jest-metadata/src/types/Emitter.ts

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export interface ReadonlyEmitter<Event extends { type: string }> {
  on(type: '*', listener: (event: Event) => unknown): this;
  on<E extends Event>(type: E['type'], listener: (event: E) => unknown): this;
  once(type: '*', listener: (event: Event) => unknown): this;
  once<E extends Event>(type: E['type'], listener: (event: E) => unknown): this;
  off(type: '*', listener: (event: Event) => unknown): this;
  off<E extends Event>(type: E['type'], listener: (event: E) => unknown): this;
}

export interface ReadonlyAsyncEmitter<Event extends { type: string }> {
  on<E extends Event>(
    type: E['type'] | '*',
    listener: (event: E) => void | Promise<void>,
    weight?: number,
  ): this;
  once<E extends Event>(
    type: E['type'] | '*',
    listener: (event: E) => void | Promise<void>,
    weight?: number,
  ): this;
  off<E extends Event>(type: E['type'] | '*', listener: (event: E) => void | Promise<void>): this;
}

export interface Emitter<Event extends { type: string }> extends ReadonlyEmitter<Event> {
  emit(event: Event): void;
}

export interface AsyncEmitter<Event extends { type: string }> extends ReadonlyAsyncEmitter<Event> {
  emit(event: Event): Promise<void>;
}