将枚举值映射到类型 [英] Mapping enum values to types

问题描述

假设我有这样的代码：

// Events we might receive:
enum EventType { PlaySong, SeekTo, StopSong };

// Callbacks we would handle them with:
type PlaySongCallback = (name: string) => void;
type SeekToCallback = (seconds: number) => void;
type StopSongCallback = () => void;

在给出的API中，我可以使用$ $ b注册这样的回调
$ b

In the API I'm given, I can register such a callback with

declare function registerCallback(t: EventType, f: (...args: any[]) => void);

但是我想摆脱任何 any [] ，并确保我无法注册错误类型的回调函数。

But I want to get rid of that any[] and make sure I can't register an ill-typed callback function.

我意识到我可以做到：

type CallbackFor<T extends EventType> =
    T extends EventType.PlaySong
        ? PlaySongCallback
        : T extends EventType.SeekTo
            ? SeekToCallback
            : T extends EventType.StopSong
                ? StopSongCallback
                : never;

declare function registerCallback<T extends EventType>(t: T, f: CallbackFor<T>);

// Rendering this valid:
registerCallback(EventType.PlaySong, (name: string) => { /* ... */ })

// But these invalid:
// registerCallback(EventType.PlaySong, (x: boolean) => { /* ... */ })
// registerCallback(EventType.SeekTo, (name: string) => { /* ... */ })

这真的很漂亮，强大！感觉就像我在使用依赖类型：我基本上是在这里写了一个将值映射到类型的函数。

This is really nifty and powerful! It feels like I'm using dependent types: I basically wrote myself a function mapping values to types, here.

但是，我不知道TypeScript的全部功能类型系统，也许还有更好的方法将枚举值映射到这样的类型。

However, I don't know the full strength of TypeScript's type system and maybe there is an even better way to map enum values to types like this.

是否有更好的方法将枚举值映射到这样的类型？是否可以避免上述很大的条件类型？ （实际上，我发生了很多事件，这简直是一团糟：当我将鼠标悬停在 CallbackFor 上时，VS Code表现出了巨大的表情，而我的棉绒呢真的想在每次：。）

Is there a better way to map enum values to types like this? Can I avoid a really big conditional type as above? (In reality I have many events, and it's kind of a mess: VS Code shows a huge expression when I hover over CallbackFor, and my linter really wants to indent after every :.)

我很想编写一个将枚举值映射为类型的对象，因此可以声明 registerCallback 使用 T 和 CallbackFor [T] ，但这不是似乎是一回事。

I'd love to write an object mapping enum values to types, so I can declare registerCallback using T and CallbackFor[T], but that doesn't seem to be a thing. Any insights are appreciated!

推荐答案

我们可以创建一个在枚举成员和回调类型之间映射的类型，但是如果直接在 registerCallback 中使用它，我们将无法正确推断出回调参数类型：

We can create a type that maps between the enum members and the callback types, but the if we use it directly in registerCallback we will not get correct inference for callback argument types:

type EventTypeCallbackMap = {
    [EventType.PlaySong] : PlaySongCallback,
    [EventType.SeekTo] : SeekToCallback,
    [EventType.StopSong] : StopSongCallback,
}

declare function registerCallback
    <T extends EventType>(t: T, f: EventTypeCallbackMap[T]): void;

registerCallback(EventType.PlaySong, n => { }) // n is any

如果只有3个事件类型，则多个重载实际上是一个很好的解决方案：

If you have just 3 event types, multiple overloads are actually a pretty good solution:

declare function registerCallback(t: EventType.PlaySong, f: PlaySongCallback): void;
declare function registerCallback(t: EventType.SeekTo, f: SeekToCallback): void;
declare function registerCallback(t: EventType.StopSong, f: StopSongCallback): void;

registerCallback(EventType.PlaySong, n => { }) // n is string

如果您有很多枚举成员，您还可以自动生成重载签名：

If you have a lot of enum member you could also generate the overload signature automatically:

type EventTypeCallbackMap = {
    [EventType.PlaySong]: PlaySongCallback,
    [EventType.SeekTo]: SeekToCallback,
    [EventType.StopSong]: StopSongCallback,
}

type UnionToIntersection<U> = 
(U extends any ? (k: U)=>void : never) extends ((k: infer I)=>void) ? I : never
declare let registerCallback: UnionToIntersection<
    EventType extends infer T ?
    T extends T ? (t: T, f: EventTypeCallbackMap[T]) => void :
    never: never
> 


registerCallback(EventType.PlaySong, n => { }) // n is string

请参见此处（并进行投票）答案）以解释 UnionToIntersection

See here (and up-vote the answer) for an explanation of UnionToIntersection

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