Question

In the past, I have used libraries that would allow me to register a callback so that the library can call my method when some event happens (e.g. it is common to see in code that use GUI libraries to look like button.onClick(clickHandler)).

Naively, I suppose the library’s handling mechanism could be implemented like:

while(1){
  if (event1) { event1Handler(); }
  if (event2) { event2Handler(); }
  ...
}

but that would be really wasteful right? Or is that really how it is done (for instance do well known GUI libraries like java swing, or GTK+ do it this way)?

background:

This question hadn’t really occured to me until I encountered curses. I thought about implementing my own callback system, until I realized I didn’t know how.

Answer 1

The while loop will typically wait for an interrupt from the user (GetMessage in Windows). When an interrupt arrives GetMessage returns and then it ends up in the callback function. The if statements are typically implemented as a switch-case. See Windows Message Loop on Wikipedia.

In more detail, what happens is the following:

The user application calls GetMessage, which forces the process to sleep until an input message for that application arrives from the systems queue. When a message arrives, the user app calls DispatchMessage, which calls the callback function associated with the window that the message was aimed at.

Windows API uses one callback which handles all events in a switch case. Other libraries use one callback per event class instead.

The function pointers themselves are stored together with other window data in a struct.