swift : What does "_, _ " means?

Question

我在对 Alamofire 进行一些研究时发现了这段代码:

    switch encodingResult {

    case .Success(let upload, _, _):

    upload.responseJSON { response in

        if let info = response.result.value as? Dictionary<String, AnyObject> {

            if let links = info["links"] as? Dictionary<String, AnyObject> {

                if let imgLink = links["image_link"] as? String {

                    print("LINK: \(imgLink)")
                }
            }
        }

    } case .Failure(let error):
        print(error)
}

我可以知道 _, _ 是什么意思吗？

我见过像 let _ = "xyz" 这样的用法，但我以前从未见过它像上面的代码那样使用。

这是否意味着它有2个参数没有被使用？

提前致谢

Answer 1

是的，这基本上意味着它们是未使用的值，在 Apples documentation 中有描述.

Wildcard Pattern

A wildcard pattern matches and ignores any value and consists of an underscore (_). Use a wildcard pattern when you don’t care about the values being matched against. For example, the following code iterates through the closed range 1...3, ignoring the current value of the range on each iteration of the loop:

for _ in 1...3 {
  // Do something three times.
}