回调函数的意思是：“把函数A作为参数传递给另一个函数B”。
维基百科的解释：
a callback is a piece of executable code that is passed as an argument to other code, which is expected to call back(execute) the argument at some convenient time.
作者：lee philip
链接：https://www.zhihu.com/question/19801131/answer/23318523
