Method tap

packages/markmap-common/src/hook.ts:6–9

(fn: HookCallback<T>)

  protected listeners: Array<HookCallback<T>> = [];

  tap(fn: HookCallback<T>): () => void {
    this.listeners.push(fn);
    return () => this.revoke(fn);
  }

  revoke(fn: HookCallback<T>): void {
    const i = this.listeners.indexOf(fn);