<div id="root">
  <transition name="fade"
    @before-enter="handleBeforeEnter"    //动画出现前的事件
  >
    <div v-if="show">hello world</div>
  </transition>
  <button @click="handleClick">切换</button>
</div>
let vm = new Vue({
  el: '#root',
  data: {
    show:true
  },
  methods: {
    handleClick(){
      this.show = !this.show
    },
    handleBeforeEnter(el){     //接收一个参数 el，为 dom 元素
      el.style.color = 'red'
    }
  }
})

<div id="root">
  <transition name="fade"
    @before-enter="handleBeforeEnter"
    @enter="handleEnter"    //before-enter 事件结束后执行
  >
    <div v-if="show">hello world</div>
  </transition>
  <button @click="handleClick">切换</button>
</div>
let vm = new Vue({
  el: '#root',
  data: {
    show:true
  },
  methods: {
    handleClick(){
      this.show = !this.show
    },
    handleBeforeEnter(el){
      el.style.color = 'red'
    }
    handleEnter(el，done){      //接收两个参数，第一个参数 el 为 dom 元素，第二个参数是 done 是个回调函数
      setTimeout(()=>{
        el.style.color = 'green'
        done()         //done 回调函数非常重要，在执行完动画后需手动执行下这个函数，相当于告诉 Vue 这个动画已经执行完了，会再去调用 after-enter 事件
      },2000)
    }
  }
})

<div id="root">
  <transition name="fade"
        @before-enter="handleBeforeEnter"
        @enter="handleEnter"
        @after-enter="handleAfterEnter"   //enter 事件中 done 被调用后，after-enter 事件会被触发
  >
    <div v-if="show">hello world</div>
  </transition>
  <button @click="handleClick">切换</button>
</div>
let vm = new Vue({
  el: '#root',
  data: {
    show: true
  },
  methods: {
    handleClick() {
      this.show = !this.show
    },
    handleBeforeEnter(el) {
      el.style.color = 'red'
    },
    handleEnter(el, done) {
      setTimeout(() => {
        el.style.color = 'green'
        done()
      }, 2000)
    },
    handleAfterEnter(el) {   //接收一个参数 el，为 dom 元素
      setTimeout(()=>{
        el.style.color = 'blue'
      },2000)
    }
  }
})

<div id="root">
  <transition name="fade"
        @before-enter="handleBeforeEnter"
        @enter="handleEnter"
        @after-enter="handleAfterEnter"   //enter 事件中 done 被调用后，after-enter 事件会被触发
  >
    <div v-if="show">hello world</div>
  </transition>
  <button @click="handleClick">切换</button>
</div>
let vm = new Vue({
  el: '#root',
  data: {
    show: true
  },
  methods: {
    handleClick() {
      this.show = !this.show
    },
    handleBeforeEnter(el) {
      el.style.opacity = 0
    },
    handleEnter(el, done) {
      Velocity(el, {     //第一个参数是要操作的 dom 元素
        opacity: 1
      }, {
        duration: 1000,   //第三参数中需要配置一个 complete 属性，值为 done，因为 Velocity 执行完后会自动执行 complete 属性，这里设置为 done 是让它去触发 after-enter 事件
        complete: done
      })
    },
    handleAfterEnter(el) {
      console.log('动画结束')
    }
  }
})