This function sets up a filter to process all events before they are posted to the event queue. This is a very powerful and flexible feature. The filter is prototyped as:
typedef int (*SDL_EventFilter)(const SDL_Event *event);
If the filter returns 1, then the event will be added to the internal queue. If it returns 0, then the event will be dropped from the queue. This allows selective filtering of dynamically.
There is one caveat when dealing with the SDL_QUITEVENT event type. The event filter is only called when the window manager desires to close the application window. If the event filter returns 1, then the window will be closed, otherwise the window will remain open if possible. If the quit event is generated by an interrupt signal, it will bypass the internal queue and be delivered to the application at the next event poll.
Events pushed onto the queue with SDL_PushEvent or SDL_PeepEvents do not get passed through the event filter.
Be Careful! The event filter function may run in a different thread so be careful what you do within it.