export class EventType<Payload = never> {
  // we have to refer to the type `Payload`, otherwise ts will have weird behaviors
  // @see https://github.com/Microsoft/TypeScript/issues/20300
  payload: Payload | undefined;
  symbol: symbol;
  // don't use the description in symbol, to keep the structure extendable
  constructor(public description: string = '') {
    this.symbol = Symbol();

    // symbol 加上 description 方便调试
    if (process.env.NODE_ENV === 'development') {
      this.symbol = Symbol(description);
    }
  }
}