src/core/overlay.js

import Position from './position';
import { ID_OVERLAY, OVERLAY_ZINDEX } from '../common/constants';

/**
 * Responsible for overlay creation and manipulation i.e.
 * cutting out the visible part, animating between the sections etc
 */
export default class Overlay {
  /**
   * @param {Object} options
   * @param {Window} window
   * @param {Document} document
   */
  constructor(options, window, document) {
    this.options = options;

    this.overlayAlpha = 0;                       // Is used to animate the layover
    this.positionToHighlight = new Position({}); // position at which layover is to be patched at
    this.highlightedPosition = new Position({}); // position at which layover is patched currently
    this.redrawAnimation = null;                 // used to cancel the redraw animation
    this.highlightedElement = null;              // currently highlighted dom element (instance of Element)
    this.lastHighlightedElement = null;          // element that was highlighted before current one

    this.draw = this.draw.bind(this);  // To pass the context of class, as it is to be used in redraw animation callback

    this.window = window;
    this.document = document;

    this.resetOverlay();
    this.setSize();
  }

  /**
   * Prepares the overlay
   */
  resetOverlay() {
    // Check and remove the canvas if it already exists
    const canvasOverlay = this.document.getElementById(ID_OVERLAY);
    if (canvasOverlay && canvasOverlay.parentNode) {
      canvasOverlay.parentNode.removeChild(canvasOverlay);
    }

    const overlay = this.document.createElement('canvas');

    this.overlay = overlay;
    this.context = overlay.getContext('2d');

    this.overlay.id = ID_OVERLAY;
    this.overlay.style.pointerEvents = 'none';
    this.overlay.style.background = 'transparent';
    this.overlay.style.position = 'fixed';
    this.overlay.style.top = '0';
    this.overlay.style.left = '0';
    this.overlay.style.zIndex = OVERLAY_ZINDEX;
  }

  /**
   * Highlights the dom element on the screen
   * @param {Element} element
   * @param {boolean} animate
   */
  highlight(element, animate = true) {
    if (!element || !element.node) {
      console.warn('Invalid element to highlight. Must be an instance of `Element`');
      return;
    }

    // Trigger the hook for highlight started
    element.onHighlightStarted();

    // Old element has been deselected
    if (this.highlightedElement) {
      this.highlightedElement.onDeselected();
    }

    // get the position of element around which we need to draw
    const position = element.getCalculatedPosition();
    if (!position.canHighlight()) {
      return;
    }

    this.lastHighlightedElement = this.highlightedElement;
    this.highlightedElement = element;
    this.positionToHighlight = position;

    // If animation is not required then set the last path to be same
    // as the current path so that there is no easing towards it
    if (!this.options.animate || !animate) {
      this.highlightedPosition = this.positionToHighlight;
    }

    this.draw();
  }

  /**
   * Returns the currently selected element
   * @returns {null|*}
   */
  getHighlightedElement() {
    return this.highlightedElement;
  }

  /**
   * Gets the element that was highlighted before current element
   * @returns {null|*}
   */
  getLastHighlightedElement() {
    return this.lastHighlightedElement;
  }

  /**
   * Removes the overlay and cancel any listeners
   */
  clear() {
    this.positionToHighlight = new Position();
    if (this.highlightedElement) {
      this.highlightedElement.onDeselected();
    }

    this.highlightedElement = null;
    this.lastHighlightedElement = null;

    this.draw();
  }

  /**
   * `draw` is called for every frame . Puts back the
   * filled overlay on body (i.e. while removing existing highlight if any) and
   * Slowly eases towards the item to be selected.
   */
  draw() {
    // Cache the response of this for re-use below
    const canHighlight = this.positionToHighlight.canHighlight();

    // Remove the existing cloak from the body
    // it might be torn i.e. have patches from last highlight
    this.removeCloak();
    // Add the overlay on top of the whole body
    this.addCloak();

    const isFadingIn = this.overlayAlpha < 0.1;

    if (canHighlight) {
      if (isFadingIn) {
        // Ignore the animation, just highlight the item at its current position
        this.highlightedPosition = this.positionToHighlight;
      } else {
        // Slowly move towards the position to highlight
        this.highlightedPosition.left += (this.positionToHighlight.left - this.highlightedPosition.left) * 0.18;
        this.highlightedPosition.top += (this.positionToHighlight.top - this.highlightedPosition.top) * 0.18;
        this.highlightedPosition.right += (this.positionToHighlight.right - this.highlightedPosition.right) * 0.18;
        this.highlightedPosition.bottom += (this.positionToHighlight.bottom - this.highlightedPosition.bottom) * 0.18;
      }
    }

    // Cut the chunk of overlay that is over the highlighted item
    this.removeCloak({
      posX: this.highlightedPosition.left - this.window.scrollX - this.options.padding,
      posY: this.highlightedPosition.top - this.window.scrollY - this.options.padding,
      width: (this.highlightedPosition.right - this.highlightedPosition.left) + (this.options.padding * 2),
      height: (this.highlightedPosition.bottom - this.highlightedPosition.top) + (this.options.padding * 2),
    });

    // Fade the overlay in if we can highlight
    if (canHighlight) {
      if (!this.options.animate) {
        this.overlayAlpha = this.options.opacity;
      } else {
        this.overlayAlpha += (this.options.opacity - this.overlayAlpha) * 0.08;
      }
    } else {
      // otherwise fade out
      this.overlayAlpha = Math.max((this.overlayAlpha * 0.85) - 0.02, 0);
    }

    // cancel any existing animation frames
    // to avoid the overlapping of frames
    this.window.cancelAnimationFrame(this.redrawAnimation);

    // Continue drawing while we can highlight or we are still fading out
    if (canHighlight || this.overlayAlpha > 0) {
      // Add the overlay if not already there
      if (!this.overlay.parentNode) {
        this.document.body.appendChild(this.overlay);
      }

      // Stage a new animation frame only if the position has not been reached
      // or the alpha has not yet fully reached fully required opacity
      if (!this.hasPositionHighlighted()) {
        this.redrawAnimation = this.window.requestAnimationFrame(this.draw);
      } else if (!this.options.animate && isFadingIn) {
        this.redrawAnimation = this.window.requestAnimationFrame(this.draw);
      } else {
        // Element has been highlighted
        this.highlightedElement.onHighlighted();
      }
    } else if (this.overlay.parentNode) {
      // Otherwise if the overlay is there, remove it
      this.document.body.removeChild(this.overlay);
    }
  }

  /**
   * Checks if there as any position highlighted
   * @returns {boolean}
   */
  hasPositionHighlighted() {
    return this.positionToHighlight.equals(this.highlightedPosition) &&
      this.overlayAlpha > (this.options.opacity - 0.05);
  }

  /**
   * Removes the cloak from the given position
   * i.e. cuts the chunk of layout which is over the element
   * to be highlighted
   *
   * @param {number} posX
   * @param {number} posY
   * @param {number} width
   * @param {number} height
   */
  removeCloak({
    posX = 0,
    posY = 0,
    width = this.overlay.width,
    height = this.overlay.height,
  } = {}) {
    this.context.clearRect(posX, posY, width, height);
  }

  /**
   * Adds the overlay i.e. to cover the given
   * position with dark overlay
   *
   * @param {number} posX
   * @param {number} posY
   * @param {number} width
   * @param {number} height
   */
  addCloak({
    posX = 0,
    posY = 0,
    width = this.overlay.width,
    height = this.overlay.height,
  } = {}) {
    this.context.fillStyle = `rgba( 0, 0, 0, ${this.overlayAlpha} )`;
    this.context.fillRect(posX, posY, width, height);
  }

  /**
   * Sets the size for the overlay
   *
   * @param {number|null} width
   * @param {number|null} height
   */
  setSize(width = null, height = null) {
    // By default it is going to cover the whole page and then we will
    // cut out a chunk for the element to be visible out of it
    this.overlay.width = width || this.window.innerWidth;
    this.overlay.height = height || this.window.innerHeight;
  }

  /**
   * Refreshes the overlay i.e. sets the size according to current window size
   * And moves the highlight around if necessary
   *
   * @param {boolean} animate
   */
  refresh(animate = true) {
    this.setSize();

    // If the highlighted element was there Cancel the
    // existing animation frame if any and highlight it again
    // as its position might have been changed
    if (this.highlightedElement) {
      this.window.cancelAnimationFrame(this.redrawAnimation);
      this.highlight(this.highlightedElement, animate);
      this.highlightedElement.onHighlighted();
    }
  }
}