Vue之keep-alive原理

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keep-alive是什么？

keep-alive是Vue的一个内置组件，用来缓存keep-alive内节点，当下次在访问的时候会命中缓存，降低请求数量及防止重复渲染dom。接受三个属性，include,exclude,max。
keep-alive是一个抽象组件，根据abstract: true判断：其节点不会渲染到页面中。
含在 keep-alive 中创建的组件，会多出两个生命周期的钩子: activated 与 deactivated

  1. activated 当 keepalive 包含的组件再次渲染的时候触发
  2. deactivated 当 keepalive 包含的组件销毁的时候触发

keep-alive源码

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export default {
  name: 'keep-alive',
  abstract: true,

  props: {
    include: patternTypes,
    exclude: patternTypes,
    max: [String, Number]
  },

  created () {
    this.cache = Object.create(null)     //创建一个缓存的空对象
    this.keys = []
  },

  destroyed () {
    for (const key in this.cache) {
        //  销毁所有组件
      pruneCacheEntry(this.cache, key, this.keys)
    }
  },

  mounted () {
      //监听 include 数据是否有变化
    this.$watch('include', val => {
      //判断include 对象中 name 不存在了 就 调用 检测缓存中的组件，如果不是当前激活的组件则销毁
      pruneCache(this, name => matches(val, name))
    })
    //监听 exclude 数据是否有变化
    this.$watch('exclude', val => {
      //如果exclude 对象中存在name 不存在了 就 调用 检测缓存中的组件，如果不是当前激活的组件则销毁
      pruneCache(this, name => !matches(val, name))
    })
  },

  render () {
      
    const slot = this.$slots.default    //获取插槽
    const vnode: VNode = getFirstComponentChild(slot) // 获取插槽子组件
    const componentOptions: ?VNodeComponentOptions = vnode && vnode.componentOptions    //获取组件参数
    if (componentOptions) {
      // check pattern
      const name: ?string = getComponentName(componentOptions) //获取组件名称
      const { include, exclude } = this //获取include exclude
      if (
        // not included
        (include && (!name || !matches(include, name))) ||
        // excluded
        (exclude && name && matches(exclude, name))
      ) {
        return vnode
      }

      const { cache, keys } = this //缓存的对象、keys
      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
      if (cache[key]) { //获取值 如果key存在
        vnode.componentInstance = cache[key].componentInstance //直接获取组件实例化
        // make current key freshest
        remove(keys, key)   //把key添加到末端
        keys.push(key)
      } else {
        //将虚拟dom缓存起来
        cache[key] = vnode
        keys.push(key) //key缓存起来
        // prune oldest entry
        if (this.max && keys.length > parseInt(this.max)) {
          pruneCacheEntry(cache, keys[0], keys, this._vnode)
        }
      }

      vnode.data.keepAlive = true
    }
    return vnode || (slot && slot[0])
  }
}

https://juejin.im/post/6844903837770203144