如何解开双选项? [英] How to unwrap double optionals?

问题描述

你如何解开一个返回的字符串:

How do you unwrap a string that is returned as:

(可选(可选蓝色"))

(Optional(Optional "blue"))

var cityName = String()

if let cityAnno = annotation as MGLAnnotation! {
    cityName = String(stringInterpolationSegment: cityAnno.title!)
}

cityLabelName.text = ("\(cityName), \(county)")

cityLabelName 打印为 (可选纽约")

cityLabelName prints out as (Optional "New York")

推荐答案

给定一个双可选的，比如这个双包裹的String:

Given a double optional such as this doubly wrapped String:

let a: String?? = "hello"
print(a as Any) // "Optional(Optional("hello"))\n"

@Leo，表明您可以使用两次可选绑定:

@Leo, showed that you could use optional binding twice:

if let temp = a, let value = temp {
    print(value) // "hello\n"
}

或强制解包两次:

print(value!!)  // don't do this - you're just asking for a crash

<小时>

这里有另外 5 种方法可以用来安全地解开双可选:

---

Here are 5 more methods you can use to safely unwrap a double optional:

方法一:

您也可以使用模式匹配:

if case let value?? = a {
    print(value) // "hello\n"
}

正如@netigger 在他们的回答中所指出的，这也可以写成:

As @netigger noted in their answer, this can also be written as:

if case .some(.some(let value)) = a {
    print(value) // "hello\n"
}

虽然不那么简洁可能更容易阅读.

which while less concise might be a bit easier to read.

方法二:

或者，您可以使用 nil 合并运算符 ?? 两次:

Alternatively, you can use the nil coalescing operator ?? twice:

print((a ?? "") ?? "")  // "hello\n"

注意:与此处介绍的其他方法不同，这将始终产生一个值."" (空 String) 如果任一选项为 nil，则使用.

Note: Unlike the other methods presented here, this will always produce a value. "" (empty String) is used if either of the optionals is nil.

方法 3:

或者你可以使用 nil 合并运算符 ?? 和 可选绑定:

Or you can use the nil coalescing operator ?? with optional binding:

if let value = a ?? nil {
    print(value)  // "hello\n"
}

这是如何工作的?

使用双重包装的可选项，变量保存的值可能是以下三件事之一:Optional(Optional("some string")), Optional(nil) 如果内部可选是 nil，或者 nil 如果外部可选是 nil.所以一个 ??nil 解开外部可选.如果外部可选是 nil，则 ?? 将其替换为默认值 nil.如果 a 是 Optional(nil)，那么 ?? 将解开外部的可选内容，留下 nil.此时，如果内部或外部可选是 nil，您将拥有一个 String?，即 nil.如果里面有String，你会得到Optional("some string").

With a doubly wrapped optional, the value held by the variable could be one of 3 things: Optional(Optional("some string")), Optional(nil) if the inner optional is nil, or nil if the outer optional is nil. So a ?? nil unwraps the outer optional. If the outer optional is nil, then ?? replaces it with the default value of nil. If a is Optional(nil), then ?? will unwrap the outer optional leaving nil. At this point you will have a String? that is nil if either the inner or outer optional is nil. If there is a String inside, you get Optional("some string").

最后，可选绑定(if let)解开Optional("some string")得到"some string" 或 optional binding 失败，如果其中一个 optional 是 nil 并跳过块.

Finally, the optional binding (if let) unwraps Optional("some string") to get "some string" or the optional binding fails if either of the optionals is nil and skips the block.

方法 4:

此外，您可以将 flatMap 与可选绑定一起使用:

Also, you can use flatMap with optional binding:

if let value = a.flatMap({ $0 }) {
    print(value)  // "hello\n"
}

<小时>

方法 5:

有条件地将值转换为类型.令人惊讶的是，这将删除所有级别的选项:

Conditionally cast the value to the type. Surprisingly, this will remove all levels of optionals:

let a: String?? = "hello"
let b: String??????? = "bye"

if let value = a as? String {
    print(value)  // "hello\n"
}

print(b as Any)  // "Optional(Optional(Optional(Optional(Optional(Optional(Optional("bye")))))))\n"

if let value = b as? String {
    print(value)  // "bye\n"
}

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