How to make SDL_Keypress detect new Keypress during keypress-loop? - c

Question

I'm writing Conways game of life in C with SDL. The game logic and everything works fine but I want to have it when you press p for example that the game automatically updates until p is pressed again and the loop stops. Somehow it only repeats once and doesn't even register the second time p is pressed.
else if (e.key.keysym.sym == SDLK_p){
bool stop = false;
while (!stop){
nextEpoch();
updateGame(window, renderer, r);
msleep(500);
if (e.type == SDL_KEYDOWN){
if (e.key.keysym.sym == SDLK_p){
stop = true;
printf("s\n");
}
}
}
}
It doesn't register that p is pressed when it in the while-loop.
Here's the full code: https://gist.github.com/PhilippRados/2df8760cc55822c2eac62addafb24403

Answer 1

As already pointed out by someone else in the comments section, you are not updating e in the inner loop. If you want to update e with a new event, you must call SDL_PollEvent( &e ) again to fill it with a new event.
In your linked code, you seem to be attempting to implement a new state of your program outside the main event loop, which represents the running state of the program, whereas the main event loop represents the paused state of the program. That way, you effectively have two separate event loops, one for each state of the program. While it is possible to make your program work this way, I don't recommend it.
For example, the way you have designed your program, your program won't respond to SDL_QUIT events in the running state. It will only do so in the paused state.
Therefore, it would make more sense to have a single event loop for both the running and the paused states of your program.
I don't recommend that you call usleep or SDL_Delay for waiting until it is time to render the next frame, as your program will not be responding to user input during this time. Especially since you have a very low frame rate of 2 frames per second, this means that it will take up to half a second for your program to respond to user input (for example if the user resizes the window or attempts to close it). Instead, I recommend that you set up a timer using SDL_AddTimer. You can program the timer callback function to give you a SDL_USEREVENT event twice per second. That way, when you receive this event, you will know that it is time to update the game and render the next frame. While waiting for this event, your program will still be able to respond to other events.
Note that in order to use SDL timers, you must initialize the corresponding subsystem using the SDL_INIT_TIMER flag when calling SDL_Init. Strangely, your linked code does not seem to call SDL_Init at all, so I am surprised that your code works at all. According to the documentation, you should call SDL_Init like this:
SDL_Init( SDL_INIT_TIMER | SDL_INIT_VIDEO );
Also, calling SDL_PollEvent in a loop seems like a big waste of CPU resources, as your CPU usage will likely be 100% although you are effectively doing nothing most of the time. I recommend that you call SDL_WaitEvent instead, so that the thread will sleep until it receives a new event to respond to.
Another thing worth noting is that when handling an SDL_MOUSEBUTTONDOWN event, it does not seem appropriate to use the result of SDL_GetMouseState to determine the coordinates of the mouse click, as that function will return the current mouse coordinates and not the coordinates at the time of the click. Therefore, it would be more appropriate to read these coordinates from the SDL_MouseButtonEvent structure.
Here is an example of how you could rewrite your event loop to use SDL_WaitEvent instead of SDL_PollEvent and to support both a running and a paused state in the main event loop.
Uint32 my_callbackfunc( Uint32 interval, void *param )
{
SDL_Event e;
e.user.type = SDL_USEREVENT;
e.user.code = 0;
e.user.data1 = NULL;
e.user.data2 = NULL;
SDL_PushEvent( &e );
return interval;
}
int main (int argc, char** argv)
{
[...]
//set timer to trigger twice per second
SDL_TimerID timer = SDL_AddTimer( 500, my_callbackfunc, NULL );
if ( timer == 0 ) {
//TODO: handle error
return EXIT_FAILURE;
}
//start game in a paused state
bool paused = true;
while ( SDL_WaitEvent( &e ) ) {
switch ( e.type ) {
case SDL_QUIT:
goto quit_game;
case SDL_WINDOWEVENT:
//rerender in case of window state change
updateGame( window, renderer, r );
break;
case SDL_USEREVENT:
if ( !paused ) {
nextEpoch();
updateGame(window, renderer, r);
}
break;
case SDL_MOUSEBUTTONDOWN:
mouseX = getNearestMultiple( e.button.x ) / RECT_SIZE;
mouseY = getNearestMultiple( e.button.y) / RECT_SIZE;
if ( Field[mouseX][mouseY] ) {
//Deactivate cell
Field[mouseX][mouseY] = false;
updateGame(window,renderer,r);
}
else {
//activate cell at position x,y
Field[mouseX][mouseY] = true;
updateGame(window,renderer,r);
}
break;
case SDL_KEYDOWN:
switch ( e.key.keysym.sym ) {
case SDLK_SPACE:
if ( paused ) {
nextEpoch();
updateGame(window, renderer, r);
}
break;
case SDLK_r:
memset(Field,0,sizeof(Field[0][0]) * WIDTH * HEIGHT);
memset(nextState,0,sizeof(nextState[0][0]) * WIDTH * HEIGHT);
updateGame(window,renderer, r);
break;
case SDLK_p:
paused = !paused;
}
}
}
quit_game:
SDL_DestroyWindow(window);
SDL_Quit();
return EXIT_SUCCESS;
}