Swift中的两个（或更多）选项 [英] Two (or more) optionals in Swift

问题描述

在观看Apple关于LLDB调试器的视频时，我发现了一些我无法找到解释的内容;当他写道时，他正在讨论可选值：

While watching an Apple's video about LLDB debugger I found something I can't find an explanation for; he was talking about optional values when he wrote:

var optional: String? = nil; //This is ok, a common optional
var twice_optional: String?? = nil; //What is this and why's this useful??

我开了一个游乐场并开始试用它并意识到你可以写多少？如你所愿，然后使用相同数量的！打开它们。我理解包装/展开变量的概念，但想不到我想要包装4,5或6次值的情况。

I opened a playground and started trying it out and realized that you can write as many as ? as you want, and then unwrap them with the same number of !. I understand the concept of wrapping/unwrapping a variable but can't think of a situation where I would like to wrap a value 4, 5 or 6 times.

推荐答案

（更新为Swift> = 3）

双选项可以很有用，Swift博客文章Optionals案例研究：valuesForKeys描述了一个应用程序。

"Double optionals" can be useful, and the Swift blog entry "Optionals Case Study: valuesForKeys" describes an application.

这是一个简化的例子：

let dict : [String : String?] = ["a" : "foo" , "b" : nil]

是一个带有可选字符串作为值的字典。因此

is a dictionary with optional strings as values. Therefore

let val = dict[key]

的类型为 String ?? aka 可选< Optional< String>> 。它是 .none （或 nil ）
如果字典中没有该键，并且 .some（x）否则。在第二个
的情况下， x 是一个字符串？又名可选< String> ; ，可以是 .none （或 nil ）
或 .some（s）其中 s 是一个字符串。

has the type String?? aka Optional<Optional<String>>. It is .none (or nil) if the key is not present in the dictionary, and .some(x) otherwise. In the second case, x is a String? aka Optional<String> and can be .none (or nil) or .some(s) where s is a String.

您可以使用嵌套的可选绑定来检查各种情况：

You can use nested optional binding to check for the various cases:

for key in ["a", "b", "c"] {

    let val = dict[key]
    if let x = val {
        if let s = x {
            print("\(key): \(s)")
        } else {
            print("\(key): nil")
        }
    } else {
        print("\(key): not present")
    }

}

输出：

a: foo
b: nil
c: not present

在switch-statement中使用模式匹配
可以获得同样的结果可能是有益的：

It might be instructive to see how the same can be achieved with pattern matching in a switch-statement:

let val = dict[key]
switch val {
case .some(.some(let s)):
    print("\(key): \(s)")
case .some(.none):
    print("\(key): nil")
case .none:
    print("\(key): not present")
}

或使用 x？模式作为 .some的同义词（ x）：

let val = dict[key]
switch val {
case let (s??):
    print("\(key): \(s)")
case let (s?):
    print("\(key): nil")
case nil:
    print("\(key): not present")
}

（我不知道更深层嵌套的选项的合理应用。）

(I do not know a sensible application for more deeply nested optionals.)

这篇关于Swift中的两个（或更多）选项的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！