Swift可选字典[String：String？]展开错误 [英] Swift Optional Dictionary [String: String?] unwrapping error

问题描述

所以这里我有一个基本设置

So here I have a basic setup

var preferenceSpecification = [String : String?]()
preferenceSpecification["Key"] = "Some Key"
preferenceSpecification["Some Key"] = nil
preferenceSpecification["DefaultValue"] = "Some DefaultValue"
print(preferenceSpecification)
var defaultsToRegister = [String : String]()

if let key = preferenceSpecification["Key"], let defaultValueKey = preferenceSpecification["DefaultValue"] {
    defaultsToRegister[key] = preferenceSpecification[defaultValueKey]!
}

但是错误指出了在哪里要求我强制打开包装像这样：

But the error points out where it demands that I force unwrap this, to be like this:

defaultsToRegister[key!] = preferenceSpecification[defaultValueKey!]!

这没有意义，因为 keyValue 和 defaultValue 已经解包

Which doesn't make sense, because keyValue and defaultValue already are unwrapped

推荐答案

使用下标从这样的字典中提取值时

When you extract a value from a dictionary like this using subscript

[String: String?]

您需要管理2个可选级别。第一个因为下标返回一个可选。第二个是因为字典的值是可选的字符串。

you need to manage 2 levels of optional. The first one because the subscript returns an optional. The second one because the value of you dictionary is an optional String.

因此，当您编写

if let value = preferenceSpecification["someKey"] {

}

您将值定义为可选的字符串。

you get value defined as an optional String.

以下是修复该问题的代码

Here's the code to fix that

if let
    optionalKey = preferenceSpecification["Key"],
    key = optionalKey,
    optionalDefaultValueKey = preferenceSpecification["DefaultValue"],
    defaultValueKey = optionalDefaultValueKey,
    value = preferenceSpecification[defaultValueKey] {
    defaultsToRegister[key] = value
}

建议

Suggestions

1. 您应尽可能避免强行展开。相反，您设法将3个！放在一行上。


2. 您还应该尝试对常量和变量使用更好的名称。

1. You should avoid force unwrapping as much as possible. Instead you managed to put 3 ! on a single line!
2. You should also try to use better name for your constants and variables.

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