Vue2.0 变成ref以后 _createLoop用不了了

[wjhcc2018]

请问为什么改成ref以后，

 this.$nextTick(function () {
    this.setPage(this.currentPage, true)
    this.slideEls = this.$refs.swiperWrap.children;
    this._createLoop()
  })

<div class="vswiper-wrap" ref="swiperWrap" :style="{
        'transform' : 'translate3d(' + translateX + 'px,' + translateY + 'px, 0)',
        'transition-duration': transitionDuration + 'ms'
     }" @transitionend="_onTransitionEnd">
    <slot></slot>
</div>

_createLoop () {
  var propName = this.isHorizontal() ? 'clientWidth' : 'clientHeight'
  var swiperWrapEl = this.$refs.swiperWrap
  if(swiperWrapEl[0]){
    var duplicateFirstChild = swiperWrapEl[0].cloneNode(true)
    var duplicateLastChild = swiperWrapEl.lastElementChild.cloneNode(true)
    swiperWrapEl.insertBefore(duplicateLastChild, swiperWrapEl.firstElementChild)
    swiperWrapEl.appendChild(duplicateFirstChild)
    this.translateOffset = -duplicateLastChild[propName]
  }
}

loop 为true就用不了了呢？