SDL3/SDL_SetEventFilter

Set up a filter to process all events before they change internal state and are posted to the internal event queue.

Header File

Syntax

void SDL_SetEventFilter(SDL_EventFilter filter, void *userdata);

Function Parameters

Remarks

If the filter function returns 1 when called, then the event will be added to the internal queue. If it returns 0, then the event will be dropped from the queue, but the internal state will still be updated. This allows selective filtering of dynamically arriving events.

WARNING: Be very careful of what you do in the event filter function, as it may run in a different thread!

On platforms that support it, if the quit event is generated by an interrupt signal (e.g. pressing Ctrl-C), it will be delivered to the application at the next event poll.

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.

Note: Disabled events never make it to the event filter function; see SDL_SetEventEnabled().

Note: If you just want to inspect events without filtering, you should use SDL_AddEventWatch() instead.

Note: Events pushed onto the queue with SDL_PushEvent() get passed through the event filter, but events pushed onto the queue with SDL_PeepEvents() do not.

Thread Safety

SDL may call the filter callback at any time from any thread; the application is responsible for locking resources the callback touches that need to be protected.

Version

Code Examples

#include <SDL3/SDL.h>


// Just a quick warning: this is a silly way to do things, but it
//  illustrates how event filters work. In real life, you'd just
//  handle the event when you get it from SDL_PollEvent in the main
//  loop. Generally if you find yourself using an event filter, you
//  should stop and think carefully about whether you have a good
//  reason to be doing so in the first place.


// This is the function we'll pass to SDL_SetEventFilter. If it sees
// that the user has pressed the keyboard's space bar, it toggles
// the value pointed to by `userdata`, to represent the color blue,
// between 255 and 0. The main program uses this value to clear the
// window to a specific color as the space bar is pressed.
static int SDLCALL my_event_filter(void *userdata, SDL_Event * event)
{
    if ((event->type == SDL_EVENT_KEY_DOWN) && (event->key.keysym.sym == SDLK_SPACE)) {
        Uint8 *blue = (Uint8 *) userdata;
        if (*blue == 0) {
            *blue = 255;
        } else {
            *blue = 0;
        }
    }
    return 1;  // let all events be added to the queue since we always return 1.
}

int main(int argc, char **argv)
{
    Uint8 blue = 0;
    int quit = 0;

    // Just get a window on the screen and clear it to black.
    // In real life, you should check these for errors!
    SDL_Init(SDL_INIT_VIDEO);
    SDL_Window *window = SDL_CreateWindow("Hello SDL", 640, 480, 0);
    SDL_Renderer *renderer = SDL_CreateRenderer(window, NULL, SDL_RENDERER_PRESENTVSYNC);
    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
    SDL_RenderClear(renderer);
    SDL_RenderPresent(renderer);

    // Set up an event filter...
    SDL_SetEventFilter(my_event_filter, &blue);

    // Now run the event loop mostly forever. Each event goes through the
    //  my_event_filter function before it lands here. Each frame, we fill
    //  in the window with whatever color `blue` is set to, which might be
    //  changed by the filter function, as its address is our userdata.
    while (!quit) {
        SDL_Event e;
        while (SDL_PollEvent(&e)) {
            if (e.type == SDL_EVENT_QUIT) {
                quit = 1;
            }
        }

        SDL_SetRenderDrawColor(renderer, 0, 0, blue, 255);
        SDL_RenderClear(renderer);
        SDL_RenderPresent(renderer);
    }

    SDL_Quit();
    return 0;
}

filter	An SDL_EventFilter function to call when an event happens
userdata	a pointer that is passed to `filter`

(This is the documentation for SDL3, which is under heavy development and the API is changing! SDL2 is the current stable version!)

SDL_SetEventFilter