先简单介绍下 transition 标签的使用方法
主要用于 v-show, v-if 或 router-view 的进出场动画
    <transition name="name" >
      <div v-show="show" ></div>
      <div v-if="show" ></div>
      <router-view/>
    </transition>
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1. fade 淡化进出
    .fade-enter, .fade-leave-to {
      opacity: 0
    }
    .fade-leave, .fade-enter-to {
      opacity: 1
    }
    .fade-enter-active, .fade-leave-active {
      transition: all .2s
    }
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2. scale 缩放进出
    .scale-enter, .scale-leave-to {
      transform: scale(0)
    }
    .scale-leave, .scale-enter-to {
      transform: scale(1)
    }
    .scale-enter-active, .scale-leave-active {
      transition: all .2s
    }
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3. left 左侧进出 (通常用于左侧边栏)
    .left-enter, .left-leave-to {
      transform: translate3d(-100%, 0, 0)
    }
    .left-leave, .left-enter-to {
      transform: translate3d(0, 0, 0)
    }
    .left-enter-active, .left-leave-active {
      transition: all .2s
    }
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4. right 右侧进出 (通常用于右侧边栏)
    .right-enter, .right-leave-to {
      transform: translate3d(100%, 0, 0)
    }
    .right-leave, .right-enter-to {
      transform: translate3d(0, 0, 0)
    }
    .right-enter-active, .right-leave-active {
      transition: all .2s
    }
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5. top 顶部进出 (通常用于提示弹窗)
    .top-enter, .top-leave-to {
      transform: translate3d(0, -100%, 0)
    }
    .top-leave, .top-enter-to {
      transform: translate3d(0, 0, 0)
    }
    .top-enter-active, .top-leave-active {
      transition: all .2s
    }