Swift 中的回调函数语法 [英] Callback function syntax in Swift

问题描述

我正在尝试将一个函数传递给另一个函数，然后执行传递给它的函数并将一个变量传递给它.

I am attempting pass a function to another function and then have the passed function executed passing to it a variable.

这是我的代码:

func showStandardPrompt(prompt:String,view: UIViewController,numberInput: Bool, callback: (()->(String))?) {
    let alert = UIAlertController(title: "Input Data", message: prompt, preferredStyle: .Alert)
    alert.addTextFieldWithConfigurationHandler { (textField) in
        if numberInput {
            textField.keyboardType = .NumberPad
        }
    }

    let OKAction = UIAlertAction(title: "OK", style: .Default) { (action) in
        let field = alert.textFields![0] as UITextField
        callback?(field.text!)
    }

    alert.addAction(OKAction)
    let CancelAction = UIAlertAction(title: "Cancel", style: .Default, handler: nil)
    alert.addAction(CancelAction)
    view.presentViewController(alert,animated: true, completion: nil)
}

我得到的错误是

callback?(field.text!)

错误是无法将 'String' 的值类型转换为预期的参数类型 '()'.我知道这是一个语法问题 - 只是不知道它应该是什么.

The error is "Cannot convert value type of 'String' to expected argument type '()'. I know it's a matter of syntax - just don't know what it should be.

推荐答案

Rob 的回答是正确的，虽然我想分享一个简单的工作回调/完成处理程序的示例，您可以下载下面的示例项目并进行实验getBoolValue 的输入.

Rob's answer is correct, though I'd like to share an example of a simple working callback / completion handler, you can download an example project below and experiment with the getBoolValue's input.

func getBoolValue(number : Int, completion: (Bool)->()) {
    if number > 5 {
        completion(true)
    } else {
        completion(false)
    }
}

getBoolValue(number: 2) { (result) -> () in
    // do stuff with the result
    print(result)
}

重要的理解:

(String)->() // takes a String returns void
()->(String) // takes void returns a String

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