将类型 '(_) -> ()' 的值转换为预期的参数类型 '(() -> 无效)？'

并且错误出现在以下代码行：

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

我不确定如何解决该错误。 任何提供的帮助将不胜感激

    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 )

【参考方案1】：

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，如果要声明completion，则不需要_ 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)  

【参考方案2】：

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)