SDL Examples

SDL3 example: input/01-joystick-polling

This example code looks for the current joystick state once per frame,
and draws a visual representation of it.
/* joystick-polling.c ... */

/*
 * This example code looks for the current joystick state once per frame,
 * and draws a visual representation of it.
 *
 * This code is public domain. Feel free to use it for any purpose!
 */

/* Joysticks are low-level interfaces: there's something with a bunch of
   buttons, axes and hats, in no understood order or position. This is
   a flexible interface, but you'll need to build some sort of configuration
   UI to let people tell you what button, etc, does what. On top of this
   interface, SDL offers the "gamepad" API, which works with lots of devices,
   and knows how to map arbitrary buttons and such to look like an
   Xbox/PlayStation/etc gamepad. This is easier, and better, for many games,
   but isn't necessarily a good fit for complex apps and hardware. A flight
   simulator, a realistic racing game, etc, might want this interface instead
   of gamepads. */

/* SDL can handle multiple joysticks, but for simplicity, this program only
   deals with the first stick it sees. */

#define SDL_MAIN_USE_CALLBACKS 1  /* use the callbacks instead of main() */
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>

/* We will use this renderer to draw into this window every frame. */
static SDL_Window *window = NULL;
static SDL_Renderer *renderer = NULL;
static SDL_Joystick *joystick = NULL;
static SDL_Color colors[64];

/* This function runs once at startup. */
SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
{
    int i;

    SDL_SetAppMetadata("Example Input Joystick Polling", "1.0", "com.example.input-joystick-polling");

    if (!SDL_Init(SDL_INIT_VIDEO | SDL_INIT_JOYSTICK)) {
        SDL_Log("Couldn't initialize SDL: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    if (!SDL_CreateWindowAndRenderer("examples/input/joystick-polling", 640, 480, 0, &window, &renderer)) {
        SDL_Log("Couldn't create window/renderer: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    for (i = 0; i < SDL_arraysize(colors); i++) {
        colors[i].r = SDL_rand(255);
        colors[i].g = SDL_rand(255);
        colors[i].b = SDL_rand(255);
        colors[i].a = 255;
    }

    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs when a new event (mouse input, keypresses, etc) occurs. */
SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
{
    if (event->type == SDL_EVENT_QUIT) {
        return SDL_APP_SUCCESS;  /* end the program, reporting success to the OS. */
    } else if (event->type == SDL_EVENT_JOYSTICK_ADDED) {
        /* this event is sent for each hotplugged stick, but also each already-connected joystick during SDL_Init(). */
        if (joystick == NULL) {  /* we don't have a stick yet and one was added, open it! */
            joystick = SDL_OpenJoystick(event->jdevice.which);
            if (!joystick) {
                SDL_Log("Failed to open joystick ID %u: %s", (unsigned int) event->jdevice.which, SDL_GetError());
            }
        }
    } else if (event->type == SDL_EVENT_JOYSTICK_REMOVED) {
        if (joystick && (SDL_GetJoystickID(joystick) == event->jdevice.which)) {
            SDL_CloseJoystick(joystick);  /* our joystick was unplugged. */
            joystick = NULL;
        }
    }
    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs once per frame, and is the heart of the program. */
SDL_AppResult SDL_AppIterate(void *appstate)
{
    int winw = 640, winh = 480;
    const char *text = "Plug in a joystick, please.";
    float x, y;
    int i;

    if (joystick) {  /* we have a stick opened? */
        text = SDL_GetJoystickName(joystick);
    }

    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
    SDL_RenderClear(renderer);
    SDL_GetWindowSize(window, &winw, &winh);

    /* note that you can get input as events, instead of polling, which is
       better since it won't miss button presses if the system is lagging,
       but often times checking the current state per-frame is good enough,
       and maybe better if you'd rather _drop_ inputs due to lag. */

    if (joystick) {  /* we have a stick opened? */
        const float size = 30.0f;
        int total;

        /* draw axes as bars going across middle of screen. We don't know if it's an X or Y or whatever axis, so we can't do more than this. */
        total = SDL_GetNumJoystickAxes(joystick);
        y = (float) ((winh - (total * size)) / 2);
        x = ((float) winw) / 2.0f;
        for (i = 0; i < total; i++) {
            const SDL_Color *color = &colors[i % SDL_arraysize(colors)];
            const float val = (((float) SDL_GetJoystickAxis(joystick, i)) / 32767.0f);  /* make it -1.0f to 1.0f */
            const float dx = x + (val * x);
            const SDL_FRect dst = { dx, y, x - SDL_fabsf(dx), size };
            SDL_SetRenderDrawColor(renderer, color->r, color->g, color->b, color->a);
            SDL_RenderFillRect(renderer, &dst);
            y += size;
        }

        /* draw buttons as blocks across top of window. We only know the button numbers, but not where they are on the device. */
        total = SDL_GetNumJoystickButtons(joystick);
        x = (float) ((winw - (total * size)) / 2);
        for (i = 0; i < total; i++) {
            const SDL_Color *color = &colors[i % SDL_arraysize(colors)];
            const SDL_FRect dst = { x, 0.0f, size, size };
            if (SDL_GetJoystickButton(joystick, i)) {
                SDL_SetRenderDrawColor(renderer, color->r, color->g, color->b, color->a);
            } else {
                SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
            }
            SDL_RenderFillRect(renderer, &dst);
            SDL_SetRenderDrawColor(renderer, 255, 255, 255, color->a);
            SDL_RenderRect(renderer, &dst);  /* outline it */
            x += size;
        }

        /* draw hats across the bottom of the screen. */
        total = SDL_GetNumJoystickHats(joystick);
        x = ((float) ((winw - (total * (size * 2.0f))) / 2.0f)) + (size / 2.0f);
        y = ((float) winh) - size;
        for (i = 0; i < total; i++) {
            const SDL_Color *color = &colors[i % SDL_arraysize(colors)];
            const float thirdsize = size / 3.0f;
            const SDL_FRect cross[] = { { x, y + thirdsize, size, thirdsize }, { x + thirdsize, y, thirdsize, size } };
            const Uint8 hat = SDL_GetJoystickHat(joystick, i);

            SDL_SetRenderDrawColor(renderer, 90, 90, 90, 255);
            SDL_RenderFillRects(renderer, cross, SDL_arraysize(cross));

            SDL_SetRenderDrawColor(renderer, color->r, color->g, color->b, color->a);

            if (hat & SDL_HAT_UP) {
                const SDL_FRect dst = { x + thirdsize, y, thirdsize, thirdsize };
                SDL_RenderFillRect(renderer, &dst);
            }

            if (hat & SDL_HAT_RIGHT) {
                const SDL_FRect dst = { x + (thirdsize * 2), y + thirdsize, thirdsize, thirdsize };
                SDL_RenderFillRect(renderer, &dst);
            }

            if (hat & SDL_HAT_DOWN) {
                const SDL_FRect dst = { x + thirdsize, y + (thirdsize * 2), thirdsize, thirdsize };
                SDL_RenderFillRect(renderer, &dst);
            }

            if (hat & SDL_HAT_LEFT) {
                const SDL_FRect dst = { x, y + thirdsize, thirdsize, thirdsize };
                SDL_RenderFillRect(renderer, &dst);
            }

            x += size * 2;
        }
    }

    x = (((float) winw) - (SDL_strlen(text) * SDL_DEBUG_TEXT_FONT_CHARACTER_SIZE)) / 2.0f;
    y = (((float) winh) - SDL_DEBUG_TEXT_FONT_CHARACTER_SIZE) / 2.0f;
    SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
    SDL_RenderDebugText(renderer, x, y, text);
    SDL_RenderPresent(renderer);

    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs once at shutdown. */
void SDL_AppQuit(void *appstate, SDL_AppResult result)
{
    if (joystick) {
        SDL_CloseJoystick(joystick);
    }

    /* SDL will clean up the window/renderer for us. */
}