update (el, binding, vnode) {
      let { src, loading, error } = this._valueFormatter(binding.value)
      src = getBestSelectionFromSrcset(el, this.options.scale) || src

      const exist = find(this.ListenerQueue, item => item.el === el)
      if (!exist) {
        this.add(el, binding, vnode)
      } else {
        exist.update({
          src,
          loading,
          error
        })
      }
      if (this._observer) {
        this._observer.unobserve(el)
        this._observer.observe(el)
      }
      this.lazyLoadHandler()
      Vue.nextTick(() => this.lazyLoadHandler())
    }
update image src
@param  {DOM} el
@param  {object} vue directive binding
@return