为什么我仍然需要解开 Swift 字典值? [英] Why do I still need to unwrap Swift dictionary value?

问题描述

class X {
    static let global: [String:String] = [
        "x":"x data",
        "y":"y data",
        "z":"z data"
    ]

    func test(){
        let type = "x"
        var data:String = X.global[type]!
    }
}

我收到错误:可选类型字符串?"的值未解包.

为什么我需要在 X.global[type] 之后使用 !?我的字典中没有使用任何可选的?

Why do I need to use ! after X.global[type]? I'm not using any optional in my dictionary?

已编辑:

即使该类型可能不存在 X.global[type]，强制解包仍然会在运行时崩溃.更好的方法可能是:

Even if X.global[type] may not exist for the type, force unwrapping will still crash on runtime. A better approach may be:

if let valExist = X.global[type] {
}

但是 Xcode 通过暗示可选类型给了我错误的想法.

but Xcode is giving me the wrong idea by hinting about optional type.

推荐答案

字典访问器返回其值类型的可选，因为它不知道"运行时字典中是否存在某个键.如果存在，则返回关联的值，如果不存在，则返回 nil.

Dictionary accessor returns optional of its value type because it does not "know" run-time whether certain key is there in the dictionary or not. If it's present, then the associated value is returned, but if it's not then you get nil.

来自 文档:

您还可以使用下标语法从字典中检索特定键的值.因为可以请求不存在值的键，所以字典的下标返回字典值类型的可选值.如果字典包含请求键的值，则下标返回一个可选值，其中包含该键的现有值.否则下标返回nil...

You can also use subscript syntax to retrieve a value from the dictionary for a particular key. Because it is possible to request a key for which no value exists, a dictionary’s subscript returns an optional value of the dictionary’s value type. If the dictionary contains a value for the requested key, the subscript returns an optional value containing the existing value for that key. Otherwise, the subscript returns nil...

为了正确处理这种情况，您需要解开返回的可选项.

In order to handle the situation properly you need to unwrap the returned optional.

有几种方法:

选项 1:

func test(){
    let type = "x"
    if var data = X.global[type] {
        // Do something with data
    }
}

选项 2:

func test(){
    let type = "x"
    guard var data = X.global[type] else { 
        // Handle missing value for "type", then either "return" or "break"
    }

    // Do something with data
}

选项 3:

func test(){
    let type = "x"
    var data = X.global[type] ?? "Default value for missing keys"
}

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