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>Chapter 3. Input handling</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="GUIDEINPUTKEYBOARD"
>Handling the Keyboard</A
></H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN205"
>Keyboard Related Structures</A
></H2
><P
>It should make it a lot easier to understand this tutorial is you are familiar with the data types involved in keyboard access, so I'll explain them first.</P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN208"
>SDLKey</A
></H3
><P
><SPAN
CLASS="STRUCTNAME"
>SDLKey</SPAN
> is an enumerated type defined in SDL/include/SDL_keysym.h and detailed <A
HREF="sdlkey.html"
>here</A
>. Each <SPAN
CLASS="STRUCTNAME"
>SDLKey</SPAN
> symbol represents a key, <TT
CLASS="LITERAL"
>SDLK_a</TT
> corresponds to the 'a' key on a keyboard, <TT
CLASS="LITERAL"
>SDLK_SPACE</TT
> corresponds to the space bar, and so on.</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN216"
>SDLMod</A
></H3
><P
>SDLMod is an enumerated type, similar to <SPAN
CLASS="STRUCTNAME"
>SDLKey</SPAN
>, however it enumerates keyboard modifiers (Control, Alt, Shift). The full list of modifier symbols is <A
HREF="sdlkey.html#SDLMOD"
>here</A
>. <SPAN
CLASS="STRUCTNAME"
>SDLMod</SPAN
> values can be AND'd together to represent several modifiers.</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN222"
>SDL_keysym</A
></H3
><PRE
CLASS="PROGRAMLISTING"
>typedef struct{
  Uint8 scancode;
  SDLKey sym;
  SDLMod mod;
  Uint16 unicode;
} SDL_keysym;</PRE
><P
>The <SPAN
CLASS="STRUCTNAME"
>SDL_keysym</SPAN
> structure describes a key press or a key release. The <TT
CLASS="STRUCTFIELD"
><I
>scancode</I
></TT
> field is hardware specific and should be ignored unless you know what your doing. The <TT
CLASS="STRUCTFIELD"
><I
>sym</I
></TT
> field is the <SPAN
CLASS="STRUCTNAME"
>SDLKey</SPAN
> value of the key being pressed or released. The <TT
CLASS="STRUCTFIELD"
><I
>mod</I
></TT
> field describes the state of the keyboard modifiers at the time the key press or release occurred. So a value of <TT
CLASS="LITERAL"
>KMOD_NUM | KMOD_CAPS | KMOD_LSHIFT</TT
> would mean that Numlock, Capslock and the left shift key were all press (or enabled in the case of the lock keys). Finally, the <TT
CLASS="STRUCTFIELD"
><I
>unicode</I
></TT
> field stores the 16-bit unicode value of the key.</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>It should be noted and understood that this field is only valid when the <SPAN
CLASS="STRUCTNAME"
>SDL_keysym</SPAN
> is describing a key press, not a key release. Unicode values only make sense on a key press because the unicode value describes an international character and only key presses produce characters. More information on Unicode can be found at <A
HREF="http://www.unicode.org"
TARGET="_top"
>www.unicode.org</A
></P
></BLOCKQUOTE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>Unicode translation must be enabled using the <A
HREF="sdlenableunicode.html"
><TT
CLASS="FUNCTION"
>SDL_EnableUNICODE</TT
></A
> function.</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN241"
>SDL_KeyboardEvent</A
></H3
><PRE
CLASS="PROGRAMLISTING"
>typedef struct{
  Uint8 type;
  Uint8 state;
  SDL_keysym keysym;
} SDL_KeyboardEvent;</PRE
><P
>The <SPAN
CLASS="STRUCTNAME"
>SDL_KeyboardEvent</SPAN
> describes a keyboard event (obviously). The <TT
CLASS="STRUCTFIELD"
><I
>key</I
></TT
> member of the <A
HREF="sdlevent.html"
><SPAN
CLASS="STRUCTNAME"
>SDL_Event</SPAN
></A
> union is a <SPAN
CLASS="STRUCTNAME"
>SDL_KeyboardEvent</SPAN
> structure. The <TT
CLASS="STRUCTFIELD"
><I
>type</I
></TT
> field specifies whether the event is a key release (<TT
CLASS="LITERAL"
>SDL_KEYUP</TT
>) or a key press (<TT
CLASS="LITERAL"
>SDL_KEYDOWN</TT
>) event. The <TT
CLASS="STRUCTFIELD"
><I
>state</I
></TT
> is largely redundant, it reports the same information as the <TT
CLASS="STRUCTFIELD"
><I
>type</I
></TT
> field but uses different values (<TT
CLASS="LITERAL"
>SDL_RELEASED</TT
> and <TT
CLASS="LITERAL"
>SDL_PRESSED</TT
>). The <TT
CLASS="STRUCTFIELD"
><I
>keysym</I
></TT
> contains information of the key press or release that this event represents (see above).</P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN258"
>Reading Keyboard Events</A
></H2
><P
>Reading keybaord events from the event queue is quite simple (the event queue and using it is described <A
HREF="sdlevent.html"
>here</A
>). We read events using <A
HREF="sdlpollevent.html"
><TT
CLASS="FUNCTION"
>SDL_PollEvent</TT
></A
> in a <TT
CLASS="LITERAL"
>while()</TT
> loop and check for <TT
CLASS="LITERAL"
>SDL_KEYUP</TT
> and <TT
CLASS="LITERAL"
>SDL_KEYDOWN</TT
> events using a <TT
CLASS="LITERAL"
>switch</TT
> statement, like so:
<PRE
CLASS="PROGRAMLISTING"
>  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( &#38;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;
    }
  }
  .
  .</PRE
>
This is a very basic example. No information about the key press or release is interpreted. We will explore the other extreme out our first full example below - reporting all available information about a keyboard event.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN269"
>A More Detailed Look</A
></H2
><P
>Before we can read events SDL must be initialised with <A
HREF="sdlinit.html"
><TT
CLASS="FUNCTION"
>SDL_Init</TT
></A
> and a video mode must be set using <A
HREF="sdlsetvideomode.html"
><TT
CLASS="FUNCTION"
>SDL_SetVideoMode</TT
></A
>. There are, however, two other functions we must use to obtain all the information required. We must enable unicode translation by calling <TT
CLASS="FUNCTION"
>SDL_EnableUNICODE(1)</TT
> and we must convert <SPAN
CLASS="STRUCTNAME"
>SDLKey</SPAN
> values into something printable, using <A
HREF="sdlgetkeyname.html"
><TT
CLASS="FUNCTION"
>SDL_GetKeyName</TT
></A
></P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>It is useful to note that unicode values &#60; 0x80 translate directly a characters ASCII value. THis is used in the example below</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN282"
></A
><P
><B
>Example 3-1. keys.c - Key event information</B
></P
><PRE
CLASS="PROGRAMLISTING"
>&#13;    #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( &#38;event ) ){
                
                switch( event.type ){
                    /* Keyboard event */
                    /* Pass the event data onto PrintKeyInfo() */
                    case SDL_KEYDOWN:
                    case SDL_KEYUP:
                        PrintKeyInfo( &#38;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-&#62;type == SDL_KEYUP )
            printf( "Release:- " );
        else
            printf( "Press:- " );

        /* Print the hardware scancode first */
        printf( "Scancode: 0x%02X", key-&#62;keysym.scancode );
        /* Print the name of the key */
        printf( ", Name: %s", SDL_GetKeyName( key-&#62;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-&#62;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-&#62;keysym.unicode &#60; 0x80 &#38;&#38; key-&#62;keysym.unicode &#62; 0 ){
                printf( "%c (0x%04X)", (char)key-&#62;keysym.unicode,
                        key-&#62;keysym.unicode );
            }
            else{
                printf( "? (0x%04X)", key-&#62;keysym.unicode );
            }
        }
        printf( "\n" );
        /* Print modifier info */
        PrintModifiers( key-&#62;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 &#38; KMOD_NUM ) printf( "NUMLOCK " );
        if( mod &#38; KMOD_CAPS ) printf( "CAPSLOCK " );
        if( mod &#38; KMOD_LCTRL ) printf( "LCTRL " );
        if( mod &#38; KMOD_RCTRL ) printf( "RCTRL " );
        if( mod &#38; KMOD_RSHIFT ) printf( "RSHIFT " );
        if( mod &#38; KMOD_LSHIFT ) printf( "LSHIFT " );
        if( mod &#38; KMOD_RALT ) printf( "RALT " );
        if( mod &#38; KMOD_LALT ) printf( "LALT " );
        if( mod &#38; KMOD_CTRL ) printf( "CTRL " );
        if( mod &#38; KMOD_SHIFT ) printf( "SHIFT " );
        if( mod &#38; KMOD_ALT ) printf( "ALT " );
        printf( "\n" );
    }</PRE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN285"
>Game-type Input</A
></H2
><P
>I have found that people using keyboard events for games and other interactive applications don't always understand one fundemental point.</P
><A
NAME="AEN288"
></A
><BLOCKQUOTE
CLASS="BLOCKQUOTE"
><P
>Keyboard events <I
CLASS="EMPHASIS"
>only</I
> take place when a keys state changes from being unpressed to pressed, and vice versa.</P
></BLOCKQUOTE
><P
>Imagine you have an image of an alien that you wish to move around using the cursor keys - when you pressed the left arrow key you want him to slide over to the left, when you press the down key you want him to slide down the screen. Examine the following code, it highlights and error that many people have made.
<PRE
CLASS="PROGRAMLISTING"
>    /* 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( &#38;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;
                }
            }
        }
    }
    .
    .</PRE
>
At first glance you may think this is a perfectly reasonable piece of code for the task, but it isn't. Like I said keyboard events only occur when a key changes state, so the user would have to press and release the left cursor key 100 times to move the alien 100 pixels to the left.</P
><P
>To get around this problem we must not use the events to change the position of the alien, we use the events to set flags which are then used in a seperate section of code to move the alien. Something like this:
<PRE
CLASS="PROGRAMLISTING"
>    /* 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( &#38;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 &#60; 0 )
                            alien_xvel = 0;
                        break;
                    case SDLK_RIGHT:
                        if( alien_xvel &#62; 0 )
                            alien_xvel = 0;
                        break;
                    case SDLK_UP:
                        if( alien_yvel &#60; 0 )
                            alien_yvel = 0;
                        break;
                    case SDLK_DOWN:
                        if( alien_yvel &#62; 0 )
                            alien_yvel = 0;
                        break;
                    default:
                        break;
                }
                break;
            
            default:
                break;
        }
    }
    .
    .
    /* Update the alien position */
    alien_x += alien_xvel;
    alien_y += alien_yvel;</PRE
>
As can be seen, we use two extra variables, alien_xvel and alien_yvel, which represent the motion of the ship, it is these variables that we update when we detect keypresses and releases.</P
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