css实现折叠面板动画本是一件容易的事情，但是，若使用height控制，需要height已知，（当然，使用max-height控制，然后给一个足够大的max-height也是可以的）；若使用scale，父容器的高度又未能随动画一起变化，本文讲述如何在vue中使用js动画实现折叠面板动画功能。

1. 理论基础

• vue transition
• 函数式组件

2. 样式

// collapse-transition.css
.collapse-transition {
    transition: height .3s ease;
    overflow: hidden;
}

3. 动画函数

// TransitionFunction.js
function Transition () {
  if (new.target !== Transition) {
    throw new TypeError('Cannot call a class as a function')
  }
}

Transition.CLASS_NAME = 'collapse-transition'

Transition.prototype.beforeEnter = function (el) {
  el.classList.add(Transition.CLASS_NAME)

  el.style.height = 0
}

Transition.prototype.enter = function (el) {
  if (el.scrollHeight !== 0) {
    el.style.height = el.scrollHeight + 'px'
  } else {
    el.style.height = 0
  }
}

Transition.prototype.afterEnter = function (el) {
  el.classList.remove(Transition.CLASS_NAME)

  el.style.height = ''
}

Transition.prototype.beforeLeave = function (el) {
  el.classList.add(Transition.CLASS_NAME)

  el.style.height = el.scrollHeight + 'px'
}

Transition.prototype.leave = function (el) {
  if (el.scrollHeight !== 0) {
    el.style.height = 0
  }
}

Transition.prototype.afterLeave = function (el) {
  el.classList.remove(Transition.CLASS_NAME)

  el.style.height = ''
}

export default Transition

4. 折叠组件

// CoreCollapseTransition.js
import './collapse-transition.css'
import Transition from './TransitionFunction'

export default {
  name: 'CoreCollapseTransition',
  functional: true,
  render: function (createElement, context) {
    const data = {
      on: new Transition()
    }

    return createElement('transition', data, context.children)
  }
}

5. 面板组件

// CoreCollapse.vue
<template>
  <div>
    <div @click="collapse = !collapse">
      {{ collapse ? '折叠' : '展开' }}
    </div>

    <core-collapse-transition>
      <div v-show="collapse">
        <slot></slot>
      </div>
    </core-collapse-transition>
  </div>
</template>

<script>
import CoreCollapseTransition from './CoreCollapseTransition'

export default {
  components: {
    CoreCollapseTransition
  },
  data () {
    return {
      collapse: true
    }
  }
}
</script>

6. 使用示例

// Demo.vue
<template>
  <div>
    <core-collapse>
      <h1>Hello world</h1>
    </core-collapse>

    <core-collapse>
      <h1>Hello world</h1>
    </core-collapse>

    <core-collapse>
      <h1>Hello world</h1>
    </core-collapse>
  </div>
</template>

<script>
import CoreCollapse from '../components/CoreCollapse'

export default {
  components: {
    CoreCollapse
  }
}
</script>

7. 踩坑记录

  本人曾尝试使用es6的class语法来实现步聚3中的TransitionFunction，一直未能实现动画功能，尚未发现具体问题在哪里，代码如下，欢迎留言指正。

// 有问题的版本，尚不清楚问题在哪里
class Transition {
  static CLASS_NAME = 'collapse-transition'

  beforeEnter (el) {
    el.classList.add(Transition.CLASS_NAME)

    el.style.height = 0
  }

  enter (el) {
    if (el.scrollHeight !== 0) {
      el.style.height = el.scrollHeight + 'px'
    } else {
      el.style.height = 0
    }
  }

  afterEnter (el) {
    el.classList.remove(Transition.CLASS_NAME)

    el.style.height = ''
  }

  beforeLeave (el) {
    el.classList.add(Transition.CLASS_NAME)

    el.style.height = el.scrollHeight + 'px'
  }

  leave (el) {
    if (el.scrollHeight !== 0) {
      el.style.height = 0
    }
  }

  afterLeave (el) {
    el.classList.remove(Transition.CLASS_NAME)

    el.style.height = ''
  }
}

export default Transition