Vue keep-alive的实现原理是什么

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keep-alive的实现原理

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。

keep-alive的使用总结

在平常开发中，有些组件只需要加载一次，后面的数据将不存在变化，亦或者是组件需要缓存状态，滚动条位置等，这个时候，keep-alive的用处就立刻凸显出来了。

1.App.vue中使用keep-alive

include表示需要缓存的页面，exclude表示不需要缓存的页面，你可以只设置其中一个即可，但两个同时设置的时候，切记exclude优先级高于include，例如a组件在exclude中和include中都存在，那么，a组件是不会被缓存的

2.App.vue中配合router进行使用

将需要缓存的组件的$route.meta中的keepAlive设置为true，反之为false

 {      path:'/login',      name:'login',      component:resolve=>require(['@/pages/login'],resolve),      meta:{        keepAlive:true,        title:'登录',        savedPosition:true,      }    },

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