Swift 3 中的 Any 、 Hashable 、 AnyHashable 有什么区别? [英] What is difference between Any , Hashable , AnyHashable in Swift 3?

问题描述

我通过大量教程摸索以了解上述 3 个术语之间的区别并找到新术语 type erased 容器，现在这让我感到困惑.这引发了很多问题.

I scratch my head through lots of tutorials to understand the difference between the above 3 terms and find new term type erased container, now it becomes confusing to me. It raises lots of question.

为什么 Swift 引入了 AnyHashable ?

Why does Swift introduce AnyHashable ?

这三个术语之间的根本区别是什么?

What is the fundamental difference between these 3 terms?

Any 和 AnyHashable 的区别?

Hashable 和 AnyHashable 的区别?

何时使用 Hashable 以及何时使用 AnyHashable ?

When to use Hashable and when to use AnyHashable ?

最后但最令人困惑的是，AnyHashable 上下文中type erased 术语的含义是什么?

Last but most confusing, what is the meaning of type erased term in the context of AnyHashable ?

作为上下文，我遵循了 Swift Evolution ProposalSE-0131.

As a context, I followed Swift Evolution Proposal SE-0131.

推荐答案

理解它们是什么比它们之间有什么区别更重要.

It's more important to understand what they are than what are the differences between them.

Any 表示任何东西"，包括 swift 枚举、元组、闭包、结构、类、协议等等.每种类型都可以分配给 Any 类型的变量.

Any means "anything", ranging from swift enums, tuples, closures, structs, classes, protocols, whatever. Every type can be assigned to a variable of type Any.

Hashable 是一个协议，它说这个对象可以被散列，即有一个散列码".如果您的对象可以散列，请实现此协议，因为很多数据结构(即字典和集合)都需要它.

Hashable is protocol that says "this object can be hashed i.e. has a hashcode". If your object can be hashed, implement this protocol, because lots of data structures (namely dictionaries and sets) need it.

那么什么是AnyHashable?

通常，如果您尝试这样做:

Normally, if you try to do this:

let a: Set<Hashable>?

它不会编译.这是因为 Hashable 继承自包含 Self 的 Equatable.

it does not compile. This is because Hashable inherits from Equatable which contains Self.

现在，假设您想将一个方法从 Objective-C 移植到 swift.该方法采用 NSSet 类型的参数.在 Swift 中，这会变成一个 Set，但它的泛型参数是什么?如果我们像处理 NSArray 那样只放置 Any，它是行不通的，因为 Set 的对象必须是可散列的.但是如果我们放置 Set 它也不起作用，因为 Hashable 只能用作通用约束.这就是为什么他们用 AnyHashable 包裹了 Hashable，不 使用 Self，因此可以用作通用参数.

Now, let's say you want to port a method from Objective-C to swift. That method takes a parameter of type NSSet. In Swift, this will turn into a Set, but what is its generic parameter? If we just put Any like we do with NSArrays, it does not work because Set's objects must be hashable. But if we put Set<Hashable> it does not work either because Hashable can only be used as a generic constraint. That's why they wrapped Hashable with an AnyHashable that does not use Self and so can be used as a generic parameter.

关于类型擦除"的含义:

Regarding what "type erased" means:

在协议中包含 Self 有点像 就像一个带有泛型参数的协议，泛型参数总是符合的类.这导致协议不能像 Set 那样单独使用，因为通用参数"是未知的.AnyHashable 完全不使用 Self 解决了这个问题，所以它现在变成了一个普通的结构体.它擦除"了通用的 Self 类型.

Having Self in a protocol is kind of like a protocol with a generic parameter, and the generic parameter is always the conforming class. This causes the protocols to be unable to be used on its own like Set<Hashable> because the "generic parameter" is unknown. AnyHashable solves this problem by not using Self at all so it now becomes a normal struct. It "erases" the generic Self type.

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