export function mountComponent (
  vm: Component,
  el: ?Element,
  hydrating?: boolean
): Component {
  vm.$el = el
  if (!vm.$options.render) {
    vm.$options.render = createEmptyVNode
    if (process.env.NODE_ENV !== 'production') {
      // 警告
    }
  }
  callHook(vm, 'beforeMount')

  // 定义 updateComponent
  let updateComponent
  if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
    updateComponent = () => {
      const name = vm._name
      const id = vm._uid
      const vnode = vm._render()
      vm._update(vnode, hydrating)
    }
  } else {
    updateComponent = () => {
      vm._update(vm._render(), hydrating)
    }
  }
  // 把 updateComponent 加入订阅
  new Watcher(vm, updateComponent, noop, {
    before () {
      if (vm._isMounted && !vm._isDestroyed) {
        callHook(vm, 'beforeUpdate')
      }
    }
  }, true /* isRenderWatcher */)
  hydrating = false
  // 执行 mounted 钩子
  if (vm.$vnode == null) {
    vm._isMounted = true
    callHook(vm, 'mounted')
  }
  return vm
}