用法

使用keep-alive包裹动态组件时, 会对组件进行缓存。避免组件的重新创建

<keep-alive :include="whiteList" :exclude="blackList" :max="count">
  <component :is="component"></component>
</keep-alive>

// 或
<keep-alive :include="whiteList" :exclude="blackList" :max="count">
  <router-view></router-view>
</keep-alive>

实现原理

export default {
  name: 'keep-alive',
  abstract: true, // 不会放到对应的lifecycle
  props: {
    include: patternTypes, // 白名单
    exclude: patternTypes, // 黑名单
    max: [String, Number] // 缓存的最大个数
  },
  created () {
    this.cache = Object.create(null) // 缓存列表
    this.keys = []  // 缓存的key列表
 },
  destroyed () {
    for (const key in this.cache) { // keep-alive销毁时 删除所有缓存
      pruneCacheEntry(this.cache, key, this.keys)
    }
 },
  mounted () { // 监控缓存列表
    this.$watch('include', val => {
      pruneCache(this, name => matches(val, name))
   })
    this.$watch('exclude', val => {
      pruneCache(this, name => !matches(val, name))
   })
 },
  render () {
    const slot = this.$slots.default
    const vnode: VNode = getFirstComponentChild(slot) // 获得第一个组件
    const componentOptions: ?VNodeComponentOptions = vnode && vnode.componentOptions
    if (componentOptions) {
      // check pattern
      const name: ?string = getComponentName(componentOptions)
      const { include, exclude } = this
      if ( // 获取组件名 看是否需要缓存，不需要缓存则直接返回
        // not included
        (include && (!name || !matches(include, name))) ||
        // excluded
       (exclude && name && matches(exclude, name))
      ) {
        return vnode
      }
      const { cache, keys } = this
      const key: ?string = vnode.key == null
        // same constructor may get registered as different local components
        // so cid alone is not enough (#3269)
        ? componentOptions.Ctor.cid + (componentOptions.tag ? `::${componentOptions.tag}` : '')
        : vnode.key // 生成缓存的key
      if (cache[key]) { // 如果有key 将组件实例直接复用
        vnode.componentInstance = cache[key].componentInstance
        // make current key freshest
        remove(keys, key)
        keys.push(key) // lru算法
      } else {
        cache[key] = vnode // 缓存组件
        keys.push(key)
        // prune oldest entry
        if (this.max && keys.length > parseInt(this.max)) {
          pruneCacheEntry(cache, keys[0], keys, this._vnode) // 超过最大限制删除第一个
       }
      }
      vnode.data.keepAlive = true // 在firstComponent的vnode中增加keep-alive属性
    }
    return vnode || (slot && slot[0])
  }
}