Move image on screen - c++

Question

I have an image which I want to be moveable. My problem is that the image isn't really moving, it's just dublicated and a new image loads on the new place while the old position still contain the image.
void draw_surface(int srcX, int srcY, int dstX, int dstY, int width, int height, SDL_Surface *source, SDL_Surface *destination)
{
SDL_Rect src;
src.x = srcX;
src.y = srcY;
src.w = width;
src.h = height;
SDL_Rect dst;
dst.x = dstX;
dst.y = dstY;
dst.w = width;
dst.h = height;
SDL_BlitSurface(source, &src, destination, &dst);
}
in the main function:
while (gameRunning)
{
if (SDL_PollEvent(&event))
{
if (event.type == SDL_QUIT)
{
gameRunning = false;
}
if (event.type == SDL_KEYDOWN)
{
if (event.key.keysym.sym == SDLK_DOWN)
{
dstY += 10; //new position
}
}
//apply
apply_surface(0, 10, background, screen);
draw_surface(srcX, srcY, dstX, dstY, width, heigth, background, screen);
}
//update screen
SDL_Flip(screen);
}
What's wrong with this code?

Answer 1

You should use a while loop to poll for events
Moving the drawing outside of the event loop block is a good idea
You need to clear your screen before drawing to it or else the old back buffer data will still be on it
Uint32 black = SDL_MapRGB(screen->format, 0, 0, 0);
while (gameRunning) {
while (SDL_PollEvent(&event)) {
if (event.type == SDL_QUIT) {
gameRunning = false;
}
if (event.type == SDL_KEYDOWN) {
if (event.key.keysym.sym == SDLK_DOWN) {
dstY += 10; //new position
}
}
}
// Clear screen
SDL_FillRect(screen, NULL, black);
//apply
apply_surface(0, 10, background, screen);
draw_surface(srcX, srcY, dstX, dstY, width, heigth, background, screen);
//update screen
SDL_Flip(screen);
}