What is a callback function? Understanding of the word "back"

Callback function

There are many explanations on the Internet , But the “ return ” Interpreted as “ come back ”, It's hard to understand , in my opinion ,“ return ” Understood as a “ In turn, ” Better , It means the call of role exchange .

Let's explain it ：
The program is divided into Applications and Library function

When developing applications, we often call library functions , At this time, if the parameters of the library function require you to pass in a function defined by yourself , The library function will call the function you wrote later , And this process is made up of
You call library functions Turned into The library function calls the function you write , in my opinion , This role exchange is called “ Callback function ” What's the essence of .

Look at it from another Angle , The application belongs to the high level , Library functions belong to the bottom , Generally speaking , yes High level call low level , Now the reverse is Low level calls high level , This transformation of hierarchical roles also reflects “ return ” The meaning of the word calling in reverse
（ This is a network diagram , Cut a random one , The source is unknown , It can help understand the calling process ）

significance ：

Understand the essence of callback function , His significance is obvious , That is to embed your own code into the underlying function （ Or library functions ） in , Because the underlying function （ Or library functions ） It is encapsulated before application development , Some functions need to vary from person to person , What shall I do? ？ So only the interface can be preserved , Pass the user-defined function in the form of parameters , Library functions call custom functions in turn , The callback function is just the embodiment of this form .

ps： I've heard of hook functions before （hook）, I don't quite understand , It may also achieve a similar effect .