第三章下 键盘输入
键盘相关数据结构
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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