Swift - 检查函数参数是否已传递

Question

假设我有以下功能:

func update(a a: String? = nil, b: String? = nil) {
    if a.notDefaultValue /* or something like that */ {
        // ...
    }

    if b.notDefaultValue {
        // ...
    }
}

我可以用这些方式调用它(期望像评论中那样):

update()               // Should do nothing
update(a: nil)         // Should only go into first 'if' statement
update(b: nil)         // Should only go into second 'if' statement
update(a: nil, b: nil) // Should go into both 'if' statements
update(a: "A")         // Should only go into first 'if' statement
update(b: "B")         // Should only go into second 'if' statement
update(a: "A", b: "B") // Should go into both 'if' statements

我怎样才能实现这个目标？

编辑:

我能想到的唯一方法是使用方法重载，但是当你有很多参数时这是不可行的(甚至不必那么多，4 个就已经需要 17 个方法了)。

Answer 1

我不确定你为什么要这样做，但你可以这样做:

let argumentPlaceholder = "__some_string_value__"

func update(a a: String? = argumentPlaceholder, b: String? = argumentPlaceholder) {
     if a == argumentPlaceholder {
        // Default argument was passed, treat it as nil.
    }

     if b == argumentPlaceholder {
        // Default argument was passed, treat it as nil.
    }
}