Vue3 KeepAlive实现原理解析

const cache: Cache = new Map()
const keys: Keys = new Set()
// cache sub tree after render
let pendingCacheKey: CacheKey | null = null
const cacheSubtree = () => {
  // fix #1621, the pendingCacheKey could be 0
  if (pendingCacheKey != null) {
    cache.set(pendingCacheKey, getInnerChild(instance.subTree))
  }
}
// 组件挂载和更新的时候就会缓存最新的组件
onMounted(cacheSubtree)
onUpdated(cacheSubtree)

  const comp = vnode.type as ConcreteComponent
  const key = vnode.key == null ? comp : vnode.key
  pendingCacheKey = key
  const cachedVNode = cache.get(key)
  
  // tips： keys 仅仅是用于收集组件的 key 组件实际收集在cache中
  if (cachedVNode) { // 节点存在缓存中
    // copy over mounted state（复制过去的状态）
    vnode.el = cachedVNode.el
    vnode.component = cachedVNode.component

    // 将节点放到队尾
    keys.delete(key) 
    keys.add(key)
  } else { // 不存在
    keys.add(key) // 加入缓存队列
    // prune oldest entry
    // 超过最大值了，将最久没有使用的组件删除，也就是队头组件
    if (max && keys.size > parseInt(max as string, 10)) { 
      pruneCacheEntry(keys.values().next().value)
    }
  }

if (shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE) {
  ;(parentComponent!.ctx as KeepAliveContext).deactivate(vnode)
  return
}

if (
  initialVNode.shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE ||
  (parent &&
    isAsyncWrapper(parent.vnode) &&
    parent.vnode.shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE)
) {
  instance.a && queuePostRenderEffect(instance.a, parentSuspense)
  if (
    __COMPAT__ &&
    isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, instance)
  ) {
    queuePostRenderEffect(
      () => instance.emit('hook:activated'),
      parentSuspense
    )
  }
}