rectChecker(checker) {
    if (is.func(checker)) {
      this._rectChecker = checker;

      this.getRect = element => {
        const rect = extend({}, this._rectChecker(element));

        if (!('width' in rect)) {
          rect.width = rect.right - rect.left;
          rect.height = rect.bottom - rect.top;
        }

        return rect;
      };

      return this;
    }

    if (checker === null) {
      delete this.getRect;
      delete this._rectChecker;
      return this;
    }

    return this.getRect;
  }
Returns or sets the function used to calculate the interactable's
element's rectangle

@param {function} [checker] A function which returns this Interactable's
bounding rectangle. See {@link Interactable.getRect}
@return {function | object} The checker function or this Interactable