使用vue的时候，想必大家都是用过keep-alive，其作用就是缓存页面以及其状态。使用了这么久vue只知道如何使用但不明白其中原理，昨天翻看实现代码，这里做个笔记。

这里以vue3为例，整个组件的源码为：

const KeepAliveImpl = {
  name: `KeepAlive`,

  // Marker for special handling inside the renderer. We are not using a ===
  // check directly on KeepAlive in the renderer, because importing it directly
  // would prevent it from being tree-shaken.
  __isKeepAlive: true,

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

  setup(props: KeepAliveProps, { slots }: SetupContext) {
    const cache: Cache = new Map()
    const keys: Keys = new Set()
    let current: VNode | null = null

    const instance = getCurrentInstance()!
    // console.log('instance',instance)
    // KeepAlive communicates with the instantiated renderer via the "sink"
    // where the renderer passes in platform-specific functions, and the
    // KeepAlive instance exposes activate/deactivate implementations.
    // The whole point of this is to avoid importing KeepAlive directly in the
    // renderer to facilitate tree-shaking.
    const sink = instance.sink as KeepAliveSink
    const {
      renderer: {
        move,
        unmount: _unmount,
        options: { createElement }
      },
      parentSuspense
    } = sink
    const storageContainer = createElement('div')
    // console.log('sink',sink)
    sink.activate = (vnode, container, anchor) => {
      move(vnode, container, anchor, MoveType.ENTER, parentSuspense)
      queuePostRenderEffect(() => {
        const component = vnode.component!
        component.isDeactivated = false
        if (component.a !== null) {
          invokeHooks(component.a)
        }
      }, parentSuspense)
    }

    sink.deactivate = (vnode: VNode) => {
      move(vnode, storageContainer, null, MoveType.LEAVE, parentSuspense)
      queuePostRenderEffect(() => {
        const component = vnode.component!
        if (component.da !== null) {
          invokeHooks(component.da)
        }
        component.isDeactivated = true
      }, parentSuspense)
    }

    function unmount(vnode: VNode) {
      // reset the shapeFlag so it can be properly unmounted
      vnode.shapeFlag = ShapeFlags.STATEFUL_COMPONENT
      _unmount(vnode, instance, parentSuspense)
    }

    function pruneCache(filter?: (name: string) => boolean) {
      cache.forEach((vnode, key) => {
        const name = getName(vnode.type as Component)
        if (name && (!filter || !filter(name))) {
          pruneCacheEntry(key)
        }
      })
    }

    function pruneCacheEntry(key: CacheKey) {
      const cached = cache.get(key) as VNode
      if (!current || cached.type !== current.type) {
        unmount(cached)
      } else if (current) {
        // current active instance should no longer be kept-alive.
        // we can't unmount it now but it might be later, so reset its flag now.
        current.shapeFlag = ShapeFlags.STATEFUL_COMPONENT
      }
      cache.delete(key)
      keys.delete(key)
    }

    watch(
      () => [props.include, props.exclude],
      ([include, exclude]) => {
        include && pruneCache(name => matches(include, name))
        exclude && pruneCache(name => matches(exclude, name))
      },
      { lazy: true }
    )

    onBeforeUnmount(() => {
      cache.forEach(unmount)
    })

    return () => {
      if (!slots.default) {
        return null
      }

      const children = slots.default()
      let vnode = children[0]
      if (children.length > 1) {
        if (__DEV__) {
          warn(`KeepAlive should contain exactly one component child.`)
        }
        current = null
        return children
      } else if (
        !isVNode(vnode) ||
        !(vnode.shapeFlag & ShapeFlags.STATEFUL_COMPONENT)
      ) {
        current = null
        return vnode
      }

      const comp = vnode.type as Component
      const name = getName(comp)
      const { include, exclude, max } = props

      if (
        (include && (!name || !matches(include, name))) ||
        (exclude && name && matches(exclude, name))
      ) {
        return vnode
      }

      const key = vnode.key == null ? comp : vnode.key
      const cached = cache.get(key)

      // clone vnode if it's reused because we are going to mutate it
      if (vnode.el) {
        vnode = cloneVNode(vnode)
      }
      cache.set(key, vnode)
      if (cached) {
        // copy over mounted state
        vnode.el = cached.el
        vnode.anchor = cached.anchor
        vnode.component = cached.component
        if (vnode.transition) {
          // recursively update transition hooks on subTree
          setTransitionHooks(vnode, vnode.transition!)
        }
        // avoid vnode being mounted as fresh
        vnode.shapeFlag |= ShapeFlags.COMPONENT_KEPT_ALIVE
        // make this key the freshest
        keys.delete(key)
        keys.add(key) 
      } else {
        keys.add(key)
        // prune oldest entry
        if (max && keys.size > parseInt(max as string, 10)) { 
          pruneCacheEntry(Array.from(keys)[0])
        }
      }
      // avoid vnode being unmounted
      vnode.shapeFlag |= ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE
      current = vnode
      return vnode
    }
  }
}

很容易看出keep-alive其实就是vue自己封装的一个组件，和普通组件一样。

再讲keep-alive组件前先了解下vue组件的整个渲染，大致流程如下：

keep-alive生命周期

组件挂载：

调用setupStatefulComponent函数触发组件setup方法，其中组件的setup方法核心代码其实就几行：

return () => {
    const children = slots.default()
    let vnode = children[0]
    cache.set(key, vnode)

    if (cached) {
      vnode.el = cached.el
      vnode.anchor = cached.anchor
      vnode.component = cached.component
      vnode.shapeFlag |= ShapeFlags.COMPONENT_KEPT_ALIVE
      keys.delete(key)
      keys.add(key) 
    } else {
      keys.add(key)
    }
    return vnode
}

主要逻辑为三：1.确认需要渲染的slot、2.将其状态置入缓存或读取已存在的缓存、3.返回slot对应的vnode，紧接着调用setupRenderEffect，渲染出dom。

组件更新（slot变化）：

当slot变化后，首先会调用keep-alive组件的render即setup的返回函数，逻辑见上面setup方法。紧接着当某个slot卸载时，会调用deactivate函数，当某个slot重新挂载时，则会调用activate函数，核心代码如下：

const storageContainer = createElement('div')
sink.activate = (vnode, container, anchor) => {
      move(vnode, container, anchor, MoveType.ENTER, parentSuspense)
      queuePostRenderEffect(() => {
        const component = vnode.component!
        component.isDeactivated = false
        if (component.a !== null) {
          invokeHooks(component.a)
        }
      }, parentSuspense)
    }

    sink.deactivate = (vnode: VNode) => {
      move(vnode, storageContainer, null, MoveType.LEAVE, parentSuspense)
      queuePostRenderEffect(() => {
        const component = vnode.component!
        if (component.da !== null) {
          invokeHooks(component.da)
        }
        component.isDeactivated = true
      }, parentSuspense)
    }

逻辑也很简单，当组件卸载时，将其移入缓存的dom节点中，调用slot的deactivate生命周期，当组件重新挂载时候，将其移入至挂载的dom节点中。

总结来说，keep-alive实现原理就是将对应的状态放入一个cache对象中，对应的dom节点放入缓存dom中，当下次再次需要渲染时，从对象中获取状态，从缓存dom中移出至挂载dom节点中。