typedef struct{ Uint8 scancode; SDLKey sym; SDLMod mod; Uint16 unicode; } SDL_keysym; 用于按键和放开消息。scancode是硬件相关键码。除非你有特殊用途，否则忽略。sym指示键，mod为修饰键集合，例如KMOD_NUM | KMOD_CAPS | KMOD_LSHIFT 为Numlock、Capslock和左Shift组合。最后unicode为对应字符。注意，仅当是按键消息时unicode才有效，放开消息时无效。Unicode变换必须用SDL_EnableUNICODE()打开。 SDL_KeyboardEvent
typedef struct{ Uint8 type; Uint8 state; SDL_keysym keysym; } SDL_KeyboardEvent; 描述一个键盘消息。type指示按键（SDL_KEYDOWN）或放开（SDL_KEYUP）。state是冗余的，和含义type相同只是符号不同（SDL_PRESSED，SDL_RELEASED）。 读取键盘消息
使用消息循环用SDL_PollEvent()从消息队列里读取，并用switch-case检测SDL_KEYUP和SDL_KEYDOWN。下面是个基本的例子。
例3-10 读键盘消息SDL_Event event; . . /* Poll for events. SDL_PollEvent() returns 0 when there are no */ /* more events on the event queue, our while loop will exit when */ /* that occurs. */ while( SDL_PollEvent( &event ) ){ /* We are only worried about SDL_KEYDOWN and SDL_KEYUP events */ switch( event.type ){ case SDL_KEYDOWN: printf( "Key press detected\n" ); break; case SDL_KEYUP: printf( "Key release detected\n" ); break; default: break; } } 更详细的内容
我们已经知道要用SDL_Init和SDL_SetVideoMode初始化，但我们还得用另外两个函数取得必要信息。要调用SDL_EnableUNICODE(1)以允许unicode变换，还要用SDL_GetKeyName将SDLKey转换成可打印字符。注意：小于0x80的unicode字符直接映射到ASCII码。下例用到这一点。
例3-11 解释按键消息#include "SDL.h" /* Function Prototypes */ void PrintKeyInfo( SDL_KeyboardEvent *key ); void PrintModifiers( SDLMod mod ); /* main */ int main( int argc, char *argv[] ){ SDL_Event event; int quit = 0; /* Initialise SDL */ if( SDL_Init( SDL_INIT_VIDEO ) ){ fprintf( stderr, "Could not initialise SDL: %s\n", SDL_GetError() ); exit( -1 ); } /* Set a video mode */ if( !SDL_SetVideoMode( 320, 200, 0, 0 ) ){ fprintf( stderr, "Could not set video mode: %s\n", SDL_GetError() ); SDL_Quit(); exit( -1 ); } /* Enable Unicode translation */ SDL_EnableUNICODE( 1 ); /* Loop until an SDL_QUIT event is found */ while( !quit ){ /* Poll for events */ while( SDL_PollEvent( &event ) ){ switch( event.type ){ /* Keyboard event */ /* Pass the event data onto PrintKeyInfo() */ case SDL_KEYDOWN: case SDL_KEYUP: PrintKeyInfo( &event.key ); break; /* SDL_QUIT event (window close) */ case SDL_QUIT: quit = 1; break; default: break; } } } /* Clean up */ SDL_Quit(); exit( 0 ); } /* Print all information about a key event */ void PrintKeyInfo( SDL_KeyboardEvent *key ){ /* Is it a release or a press? */ if( key->type == SDL_KEYUP ) printf( "Release:- " ); else printf( "Press:- " ); /* Print the hardware scancode first */ printf( "Scancode: 0x%02X", key->keysym.scancode ); /* Print the name of the key */ printf( ", Name: %s", SDL_GetKeyName( key->keysym.sym ) ); /* We want to print the unicode info, but we need to make */ /* sure its a press event first (remember, release events */ /* don't have unicode info */ if( key->type == SDL_KEYDOWN ){ /* If the Unicode value is less than 0x80 then the */ /* unicode value can be used to get a printable */ /* representation of the key, using (char)unicode. */ printf(", Unicode: " ); if( key->keysym.unicode < 0x80 && key->keysym.unicode > 0 ){ printf( "%c (0x%04X)", (char)key->keysym.unicode, key->keysym.unicode ); } else{ printf( "? (0x%04X)", key->keysym.unicode ); } } printf( "\n" ); /* Print modifier info */ PrintModifiers( key->keysym.mod ); } /* Print modifier info */ void PrintModifiers( SDLMod mod ){ printf( "Modifers: " ); /* If there are none then say so and return */ if( mod == KMOD_NONE ){ 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。
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