Swift 中的可选默认参数

Question

I have a function with default parameters in swift.

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

I want to pass in variables that will be optional strings.

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

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)

Answer 1

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
}

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.

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")

Answer 2

尝试这个：

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

Answer 3

Note: Don't interpret this optional parameters as ? , this is different approach.

Totally optional parameters, ie Don't even use while calling method.

func doAddition(value1Name value1:Int = 0, value2Name value2:Int = 0, value3Name value3:Int = 0){
    print(value1+value2+value3)
}

Now all parameters are optional to add. call how you like,

doAddition(value1Name: 1)
doAddition(value1Name: 1, value2Name: 2)
doAddition(value3Name: 3)