TypeScript - 从对象创建联合

Question

我正在用打字稿创建一个 EventEmitter，但我想不出一种方法来执行以下操作：

假设我有一个这样的界面：

interface EventEmitterSubscription { dispose(): void }

// here it is
interface EventEmitter<T extends { [key: string]: any }> {
  onAnyEvent(callback: (event: { type: ???, payload: ??? }) => void): EventEmitterSubscription
  // ...
}

我找不到一种输入onAnyEvent回调的方法，例如，对于这样的 eventEmitter：

EventEmitter<{ onNumberReceived: number, onCatReceived: { cat: ICat }, onPersonNameReceived: string }>

该onAnyEvent字段将具有类型

onAnyEvent(callback: (event: { type: 'onNumberReceived', payload: number } | { type: 'onCatReceived', payload: { cat: ICat } } | { type: 'onPersonNameReceived', payload: string }) => void): EventEmitterSubscription

目前我的实现界面如下：

onAnyEvent(callback: (event: { type: keyof T, payload: T[keyof T] }) => void): EventEmitterSubscription

除了当前不起作用，例如会产生类型 onAnyEvent(callback: (event: { type: 'onNumberReceived', payload: number } | { type: 'onNumberReceived', payload: { cat: ICat } } | /* ... */) => void): EventEmitterSubscription

那么如何输入onAnyEvent字段呢？

Answer 1

创建联合的一种可能方法是拥有一个映射类型，每个属性都具有所需联合的一个元素的类型，如下所示：

type EventTypeMap<T extends { [key: string]: {} }> =
    { [K in keyof T]: { type: K; payload: T[K] } };

然后你可以定义通用联合类型

type CallbackEventTypes<T extends { [key: string]: {} }> =
     EventTypeMap<T>[keyof EventTypeMap<T>] 

并使用它

interface EventEmitterSubscription { dispose(): void }

interface EventEmitter<T extends { [key: string]: {} }> {
  onAnyEvent(callback: (event: CallbackEventTypes<T>) => void): EventEmitterSubscription
  // ...
}

interface ICat { meow() }

type Emitter = EventEmitter<{ onNumberReceived: number, onCatReceived: { cat: ICat }, onPersonNameReceived: string }>

type HandlerType = Emitter['onAnyEvent'];

// inferred as
//type HandlerType = 
// (callback: (event: { type: "onNumberReceived"; payload: number; }
//                  | { type: "onCatReceived"; payload: { cat: ICat; }; }
//                  | { type: "onPersonNameReceived"; payload: string; }
//            ) => void) => EventEmitterSubscription