Swift中的可选默认参数 [英] Optional Default Parameter in Swift

问题描述

我在swift中有一个带默认参数的函数

I have a function with default parameters in swift

func test(string: String = "", middleString: String = "", endString: String = "") -> Void {
  // do stuff
}

我想传入变量这将是可选字符串

I want to pass in variables that will be optional strings

let string: String? // 
let middleString: String?
let endString: String?

如何使参数为零，使用默认参数。如果没有，那么使用选项中的值

How do I make it so that if the parameters are nil, use the default parameters. If not, then use the values within the optionals

test（string，middleString：middleString，endString：endString）

推荐答案

你必须使用可选字符串字符串？作为你的参数类型，默认值为 nil 。然后，当你调用你的函数时，你可以提供一个字符串或保留该参数。

You'll have to use Optional strings String? as your argument type, with default values of nil. Then, when you call your function, you can supply a string or leave that argument out.

func test(string: String? = nil, middleString: String? = nil, endString: String? = nil) -> Void {
    let s = string ?? ""
    let mS = middleString ?? ""
    let eS = endString ?? ""
    // do stuff with s, mS, and eS, which are all guaranteed to be Strings
}

在你的函数中，你必须检查 nil 的每个参数，并用那里的默认值替换。使用 ?? 运算符可以轻松实现。

Inside your function, you'll have to check each argument for nil and replace with a default value there. Using the ?? operator makes this easy.

然后您可以通过提供所有参数调用您的函数，不需要参数或者只包括你要包括的那些：

You can then call your function by supplying all arguments, no arguments, or only the ones you want to include:

test(string: "foo", middleString: "bar", endString: "baz")
test()
test(string: "hello", endString: "world")

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