回调函数的概念：
A callback is a function that is passed as an argument to another function and is executed after its parent function has completed.
自己在项目里的callback的使用的例子:
(后期整理)
回调函数的概念：
A callback is a function that is passed as an argument to another function and is executed after its parent function has completed.
自己在项目里的callback的使用的例子:
(后期整理)
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