为什么@objc枚举与纯Swift枚举有不同的描述？ [英] Why does an @objc enum have a different description than a pure Swift enum?

问题描述

考虑两个Swift枚举：

Consider two Swift enums:

enum Foo: Int {
    case bar
}

@objc enum Baz: Int {
    case qux
}

如果我要这些枚举的 print 每个 case ，我会期望相同的结果。相反，我看到一些意想不到的事情：

If I were to print each case of these enums, I would expect the same result. Instead, I see something unexpected:

print(Foo.bar) // "bar\n"
print(Baz.qux) // "Baz\n"

为什么打印一个 @objc 枚举打印枚举名称，同时打印纯粹的Swift枚举的案例打印实际案例名称？添加 @objc 更改枚举的调试描述？

Why does printing a case of an @objc enum print the enum name, while printing the case of a pure Swift enum print the actual case name? Does adding @objc change the debug description of the enum?

推荐答案

那是因为 @objc 枚举是C兼容枚举，它有意地不发布任何关于他们的案例的反射信息。

That is because @objc enums are "C-compatible enums", which intentionally do not emit any reflection information about their cases.

由于Swift是开源的，我们可以自己来看看：

Since Swift is open source, we can nose around to see this for ourselves:

• @objc 枚举被视为C兼容枚举


• C兼容枚举有意地不发布案例信息反射


• 其他e nums发出案例信息

• @objc enums are treated as C-compatible enums
• C-compatible enums intentionally don't emit case info for reflection
• Other enums do emit case info

这是一个版本的为什么，以实现为重点。现在，让我们回过头来一个问题，为什么要这样实现？ C-compatible枚举的 emitCaseNames 函数的注释解释了这一点：C兼容的枚举不保证从枚举原始值映射到标签，因为与Swift本机枚举不同，它们可以有多个案例都具有相同的原始值。

That's one version of "why", the implementation-focused. Now, let's step back one level and ask, why was it implemented this way? The comment by the emitCaseNames function for the C-compatible enums explains this: C-compatible enums don't guarantee a mapping from the enum raw value back to the tag, because, unlike the Swift-native enums, they can have multiple cases that all have the same raw value.

现在，如果您尝试在Swift中声明重复原始值的枚举，那么您的手将被打上来，并导致编译错误。但是您应该可以通过在Obj / C中声明枚举来创建这样的枚举，然后将其导入桥梁中的Swift。

Now, if you try to declare an enum in Swift that duplicates raw values, you'll get your hand slapped and a compiler error. But you should be able to create such an enum by declaring the enum in Obj/C and then importing it into Swift over the bridge.

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