在 _[05.RN社区生态/ 01. RN导航/3.嵌套导航与生命周期]_ 一节

视频出现了注册`blur`事件不触发的问题, 谢老师判定是RN的BUG. 其实不然.

根据课件的场景路由栈信息大概如下

`Setting --> Profile`

当从Profile返回到Setting时(也就是返回到上一级)路由栈发生了改变

`Setting`

只剩下了Setting页面, Profile从栈中移除, 组件也被销毁, 执行了effect的销毁函数

```

useEffect(() => {

const remove1 = navigation.addListener('focus', () => {

console.log('welcome')

})

const remove2 = navigation.addListener('blur', () => {

console.log('bay')

})

return () => {

console.log('页面销毁')

remove1()

remove2()

}

}, [])

```

而是页面返回上一级时, Profile已经从栈中删除, 执行了effect的销毁函数.所以`blur`事件不会再执行.

如果将以上代码放入到Setting组件内则可以正常执行`blur`事件

> Consider a stack navigator with screens A and B. After navigating to A, its componentDidMount is called. When pushing B, its componentDidMount is also called, but A remains mounted on the stack and its componentWillUnmount is therefore not called.

When going back from B to A, componentWillUnmount of B is called, but componentDidMount of A is not because A remained mounted the whole time.