parallax.ts

import { Utils } from "./utils";
import { Component, BaseOptions, InitElements, MElement } from "./component";

export interface ParallaxOptions extends BaseOptions {
  /**
   * The minimum width of the screen, in pixels, where the parallax functionality starts working.
   * @default 0
   */
  responsiveThreshold: number;
}

let _defaults: ParallaxOptions = {
  responsiveThreshold: 0 // breakpoint for swipeable
};

export class Parallax extends Component<ParallaxOptions> {
  private _enabled: boolean;
  private _img: HTMLImageElement;
  static _parallaxes: Parallax[] = [];
  static _handleScrollThrottled: () => any;
  static _handleWindowResizeThrottled: () => any;

  constructor(el: HTMLElement, options: Partial<ParallaxOptions>) {
    super(el, options, Parallax);
    (this.el as any).M_Parallax = this;

    this.options = {
      ...Parallax.defaults,
      ...options
    };
    
    this._enabled = window.innerWidth > this.options.responsiveThreshold;
    this._img = this.el.querySelector('img');
    this._updateParallax();
    this._setupEventHandlers();
    this._setupStyles();
    Parallax._parallaxes.push(this);
  }

  static get defaults(): ParallaxOptions {
    return _defaults;
  }

  /**
   * Initializes instance of Parallax.
   * @param el HTML element.
   * @param options Component options.
   */
  static init(el: HTMLElement, options?: Partial<ParallaxOptions>): Parallax;
  /**
   * Initializes instances of Parallax.
   * @param els HTML elements.
   * @param options Component options.
   */
  static init(els: InitElements<MElement>, options?: Partial<ParallaxOptions>): Parallax[];
  /**
   * Initializes instances of Parallax.
   * @param els HTML elements.
   * @param options Component options.
   */
  static init(els: HTMLElement | InitElements<MElement>, options: Partial<ParallaxOptions> = {}): Parallax | Parallax[] {
    return super.init(els, options, Parallax);
  }

  static getInstance(el: HTMLElement): Parallax {
    return (el as any).M_Parallax;
  }

  destroy() {
    Parallax._parallaxes.splice(Parallax._parallaxes.indexOf(this), 1);
    this._img.style.transform = '';
    this._removeEventHandlers();
    (this.el as any).M_Parallax = undefined;
  }

  static _handleScroll() {
    for (let i = 0; i < Parallax._parallaxes.length; i++) {
      let parallaxInstance = Parallax._parallaxes[i];
      parallaxInstance._updateParallax.call(parallaxInstance);
    }
  }

  static _handleWindowResize() {
    for (let i = 0; i < Parallax._parallaxes.length; i++) {
      let parallaxInstance = Parallax._parallaxes[i];
      parallaxInstance._enabled =
        window.innerWidth > parallaxInstance.options.responsiveThreshold;
    }
  }

  _setupEventHandlers() {
    this._img.addEventListener('load', this._handleImageLoad);
    if (Parallax._parallaxes.length === 0) {
      if (!Parallax._handleScrollThrottled){
        Parallax._handleScrollThrottled = Utils.throttle(Parallax._handleScroll, 5);
      }
      if (!Parallax._handleWindowResizeThrottled){
        Parallax._handleWindowResizeThrottled = Utils.throttle(Parallax._handleWindowResize, 5);
      }
      window.addEventListener('scroll', Parallax._handleScrollThrottled);
      window.addEventListener('resize', Parallax._handleWindowResizeThrottled);
    }
  }

  _removeEventHandlers() {
    this._img.removeEventListener('load', this._handleImageLoad);
    if (Parallax._parallaxes.length === 0) {
      window.removeEventListener('scroll', Parallax._handleScrollThrottled);
      window.removeEventListener('resize', Parallax._handleWindowResizeThrottled);
    }
  }

  _setupStyles() {
    this._img.style.opacity = '1';
  }

  _handleImageLoad = () => {
    this._updateParallax();
  }

  private _offset(el: Element) {
    const box = el.getBoundingClientRect();
    const docElem = document.documentElement;
    return {
      top: box.top + window.pageYOffset - docElem.clientTop,
      left: box.left + window.pageXOffset - docElem.clientLeft
    };
  }

  _updateParallax() {
    const containerHeight = this.el.getBoundingClientRect().height > 0 ? (this.el.parentNode as any).offsetHeight : 500;
    const imgHeight = this._img.offsetHeight;
    const parallaxDist = imgHeight - containerHeight;
    const bottom = this._offset(this.el).top + containerHeight;
    const top = this._offset(this.el).top;
    const scrollTop = Utils.getDocumentScrollTop();
    const windowHeight = window.innerHeight;
    const windowBottom = scrollTop + windowHeight;
    const percentScrolled = (windowBottom - top) / (containerHeight + windowHeight);
    const parallax = parallaxDist * percentScrolled;

    if (!this._enabled) {
      this._img.style.transform = '';
    }
    else if (bottom > scrollTop && top < scrollTop + windowHeight) {
      this._img.style.transform = `translate3D(-50%, ${parallax}px, 0)`;
    }
  }
}