class ScrollManager {
  constructor() {
    this.subs = {};
    this._isTicking = false;

    this._scrollAmt = this._getPageScrollAmt();
  }

  _getPageScrollAmt() {
    return window.pageYOffset || document.documentElement.scrollTop;
  }

  addListeners() {
    window.addEventListener('scroll', () => this._requestTick());
  }

  removeListeners() {
    window.removeEventListener('scroll', () => this._requestTick());
  }

  publish(scrollPosition) {
    for (const key in this.subs) {
      if (this.subs.hasOwnProperty(key)) {
        this.subs[key](scrollPosition);
      }
    }
  }

  subscribe(key, fn) {
    this.subs[key] = fn;
  }

  unsubscribe(key) {
    delete this.subs[key];
  }

  _requestTick() {
    if (!this._isTicking) {
      requestAnimationFrame(() => this._onScroll());
      this._isTicking = true;
    }
  }

  _onScroll() {
    this._scrollAmt = this._getPageScrollAmt();
    this.publish(this._scrollAmt);

    this._isTicking = false;
  }
}

export default ScrollManager;