SDL Guide 中文译版(三下)[2]

printf( "none\n" ); return; } /* check for the presence of each sdlmod value */ /* this looks messy, but there really isn't */ /* a clearer way. */ if( mod & kmod_num ) printf( "numlock " ); if( mod & kmod_caps ) printf( "capslock " ); if( mod & kmod_lctrl ) printf( "lctrl " ); if( mod & kmod_rctrl ) printf( "rctrl " ); if( mod & kmod_rshift ) printf( "rshift " ); if( mod & kmod_lshift ) printf( "lshift " ); if( mod & kmod_ralt ) printf( "ralt " ); if( mod & kmod_lalt ) printf( "lalt " ); if( mod & kmod_ctrl ) printf( "ctrl " ); if( mod & kmod_shift ) printf( "shift " ); if( mod & kmod_alt ) printf( "alt " ); printf( "\n" ); }

游戏式键盘输入

键盘消息仅在键的状态在按下和放开间变化时才触发。

设想你用光标键控制飞船运动以改变眼前看到的太空景象，当你按左键并希望镜头向左转时。看看下面的代码，并注意它为什么是错的。

    /* alien screen coordinates */
    int alien_x=0, alien_y=0;
    .
    .
    /* initialise sdl and video modes and all that */
    .
    /* 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;
                }
            }
        }
    }

问题在于你必须按100次左以便得到100次键盘消息，才能向左转100像素。正确的方法是收到事件时设定标志，根据标志来移动。

例3-12 正确的运动控制

    /* alien screen coordinates */
    int alien_x=0, alien_y=0;
    int alien_xvel=0, alien_yvel=0;
    .
    .
    /* initialise sdl and video modes and all that */
    .
    /* 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;

如您所见，我们用了两个变量alien_xvel和alien_yvel来表示飞船的运动，并在响应键盘消息时更新它们。