I would like to understand the detail of callback mechanism in Excel vba.

From what I understand Excel is essentially a single thread application. I have studied the example of issuing an asynchronous HTTP callback example here.

At the completion of such call, a callback will be made back to the main thread and operation will be transferred to a different segment of code to execute the callback.

What is going on if I have issued a wait or sleep function right after issuing such asynch HTTP request and the time it asks the process to wait is far beyond the HTTP server responds? In principal, wait or sleep will ask the main process to go to sleep and no events (not even redrawing the screen) will disturb such a pause state so does it mean such callback event will be pushed until the sleep comes to conclusion?

In multithread model, we could put main UI thread to an alertable sleep and any asynch thread would be set to wake it up if packets arrive. Such thing does not seem possible in a single process model in Excel, right?

If I have to make sure packets arrive before my main process continue, I suppose the only way possible is to put a tight loop (with time limit) after I issue such asynch call to look for change of state?