回调函数的理解

<strong>问：你有事去隔壁寝室找同学，发现人不在，你怎么办呢？</strong>
<strong>方法1</strong>，每隔几分钟再去趟隔壁寝室，看人在不
<strong>方法2</strong>，拜托与他同寝室的人，看到他回来时叫一下你

前者是轮询，后者是回调。

要是你去等他，就把原来的非阻塞的异步调用变成了阻塞的同步调用。

回调函数到底是什么

A callback is a function that is passed as an argument to another function and is 
executed after its parent function has completed.

　　https://www.jianshu.com/p/6bc353e5f7a3