Simple DirectMedia Layer: Gaming Platform for C++, Java, and More

April 3, 2008
By Victor Volkman

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Running the Game

The main logic loop is, of course, all inside RunGame(). This function is about 200 lines of code, so still too much to look at all at once, but you will examine some of the more important sections by way of explaining SDL features. Where I've deleted some code, I'll indicate with an ellipsis ( ... ) in the comments.

301 void RunGame(void)
302 {
303    int i, j;
304    SDL_Event event;
305    Uint8 *keys;
306
307    /* Paint the background */
308    numupdates = 0;
309    for ( i=0; i<screen->w; i += background->w ) {
310       SDL_Rect dst;
311
312       dst.x = i;
313       dst.y = 0;
314       dst.w = background->w;
315       dst.h = background->h;
316       SDL_BlitSurface(background, NULL, screen, &dst);
317    }
318    SDL_UpdateRect(screen, 0, 0, 0, 0);

In the first section (Lines #301 to 318), you are simply blit-ing slices of background color onto the screen buffer sequentially with SDL_BlitSurface() and finally set up a repaint with SDL_UpdateRect().

319
320    /* Initialize the objects */
321    // . . .
333    CreateAlien();
334    DrawObject(&aliens[0]);
335    UpdateScreen();
336
337    while ( player.alive ) {
338       /* Wait for the next frame */
339       WaitFrame();
340
341       /* Poll input queue, run keyboard loop */
342       while ( SDL_PollEvent(&event) ) {
343          if ( event.type == SDL_QUIT )
344             return;
345       }
346       keys = SDL_GetKeyState(NULL);
347
348       /* Erase everything from the screen */
349       // . . .
366       /* Decrement the lifetime of the explosions */
367       // . . .
373       /* Create new aliens */
374       if ( (rand()%ALIEN_ODDS) == 0 ) {
375          CreateAlien();
376       }
377
378       /* Create new shots */
379       if ( ! reloading ) {
380          if ( keys[SDLK_SPACE] == SDL_PRESSED ) {
381             for ( i=0; i<MAX_SHOTS; ++i ) {
382                if ( ! shots[i].alive ) {
383                   break;
384                }
385             }
386             if ( i != MAX_SHOTS ) {
387                shots[i].x = player.x +
388                   (player.image->w-shots[i].image->w)/2;
389                   shots[i].y = player.y -
390                      shots[i].image->h;
391                   shots[i].alive = 1;
392                   Mix_PlayChannel(SHOT_WAV,
393                      sounds[SHOT_WAV], 0);
394             }
395          }
396       }

Now, look at the input-management part of the RunGame() loop. The deceptively simple SDL_PollEvent() returns an SDL_Event object with notes about interesting things that might have happened. At this time, you check for an ESC key press with the special value of SDL_QUIT. Because this is a purely keyboard-driven game, you can use SDL_GetKeyState() to get all the info you need. Remember that the PC keyboard is designed to allow for multiple simultaneous key downs, so the result is returned in an array that you index by a keyboard code value such as SDLK_SPACE (in other words, spacebar).

397        reloading = (keys[SDLK_SPACE] == SDL_PRESSED);
398
399       /* Move the player */
400       //. . .
415       /* Move the aliens and the shots */
416       // . . .
443
444       /* Detect collisions */
445          for ( j=0; j<MAX_SHOTS; ++j ) {
446          for ( i=0; i<MAX_ALIENS; ++i ) {
447          if ( shots[j].alive && aliens[i].alive &&
448                Collide(&shots[j], &aliens[i]) ) {
449             aliens[i].alive = 0;
450             explosions[i].x = aliens[i].x;
451             explosions[i].y = aliens[i].y;
452             explosions[i].alive = EXPLODE_TIME;
453             Mix_PlayChannel(EXPLODE_WAV,
454                sounds[EXPLODE_WAV], 0);
455             shots[j].alive = 0;
456             break;
457          }
458       }
459    }

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