render () {
    const slot = this.$slots.default
    // 获取第一个组件子节点
    const vnode: VNode = getFirstComponentChild(slot)
    const componentOptions: ?VNodeComponentOptions = vnode && vnode.componentOptions
    if (componentOptions) {
      // check pattern
      const name: ?string = getComponentName(componentOptions)
      const { include, exclude } = this
      if (
        // not included
        (include && (!name || !matches(include, name))) ||
        // excluded
        (exclude && name && matches(exclude, name))
      ) {
        return vnode
      }
      const { cache, keys } = this
      const key: ?string = vnode.key == null
        // same constructor may get registered as different local components
        // so cid alone is not enough (#3269)
        ? componentOptions.Ctor.cid + (componentOptions.tag ? `::${componentOptions.tag}` : '')
        : vnode.key
      // 1、如果缓存中存在该vnode，从缓存中取得该组件的实例（一个组件对应一颗vnode树，同时一个组件对应一个vue子类的实例），不再重新创建
      if (cache[key]) {
        vnode.componentInstance = cache[key].componentInstance
        // make current key freshest
        // 将当前的组件的key作为最新的缓存（更新其在keys数组中的顺序）
        remove(keys, key)
        keys.push(key)
      } else {
        // 2、如果未命中缓存，添加到缓存
        cache[key] = vnode
        keys.push(key)
        // 如果缓存超过限制，淘汰最旧的缓存
        if (this.max && keys.length > parseInt(this.max)) {
          pruneCacheEntry(cache, keys[0], keys, this._vnode)
        }
      }
      // 标记为keepAlive组件
      vnode.data.keepAlive = true
    }
    return vnode || (slot && slot[0])
  }

<div id="app">
    <keep-alive><component :is="view"></component></keep-alive>
    <button @click="view = view =='count'? 'any': 'count'">切换组件</button>
</div>

Vue.component("count", {
    data() {
        return {
            count:0
        };
    },
    template: "<div @click='count+=1'>点了我 {{count}} 次</div>"
});
Vue.component("any", {
    template: "<div>any</div>"
});
new Vue({
    el: "#app",
    data: {
      view: "count"
    }
});

cache = {1::count: {tag: "vue-component-1-count", data:{tag: "component", hook: {…}}}, componentOptions, componentInstance, ...}
keys = ["1::count"]

cache = {
    1::count: {tag: "vue-component-1-count", data:{tag: "component", hook: {…}}, componentOptions, componentInstance, ...},
    2::any: {tag: "vue-component-2-any", data:{tag: "component", hook: {…}}, componentOptions, componentInstance, ...},
}
keys = ["1::count", "2::any"]