Typescript中可扩展的强类型事件发射器接口

Question

一段时间以来，我一直在Typescript中使用强类型事件发射器接口，但是现在我需要它来支持向其添加自己的事件的子类。在某些时候，Typescript无法识别基类事件。

以下是压缩版本（Playground Link）中的代码：

type UnionToIntersection<U> =
    (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never;

type AddParameters<ListenersT, EventT> =
    ListenersT extends (...args: infer ArgsT) => void
        ? (event: EventT, ...args: ArgsT) => Promise<boolean>
        : never;

type EmitSignatures<ListenersT> =
    { [EventT in keyof ListenersT]: AddParameters<ListenersT[EventT], EventT> };
type EmitAll<ListenersT> = UnionToIntersection<EmitSignatures<ListenersT>[keyof ListenersT]>

type OnSignatures<ListenersT, ReturnT> =
    { [EventT in keyof ListenersT]: (event: EventT, listener: ListenersT[EventT]) => ReturnT };
type OnAll<ListenersT, ReturnT> =
    UnionToIntersection<OnSignatures<ListenersT, ReturnT>[keyof ListenersT]>;

type EventEmitter<ListenersT> = EmitterInterface<ListenersT>;

export interface EmitterInterface<ListenersT>
{
    emit: EmitAll<ListenersT>;
    on: OnAll<ListenersT, this>;
}

/////////////////////////////////////////////////////////////////////////////////////////////

interface VehicleEvents
{
    accelerate(acceleration: number): void;
    brake(deceleration: number): void;
}

interface BusEvents extends VehicleEvents
{
    doorStateChange(front: boolean, middle: boolean, rear: boolean): void
}

interface Vehicle<E extends VehicleEvents> extends EventEmitter<E>
{
    onSig: OnSignatures<E, this>;
    onSigs: OnSignatures<E, this>[keyof E];
}

class Vehicle<E extends VehicleEvents>
{
    public constructor()
    { this.on('brake', () => this.flashBrakeLights()); } // supposed to work?

    public flashBrakeLights(): void {}

    public hitTheGas(strength: number): void
    { this.emit('accelerate', strength * 42); } // supposed to work?

    public test(): void
    {
        this.onSig.accelerate;
        this.onSig.brake;
        this.onSigs('accelerate', (a) => undefined); // error I don't understand
        this.onSigs('brake', (d) => undefined); // error I don't understand
        this.onSigs('foo', () => undefined); // supposed to error
    }
}

interface Bus extends EventEmitter<BusEvents> {}

class Bus extends Vehicle<BusEvents>
{
    public doorState: [boolean, boolean, boolean] = [false, false, false];

    public constructor()
    {
        super();
        this.on('accelerate', () => {
            this.door(0, false);
            this.door(1, false);
            this.door(2, false);
        });
    }

    public door(index: number, state: boolean): void
    {
        this.doorState[index] = state;
        this.emit('doorStateChange', ...this.doorState);
    }
}

export const bus = new Bus();

E类型被声明为VehicleEvents的扩展，它应该足以让Typescript知道存在accelerate和brake事件，不是吗？

任何解释为何无效？关于如何解决此问题或以其他方式实现我所需要的任何想法？

Answer 1

问题是，在类内部，那些花哨的条件类型（不确定它们来自cough的位置）如果仍包含未解析的类型参数，则无法解析。因此，尽管使用通用类型参数进行可扩展性的方法似乎是一个好主意，但其效果是，它使on和emit在类中不可用。

一种解决方案是不使用类型参数，而仅使用事件接口本身。这样做的问题（无疑是您发现的）是，它使类无法扩展，因为on和emit的任何派生版本将与基本类型版本不兼容。

要解决此问题，我们可以使用从基本类型中删除on和emit的函数。这有点骇人听闻，但我认为没有更好的方法。

interface VehicleEvents {
  accelerate(acceleration: number): void;
  brake(deceleration: number): void;
}

interface BusEvents extends VehicleEvents {
  doorStateChange(front: boolean, middle: boolean, rear: boolean): void
}

interface Vehicle extends EventEmitter<VehicleEvents> {}

class Vehicle {
  public constructor() {
    this.on('brake', () => this.flashBrakeLights()); //ok 
  }

  public flashBrakeLights(): void { }

  public hitTheGas(strength: number): void { this.emit('accelerate', strength * 42); } // ok

}

type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>
interface Bus extends EventEmitter<BusEvents> { }

function extendEmitter<TBaseCtor extends new (...a: any[])=> any>(ctor: TBaseCtor){
  return ctor as (new (...a: ConstructorParameters<TBaseCtor>) => Omit<InstanceType<TBaseCtor>, 'on' | 'emit'>)
}
class Bus extends extendEmitter(Vehicle) {
  public doorState: [boolean, boolean, boolean] = [false, false, false];

  public constructor() {
    super();
    this.on('accelerate', () => {
      this.door(0, false);
      this.door(1, false);
      this.door(2, false);
    });
  }

  public door(index: number, state: boolean): void {
    this.doorState[index] = state;
    this.emit('doorStateChange', ...this.doorState);
  }

}

export const bus = new Bus();

以上版本不能确保新派生的对象正确地实现了基本事件。我们可以编写一个版本来验证这一点，但是需要对原始定义进行一些改动，以允许我们从基本类型中提取事件接口：

type UnionToIntersection<U> =
    (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never;

type AddParameters<ListenersT, EventT> =
    ListenersT extends (...args: infer ArgsT) => void
        ? (event: EventT, ...args: ArgsT) => Promise<boolean>
        : never;

type EmitSignatures<ListenersT> =
    { [EventT in keyof ListenersT]: AddParameters<ListenersT[EventT], EventT> };
type EmitAll<ListenersT> = UnionToIntersection<EmitSignatures<ListenersT>[keyof ListenersT]>

type OnSignatures<ListenersT, ReturnT> =
    { [EventT in keyof ListenersT]: (event: EventT, listener: ListenersT[EventT]) => ReturnT };
type OnAll<ListenersT, ReturnT> =
    UnionToIntersection<OnSignatures<ListenersT, ReturnT>[keyof ListenersT]>;

type EventEmitter<ListenersT> = EmitterInterface<ListenersT>;

export interface EmitterInterface<ListenersT>
{
    emit: EmitAll<ListenersT>;
    on: OnAll<ListenersT, this> & {__source: ListenersT}; // do not use __source, just here to allow EventTypes to work
}

type EventTypes<T> = T extends EventEmitter<infer U> ? U : never;

/////////////////////////////////////////////////////////////////////////////////////////////

interface VehicleEvents {
  accelerate(acceleration: number): void;
  brake(deceleration: number): void;
}

interface BusEvents extends VehicleEvents {
  doorStateChange(front: boolean, middle: boolean, rear: boolean): void
}

interface Vehicle extends EventEmitter<VehicleEvents> {}

class Vehicle {
  public constructor() {
    this.on('brake', () => this.flashBrakeLights()); //ok 
  }

  public flashBrakeLights(): void { }

  public hitTheGas(strength: number): void { this.emit('accelerate', strength * 42); } // ok

}

type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>
interface Bus extends EventEmitter<BusEvents> { }


function extendEmitter<TBaseCtor extends new (...a: any[])=> any>(ctor: TBaseCtor){
  return function<TEvents extends EventTypes<InstanceType<TBaseCtor>>>(){
    return ctor as (new (...a: ConstructorParameters<TBaseCtor>) => Omit<InstanceType<TBaseCtor>, 'on' | 'emit'> & EventEmitter<TEvents>)
  }
}

class Bus extends extendEmitter(Vehicle)<BusEvents>() {
  public doorState: [boolean, boolean, boolean] = [false, false, false];

  public constructor() {
    super();
    this.on('accelerate', () => {
      this.door(0, false);
      this.door(1, false);
      this.door(2, false);
    });
  }

  public door(index: number, state: boolean): void {
    this.doorState[index] = state;
    this.emit('doorStateChange', ...this.doorState);
  }

}

export const bus = new Bus();