in Advanced Delphi
To call most API/DLL functions from Delphi, you write a function that calls the API function, and you're done. In some cases, however, system needs to call an application-defined function in response to events generated by an API call.
Simply put, a callback function is a routine in your program that Windows calls. More generally, a callback is a means of sending a function as a parameter into another function. When the callback function has completed, control is passed back to the original function.
Read the full article to learn how to use Windows Callback Functions in Delphi code
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