Let’s talk about our understanding of Vue’s built-in component keep-alive

What is Keep-alive? The following article will talk about your understanding of the vue

1. What is Keep-alive

keep-alivevue## The built-in components in # can retain the state in memory during component switching to prevent repeated rendering DOM. [Related recommendations: vuejs video tutorial, web front-end development]

keep-alive

keep-aliveYou can set the following props

• include
• exclude
• max

About the basic usage of

keep-alive:

<keep-alive>
  <component :is="view"></component>
</keep-alive>

Use

includesexclude:

<keep-alive include="a,b">
  <component :is="view"></component>
</keep-alive>

<!-- 正则表达式 (使用 `v-bind`) -->
<keep-alive :include="/a|b/">
  <component :is="view"></component>
</keep-alive>

<!-- 数组 (使用 `v-bind`) -->
<keep-alive :include="[&#39;a&#39;, &#39;b&#39;]">
  <component :is="view"></component>
</keep-alive>

namename

Components with keep-alive cache set up will have two more life cycle hooks (

activated):

When entering the component for the first time:
• beforeRouteEnterbeforeCreatecreated> activatedbeforeRouteLeavedeactivated
When entering the component again:
• beforeRouteEnteractivatedbeforeRouteLeavedeactivated

2. Usage scenarios

Principle of use: When we do not need to reload the page in certain scenarios, we can use

keepalive

When we go from

Homepage–>List page–>Business details page–>. At this time, the list page should need to keep-alive

from the

homepage–>list page–>Business details page–>Return to the list page (requires caching)–>Return to homepage (requires caching)–>Enter the list again page (no need to cache), at this time you can control the keep-alivekeepAlive

{
  path: 'list',
  name: 'itemList', // 列表页
  component (resolve) {
    require(['@/pages/item/list'], resolve)
 },
 meta: {
  keepAlive: true,
  title: '列表页'
 }
}

Use

7c9485ff8c3cba5ae9343ed63c2dc3f7

<div id="app" class=&#39;wrapper&#39;>
    <keep-alive>
        <!-- 需要缓存的视图组件 --> 
        <router-view v-if="$route.meta.keepAlive"></router-view>
     </keep-alive>
      <!-- 不需要缓存的视图组件 -->
     <router-view v-if="!$route.meta.keepAlive"></router-view>
</div>

3. Principle Analysis

keep-alivevue

Source code location: src/core/components/keep-alive.js

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

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

  created () {
    this.cache = Object.create(null)
    this.keys = []
  },

  destroyed () {
    for (const key in this.cache) {
      pruneCacheEntry(this.cache, key, this.keys)
    }
  },

  mounted () {
    this.$watch('include', val => {
      pruneCache(this, name => matches(val, name))
    })
    this.$watch('exclude', val => {
      pruneCache(this, name => !matches(val, name))
    })
  },

  render() {
    /* 获取默认插槽中的第一个组件节点 */
    const slot = this.$slots.default
    const vnode = getFirstComponentChild(slot)
    /* 获取该组件节点的componentOptions */
    const componentOptions = vnode && vnode.componentOptions

    if (componentOptions) {
      /* 获取该组件节点的名称，优先获取组件的name字段，如果name不存在则获取组件的tag */
      const name = getComponentName(componentOptions)

      const { include, exclude } = this
      /* 如果name不在inlcude中或者存在于exlude中则表示不缓存，直接返回vnode */
      if (
        (include && (!name || !matches(include, name))) ||
        // excluded
        (exclude && name && matches(exclude, name))
      ) {
        return vnode
      }

      const { cache, keys } = this
      /* 获取组件的key值 */
      const key = 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
     /*  拿到key值后去this.cache对象中去寻找是否有该值，如果有则表示该组件有缓存，即命中缓存 */
      if (cache[key]) {
        vnode.componentInstance = cache[key].componentInstance
        // make current key freshest
        remove(keys, key)
        keys.push(key)
      }
        /* 如果没有命中缓存，则将其设置进缓存 */
        else {
        cache[key] = vnode
        keys.push(key)
        // prune oldest entry
        /* 如果配置了max并且缓存的长度超过了this.max，则从缓存中删除第一个 */
        if (this.max && keys.length > parseInt(this.max)) {
          pruneCacheEntry(cache, keys[0], keys, this._vnode)
        }
      }

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

You can see that the component does not have

templaterender, which will automatically execute the render

this.cache

this.cache = {
    'key1':'组件1',
    'key2':'组件2',
    // ...
}

Executed when the component is destroyed

pruneCacheEntryfunction

function pruneCacheEntry (
  cache: VNodeCache,
  key: string,
  keys: Array<string>,
  current?: VNode
) {
  const cached = cache[key]
  /* 判断当前没有处于被渲染状态的组件，将其销毁*/
  if (cached && (!current || cached.tag !== current.tag)) {
    cached.componentInstance.$destroy()
  }
  cache[key] = null
  remove(keys, key)
}

inincludeexclude

mounted () {
    this.$watch('include', val => {
        pruneCache(this, name => matches(val, name))
    })
    this.$watch('exclude', val => {
        pruneCache(this, name => !matches(val, name))
    })
}

If

includeexcludepruneCache

function pruneCache (keepAliveInstance, filter) {
  const { cache, keys, _vnode } = keepAliveInstance
  for (const key in cache) {
    const cachedNode = cache[key]
    if (cachedNode) {
      const name = getComponentName(cachedNode.componentOptions)
      if (name && !filter(name)) {
        pruneCacheEntry(cache, key, keys, _vnode)
      }
    }
  }
}

this.cachenamepruneCacheEntrythis.cache

About

keep-alive The most powerful caching function of renderkey

const key = vnode.key == null? 
componentOptions.Ctor.cid + (componentOptions.tag ? `::${componentOptions.tag}` : '')
: vnode.key

get# After the ##keythis.cache<pre class="brush:js;toolbar:false;">/* 如果命中缓存，则直接从缓存中拿 vnode 的组件实例 */ if (cache[key]) { vnode.componentInstance = cache[key].componentInstance /* 调整该组件key的顺序，将其从原来的地方删掉并重新放在最后一个 */ remove(keys, key) keys.push(key) }</pre><p>直接从缓存中拿 <code>vnode 的组件实例，此时重新调整该组件key的顺序，将其从原来的地方删掉并重新放在this.keys中最后一个

this.cache对象中没有该key值的情况，如下：

/* 如果没有命中缓存，则将其设置进缓存 */
else {
    cache[key] = vnode
    keys.push(key)
    /* 如果配置了max并且缓存的长度超过了this.max，则从缓存中删除第一个 */
    if (this.max && keys.length > parseInt(this.max)) {
        pruneCacheEntry(cache, keys[0], keys, this._vnode)
    }
}

表明该组件还没有被缓存过，则以该组件的key为键，组件vnode为值，将其存入this.cache中，并且把key存入this.keys中

此时再判断this.keys中缓存组件的数量是否超过了设置的最大缓存数量值this.max，如果超过了，则把第一个缓存组件删掉

四、思考题：缓存后如何获取数据

解决方案可以有以下两种：

• beforeRouteEnter
• actived

beforeRouteEnter

每次组件渲染的时候，都会执行beforeRouteEnter

beforeRouteEnter(to, from, next){
    next(vm=>{
        console.log(vm)
        // 每次进入路由执行
        vm.getData()  // 获取数据
    })
},

actived

在keep-alive缓存的组件被激活的时候，都会执行actived钩子

activated(){
   this.getData() // 获取数据
},

注意：服务器端渲染期间avtived不被调用

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