src/Draggable/Plugins/Scrollable/Scrollable.js

import AbstractPlugin from 'shared/AbstractPlugin';
import {closest} from 'shared/utils';

export const onDragStart = Symbol('onDragStart');
export const onDragMove = Symbol('onDragMove');
export const onDragStop = Symbol('onDragStop');
export const scroll = Symbol('scroll');

/**
 * Scrollable default options
 * @property {Object} defaultOptions
 * @property {Number} defaultOptions.speed
 * @property {Number} defaultOptions.sensitivity
 * @property {HTMLElement[]} defaultOptions.scrollableElements
 * @type {Object}
 */
export const defaultOptions = {
  speed: 6,
  sensitivity: 50,
  scrollableElements: [],
};

/**
 * Scrollable plugin which scrolls the closest scrollable parent
 * @class Scrollable
 * @module Scrollable
 * @extends AbstractPlugin
 */
export default class Scrollable extends AbstractPlugin {
  /**
   * Scrollable constructor.
   * @constructs Scrollable
   * @param {Draggable} draggable - Draggable instance
   */
  constructor(draggable) {
    super(draggable);

    /**
     * Scrollable options
     * @property {Object} options
     * @property {Number} options.speed
     * @property {Number} options.sensitivity
     * @property {HTMLElement[]} options.scrollableElements
     * @type {Object}
     */
    this.options = {
      ...defaultOptions,
      ...this.getOptions(),
    };

    /**
     * Keeps current mouse position
     * @property {Object} currentMousePosition
     * @property {Number} currentMousePosition.clientX
     * @property {Number} currentMousePosition.clientY
     * @type {Object|null}
     */
    this.currentMousePosition = null;

    /**
     * Scroll animation frame
     * @property scrollAnimationFrame
     * @type {Number|null}
     */
    this.scrollAnimationFrame = null;

    /**
     * Closest scrollable element
     * @property scrollableElement
     * @type {HTMLElement|null}
     */
    this.scrollableElement = null;

    /**
     * Animation frame looking for the closest scrollable element
     * @property findScrollableElementFrame
     * @type {Number|null}
     */
    this.findScrollableElementFrame = null;

    this[onDragStart] = this[onDragStart].bind(this);
    this[onDragMove] = this[onDragMove].bind(this);
    this[onDragStop] = this[onDragStop].bind(this);
    this[scroll] = this[scroll].bind(this);
  }

  /**
   * Attaches plugins event listeners
   */
  attach() {
    this.draggable
      .on('drag:start', this[onDragStart])
      .on('drag:move', this[onDragMove])
      .on('drag:stop', this[onDragStop]);
  }

  /**
   * Detaches plugins event listeners
   */
  detach() {
    this.draggable
      .off('drag:start', this[onDragStart])
      .off('drag:move', this[onDragMove])
      .off('drag:stop', this[onDragStop]);
  }

  /**
   * Returns options passed through draggable
   * @return {Object}
   */
  getOptions() {
    return this.draggable.options.scrollable || {};
  }

  /**
   * Returns closest scrollable elements by element
   * @param {HTMLElement} target
   * @return {HTMLElement}
   */
  getScrollableElement(target) {
    if (this.hasDefinedScrollableElements()) {
      return closest(target, this.options.scrollableElements) || document.documentElement;
    } else {
      return closestScrollableElement(target);
    }
  }

  /**
   * Returns true if at least one scrollable element have been defined via options
   * @param {HTMLElement} target
   * @return {Boolean}
   */
  hasDefinedScrollableElements() {
    return Boolean(this.options.scrollableElements.length !== 0);
  }

  /**
   * Drag start handler. Finds closest scrollable parent in separate frame
   * @param {DragStartEvent} dragEvent
   * @private
   */
  [onDragStart](dragEvent) {
    this.findScrollableElementFrame = requestAnimationFrame(() => {
      this.scrollableElement = this.getScrollableElement(dragEvent.source);
    });
  }

  /**
   * Drag move handler. Remembers mouse position and initiates scrolling
   * @param {DragMoveEvent} dragEvent
   * @private
   */
  [onDragMove](dragEvent) {
    this.findScrollableElementFrame = requestAnimationFrame(() => {
      this.scrollableElement = this.getScrollableElement(dragEvent.sensorEvent.target);
    });

    if (!this.scrollableElement) {
      return;
    }

    const sensorEvent = dragEvent.sensorEvent;
    const scrollOffset = {x: 0, y: 0};

    if ('ontouchstart' in window) {
      scrollOffset.y = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0;
      scrollOffset.x = window.pageXOffset || document.documentElement.scrollLeft || document.body.scrollLeft || 0;
    }

    this.currentMousePosition = {
      clientX: sensorEvent.clientX - scrollOffset.x,
      clientY: sensorEvent.clientY - scrollOffset.y,
    };

    this.scrollAnimationFrame = requestAnimationFrame(this[scroll]);
  }

  /**
   * Drag stop handler. Cancels scroll animations and resets state
   * @private
   */
  [onDragStop]() {
    cancelAnimationFrame(this.scrollAnimationFrame);
    cancelAnimationFrame(this.findScrollableElementFrame);

    this.scrollableElement = null;
    this.scrollAnimationFrame = null;
    this.findScrollableElementFrame = null;
    this.currentMousePosition = null;
  }

  /**
   * Scroll function that does the heavylifting
   * @private
   */
  [scroll]() {
    if (!this.scrollableElement || !this.currentMousePosition) {
      return;
    }

    cancelAnimationFrame(this.scrollAnimationFrame);

    const {speed, sensitivity} = this.options;

    const rect = this.scrollableElement.getBoundingClientRect();
    const bottomCutOff = rect.bottom > window.innerHeight;
    const topCutOff = rect.top < 0;
    const cutOff = topCutOff || bottomCutOff;

    const documentScrollingElement = getDocumentScrollingElement();
    const scrollableElement = this.scrollableElement;
    const clientX = this.currentMousePosition.clientX;
    const clientY = this.currentMousePosition.clientY;

    if (scrollableElement !== document.body && scrollableElement !== document.documentElement && !cutOff) {
      const {offsetHeight, offsetWidth} = scrollableElement;

      if (rect.top + offsetHeight - clientY < sensitivity) {
        scrollableElement.scrollTop += speed;
      } else if (clientY - rect.top < sensitivity) {
        scrollableElement.scrollTop -= speed;
      }

      if (rect.left + offsetWidth - clientX < sensitivity) {
        scrollableElement.scrollLeft += speed;
      } else if (clientX - rect.left < sensitivity) {
        scrollableElement.scrollLeft -= speed;
      }
    } else {
      const {innerHeight, innerWidth} = window;

      if (clientY < sensitivity) {
        documentScrollingElement.scrollTop -= speed;
      } else if (innerHeight - clientY < sensitivity) {
        documentScrollingElement.scrollTop += speed;
      }

      if (clientX < sensitivity) {
        documentScrollingElement.scrollLeft -= speed;
      } else if (innerWidth - clientX < sensitivity) {
        documentScrollingElement.scrollLeft += speed;
      }
    }

    this.scrollAnimationFrame = requestAnimationFrame(this[scroll]);
  }
}

/**
 * Returns true if the passed element has overflow
 * @param {HTMLElement} element
 * @return {Boolean}
 * @private
 */
function hasOverflow(element) {
  const overflowRegex = /(auto|scroll)/;
  const computedStyles = getComputedStyle(element, null);

  const overflow =
    computedStyles.getPropertyValue('overflow') +
    computedStyles.getPropertyValue('overflow-y') +
    computedStyles.getPropertyValue('overflow-x');

  return overflowRegex.test(overflow);
}

/**
 * Returns true if the passed element is statically positioned
 * @param {HTMLElement} element
 * @return {Boolean}
 * @private
 */
function isStaticallyPositioned(element) {
  const position = getComputedStyle(element).getPropertyValue('position');
  return position === 'static';
}

/**
 * Finds closest scrollable element
 * @param {HTMLElement} element
 * @return {HTMLElement}
 * @private
 */
function closestScrollableElement(element) {
  if (!element) {
    return getDocumentScrollingElement();
  }

  const position = getComputedStyle(element).getPropertyValue('position');
  const excludeStaticParents = position === 'absolute';

  const scrollableElement = closest(element, (parent) => {
    if (excludeStaticParents && isStaticallyPositioned(parent)) {
      return false;
    }
    return hasOverflow(parent);
  });

  if (position === 'fixed' || !scrollableElement) {
    return getDocumentScrollingElement();
  } else {
    return scrollableElement;
  }
}

/**
 * Returns element that scrolls document
 * @return {HTMLElement}
 * @private
 */
function getDocumentScrollingElement() {
  return document.scrollingElement || document.documentElement;
}