用法

keep-alive中文文档

解析

keep-alive组件的源码定义在src/core/components/目录下，它的代码不多，逻辑也不复杂：

/* @flow */

import { isRegExp, remove } from 'shared/util'
import { getFirstComponentChild } from 'core/vdom/helpers/index'

// 虚拟节点的缓存对象
type VNodeCache = { [key: string]: ?VNode };

// 获取组件的名称
function getComponentName (opts: ?VNodeComponentOptions): ?string {
  return opts && (opts.Ctor.options.name || opts.tag)
}

// exclude & include 的匹配
function matches (pattern: string | RegExp | Array<string>, name: string): boolean {
  if (Array.isArray(pattern)) {
    return pattern.indexOf(name) > -1
  } else if (typeof pattern === 'string') {
    return pattern.split(',').indexOf(name) > -1
  } else if (isRegExp(pattern)) {
    return pattern.test(name)
  }
  /* istanbul ignore next */
  return false
}

function pruneCache (keepAliveInstance: any, filter: Function) {
  const { cache, keys, _vnode } = keepAliveInstance
  for (const key in cache) {
    const cachedNode: ?VNode = cache[key]
    if (cachedNode) {
      const name: ?string = getComponentName(cachedNode.componentOptions)
      if (name && !filter(name)) {
        pruneCacheEntry(cache, key, keys, _vnode)
      }
    }
  }
}
// 修剪缓存入口
function pruneCacheEntry (
  cache: VNodeCache,
  key: string,
  keys: Array<string>,
  current?: VNode
) {
  const cached = cache[key]
  if (cached && (!current || cached.tag !== current.tag)) {
    cached.componentInstance.$destroy()
  }
  cache[key] = null
  remove(keys, key)
}

// exclude,include这两个参数的数据类型
const patternTypes: Array<Function> = [String, RegExp, Array]

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

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

  created () {
    // 初始化缓存对象和缓存的键数组
    this.cache = Object.create(null)
    this.keys = []
  },

  destroyed () {
    // 在销毁的时候，移除掉所有的缓存
    for (const key in this.cache) {
      pruneCacheEntry(this.cache, key, this.keys)
    }
  },

  mounted () {
    // 挂载的时候，观察include和exclude这两个属性，针对它们的变动来动态的改变缓存的组件
    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
    // 获取 keep-alive组件 的第一个子组件
    const vnode: VNode = getFirstComponentChild(slot)
    const componentOptions: ?VNodeComponentOptions = vnode && vnode.componentOptions
    if (componentOptions) {
      // check pattern
      const name: ?string = getComponentName(componentOptions)
      const { include, exclude } = this
      // 检查 exclude 和 include 的满足条件
      if (
        // 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)
        // 相同的构造器可能会被注册成不同的局部组件，所以单用cid作为key是不够的
        ? componentOptions.Ctor.cid + (componentOptions.tag ? `::${componentOptions.tag}` : '')
        : vnode.key
      if (cache[key]) {
        vnode.componentInstance = cache[key].componentInstance
        // make current key freshest
        remove(keys, key)
        keys.push(key)
      } else {
        cache[key] = vnode
        keys.push(key)
        // prune oldest entry
        if (this.max && keys.length > parseInt(this.max)) {
          pruneCacheEntry(cache, keys[0], keys, this._vnode)
        }
      }
      // 设置 keepAlive 标记
      vnode.data.keepAlive = true
    }
    return vnode || (slot && slot[0])
  }
}