message: optimize pulling down and pulling up
diff: :pullDownRefresh="pullDownRefresh"
  :pullDownStyle="pullDownStyle"
  :beforePullDown="beforePullDown"
  - :pulling="pulling"
  + :isPullingDown="isPullingDown"
  :bubbleY="bubbleY"
  >
  <div ref="pulldown" class="pulldown-wrapper" :style="pullDownStyle" v-if="pullDownRefresh">
  <bubble :y="bubbleY"></bubble>
  </div>
  <div class="after-trigger" v-else>
  - <div v-if="pulling" class="loading">
  + <div v-if="isPullingDown" class="loading">
  <loading></loading>
  </div>
  <div v-else><span>{{refreshTxt}}</span></div>
  beforePullDown: true,
  isRebounding: false,
  isPullingDown: false,
  - pulling: false,
  isPullUpLoad: false,
  pullUpDirty: true,
  pullDownStyle: '',
  },
  forceUpdate(dirty) {
  if (this.pullDownRefresh && this.isPullingDown) {
  - this.pulling = false
  + this.isPullingDown = false
  this._reboundPullDown().then(() => {
  this._afterPullDown()
  })
  this.scroll.on('pullingDown', () => {
  this.beforePullDown = false
  this.isPullingDown = true
  - this.pulling = true
  this.$emit('pullingDown')
  })
  setTimeout(() => {
  this.isRebounding = true
  this.scroll.finishPullDown()
  - this.isPullingDown = false
  resolve()
  }, stopTime)
  })
  