事件驱动与状态驱动

假设有这么一个场景，是用键盘的方向键去控制一个物体前进的方向，其中有down、up、right和left，大家很容易想到的是事件驱动，大概模型如下：

/* Alien screen coordinates */
    int alien_x=0, alien_y=0;
 
    /* Main game loop */
    /* Check for events */
    while( SDL_PollEvent( &event ) ){
        switch( event.type ){
            /* Look for a keypress */
            case SDL_KEYDOWN:
                /* Check the SDLKey values and move change the coords */
                switch( event.key.keysym.sym ){
                    case SDLK_LEFT:
                        alien_x -= 1;
                        break;
                    case SDLK_RIGHT:
                        alien_x += 1;
                        break;
                    case SDLK_UP:
                        alien_y -= 1;
                        break;
                    case SDLK_DOWN:
                        alien_y += 1;
                        break;
                    default:
                        break;
                }
            }
        }
    }

但是，键盘事件只有在一个按键发生状态改变时才会发生，所以，上述的模型中，要移动100个距离，就要按100次键盘。为了避免这个问题，几乎所有游戏都是采用状态驱动的模型，如下：

/* Alien screen coordinates */
    int alien_x=0, alien_y=0;
    int alien_xvel=0, alien_yvel=0;
    
    /* Main game loop */
    /* Check for events */
    while( SDL_PollEvent( &event ) ){
        switch( event.type ){
            /* Look for a keypress */
            case SDL_KEYDOWN:
                /* Check the SDLKey values and move change the coords */
                switch( event.key.keysym.sym ){
                    case SDLK_LEFT:
                        alien_xvel = -1;
                        break;
                    case SDLK_RIGHT:
                        alien_xvel =  1;
                        break;
                    case SDLK_UP:
                        alien_yvel = -1;
                        break;
                    case SDLK_DOWN:
                        alien_yvel =  1;
                        break;
                    default:
                        break;
                }
                break;
            /* We must also use the SDL_KEYUP events to zero the x */
            /* and y velocity variables. But we must also be       */
            /* careful not to zero the velocities when we shouldn't*/
            case SDL_KEYUP:
                switch( event.key.keysym.sym ){
                    case SDLK_LEFT:
                        /* We check to make sure the alien is moving */
                        /* to the left. If it is then we zero the    */
                        /* velocity. If the alien is moving to the   */
                        /* right then the right key is still press   */
                        /* so we don't tocuh the velocity            */
                        if( alien_xvel < 0 )
                            alien_xvel = 0;
                        break;
                    case SDLK_RIGHT:
                        if( alien_xvel > 0 )
                            alien_xvel = 0;
                        break;
                    case SDLK_UP:
                        if( alien_yvel < 0 )
                            alien_yvel = 0;
                        break;
                    case SDLK_DOWN:
                        if( alien_yvel > 0 )
                            alien_yvel = 0;
                        break;
                    default:
                        break;
                }
                break;
            
            default:
                break;
        }
    }

    /* Update the alien position */
    alien_x += alien_xvel;
    alien_y += alien_yvel;