Game slows down while pressing a key or moving mouse

Question

Recently I've been trying to write my own game (clone of Space Invaders) using SDL but I had a problem with events... I have 2 events, one is emited by my timer, I use it to draw moving enemy ships. Second event is my keyboard event which I use to move my ship. When I start the game, enemy ships start to move exactly as I expect but if I press key or move mouse, they move slower. I have same problem while moving my ship, if I try to move mouse, frame rate slows down a lot. My event loop:

while(!exit)
{
    while(SDL_PollEvent(&event));
    {
        if(event.type == SDL_QUIT) exit=true;
        if(event.type == SDL_KEYDOWN)
        {
            switch(event.key.keysym.sym) 
            {
            case SDLK_LEFT:
                pship.move(pship.getPosition().x - 1, pship.getPosition().y);
                break;
            case SDLK_RIGHT:
                pship.move(pship.getPosition().x + 1, pship.getPosition().y);
                break;
            }
        }

        if(event.type == SDL_USEREVENT)
        {
            switch(event.user.code)
            {
            case 1:
                static int xOffset, yOffset;
                xOffset++;
                yOffset++;

                drawEnemyShips(eship,xOffset,yOffset);
                break;
            }
        }
    }
}

I use Visual c++ 2010 express and SDL 1.2.15

David Schwartz David Schwartz · Accepted Answer · 2012-08-21T13:25:31

while(SDL_PollEvent(&event));

You definitely don't want that ; on there! The way you have it, you are eating events. If you get an event, SDL_PollEvent returns one, so you call SDL_PollEvent again. If there's no event, you break out of the while loop and process it. But if there's a second event, you never process the first.

But in any event, a spinning poll loop is broken. It's like the kid who keeps asking "Are we there yet" every minute and doesn't let the parents drive. Use SDL_WaitEvent to wait for an event rather than spinning the CPU polling for one.

A simple way to do it is to change:

while(SDL_PollEvent(&event));

to this:

while (SDL_WaitEvent(&event) != 0)

That will leave your code a bit clumsy (the while loop is still needless, when you break out of it, you just re-enter it), but at least it will not be broken.

Game slows down while pressing a key or moving mouse

1 Answers