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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="GUIDEINPUTKEYBOARD"
></A
>Handling the Keyboard</H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN271"
></A
>Keyboard Related Structures</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="AEN274"
></A
>SDLKey</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="AEN282"
></A
>SDLMod</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="AEN288"
></A
>SDL_keysym</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="AEN307"
></A
>SDL_KeyboardEvent</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="AEN324"
></A
>Reading Keyboard Events</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:</P
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN334"
></A
><P
><B
>Example 3-10. Reading Keyboard Events</B
></P
><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
></DIV
><P
>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="AEN338"
></A
>A More Detailed Look</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="AEN351"
></A
><P
><B
>Example 3-11. Interpreting 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 ) &#60; 0){
            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="AEN354"
></A
>Game-type Input</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="AEN357"
></A
><BLOCKQUOTE
CLASS="BLOCKQUOTE"
><P
>Keyboard events <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>only</I
></SPAN
> 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, and when you press the down key you want him to slide down the screen. Examine the following code; it highlights an 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:</P
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN363"
></A
><P
><B
>Example 3-12. Proper Game Movement</B
></P
><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
></DIV
><P
>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
></DIV
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	6	NAME="GENERATOR"
	7	CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
	8	"><LINK
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	50	>Prev</A
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	52	><TD
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	56	>Chapter 3. Input handling</TD
	57	><TD
	58	WIDTH="10%"
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65	></TD
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68	><HR
69	ALIGN="LEFT"
70	WIDTH="100%"></DIV
71	><DIV
72	CLASS="SECT1"
73	><H1
74	CLASS="SECT1"
75	><A
76	NAME="GUIDEINPUTKEYBOARD"
77	></A
78	>Handling the Keyboard</H1
79	><DIV
80	CLASS="SECT2"
81	><H2
82	CLASS="SECT2"
83	><A
84	NAME="AEN271"
85	></A
86	>Keyboard Related Structures</H2
87	><P
88	>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
89	><DIV
90	CLASS="SECT3"
91	><H3
92	CLASS="SECT3"
93	><A
94	NAME="AEN274"
95	></A
96	>SDLKey</H3
97	><P
98	><SPAN
99	CLASS="STRUCTNAME"
100	>SDLKey</SPAN
101	> is an enumerated type defined in SDL/include/SDL_keysym.h and detailed <A
102	HREF="sdlkey.html"
103	>here</A
104	>. Each <SPAN
105	CLASS="STRUCTNAME"
106	>SDLKey</SPAN
107	> symbol represents a key, <TT
108	CLASS="LITERAL"
109	>SDLK_a</TT
110	> corresponds to the 'a' key on a keyboard, <TT
111	CLASS="LITERAL"
112	>SDLK_SPACE</TT
113	> corresponds to the space bar, and so on.</P
114	></DIV
115	><DIV
116	CLASS="SECT3"
117	><H3
118	CLASS="SECT3"
119	><A
120	NAME="AEN282"
121	></A
122	>SDLMod</H3
123	><P
124	>SDLMod is an enumerated type, similar to <SPAN
125	CLASS="STRUCTNAME"
126	>SDLKey</SPAN
127	>, however it enumerates keyboard modifiers (Control, Alt, Shift). The full list of modifier symbols is <A
128	HREF="sdlkey.html#SDLMOD"
129	>here</A
130	>. <SPAN
131	CLASS="STRUCTNAME"
132	>SDLMod</SPAN
133	> values can be AND'd together to represent several modifiers.</P
134	></DIV
135	><DIV
136	CLASS="SECT3"
137	><H3
138	CLASS="SECT3"
139	><A
140	NAME="AEN288"
141	></A
142	>SDL_keysym</H3
143	><PRE
144	CLASS="PROGRAMLISTING"
145	>typedef struct{
146	Uint8 scancode;
147	SDLKey sym;
148	SDLMod mod;
149	Uint16 unicode;
150	} SDL_keysym;</PRE
151	><P
152	>The <SPAN
153	CLASS="STRUCTNAME"
154	>SDL_keysym</SPAN
155	> structure describes a key press or a key release. The <TT
156	CLASS="STRUCTFIELD"
157	><I
158	>scancode</I
159	></TT
160	> field is hardware specific and should be ignored unless you know what your doing. The <TT
161	CLASS="STRUCTFIELD"
162	><I
163	>sym</I
164	></TT
165	> field is the <SPAN
166	CLASS="STRUCTNAME"
167	>SDLKey</SPAN
168	> value of the key being pressed or released. The <TT
169	CLASS="STRUCTFIELD"
170	><I
171	>mod</I
172	></TT
173	> field describes the state of the keyboard modifiers at the time the key press or release occurred. So a value of <TT
174	CLASS="LITERAL"
175	>KMOD_NUM \| KMOD_CAPS \| KMOD_LSHIFT</TT
176	> 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
177	CLASS="STRUCTFIELD"
178	><I
179	>unicode</I
180	></TT
181	> field stores the 16-bit unicode value of the key.</P
182	><DIV
183	CLASS="NOTE"
184	><BLOCKQUOTE
185	CLASS="NOTE"
186	><P
187	><B
188	>Note: </B
189	>It should be noted and understood that this field is only valid when the <SPAN
190	CLASS="STRUCTNAME"
191	>SDL_keysym</SPAN
192	> 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
193	HREF="http://www.unicode.org"
194	TARGET="_top"
195	>www.unicode.org</A
196	></P
197	></BLOCKQUOTE
198	></DIV
199	><DIV
200	CLASS="NOTE"
201	><BLOCKQUOTE
202	CLASS="NOTE"
203	><P
204	><B
205	>Note: </B
206	>Unicode translation must be enabled using the <A
207	HREF="sdlenableunicode.html"
208	><TT
209	CLASS="FUNCTION"
210	>SDL_EnableUNICODE</TT
211	></A
212	> function.</P
213	></BLOCKQUOTE
214	></DIV
215	></DIV
216	><DIV
217	CLASS="SECT3"
218	><H3
219	CLASS="SECT3"
220	><A
221	NAME="AEN307"
222	></A
223	>SDL_KeyboardEvent</H3
224	><PRE
225	CLASS="PROGRAMLISTING"
226	>typedef struct{
227	Uint8 type;
228	Uint8 state;
229	SDL_keysym keysym;
230	} SDL_KeyboardEvent;</PRE
231	><P
232	>The <SPAN
233	CLASS="STRUCTNAME"
234	>SDL_KeyboardEvent</SPAN
235	> describes a keyboard event (obviously). The <TT
236	CLASS="STRUCTFIELD"
237	><I
238	>key</I
239	></TT
240	> member of the <A
241	HREF="sdlevent.html"
242	><SPAN
243	CLASS="STRUCTNAME"
244	>SDL_Event</SPAN
245	></A
246	> union is a <SPAN
247	CLASS="STRUCTNAME"
248	>SDL_KeyboardEvent</SPAN
249	> structure. The <TT
250	CLASS="STRUCTFIELD"
251	><I
252	>type</I
253	></TT
254	> field specifies whether the event is a key release (<TT
255	CLASS="LITERAL"
256	>SDL_KEYUP</TT
257	>) or a key press (<TT
258	CLASS="LITERAL"
259	>SDL_KEYDOWN</TT
260	>) event. The <TT
261	CLASS="STRUCTFIELD"
262	><I
263	>state</I
264	></TT
265	> is largely redundant, it reports the same information as the <TT
266	CLASS="STRUCTFIELD"
267	><I
268	>type</I
269	></TT
270	> field but uses different values (<TT
271	CLASS="LITERAL"
272	>SDL_RELEASED</TT
273	> and <TT
274	CLASS="LITERAL"
275	>SDL_PRESSED</TT
276	>). The <TT
277	CLASS="STRUCTFIELD"
278	><I
279	>keysym</I
280	></TT
281	> contains information of the key press or release that this event represents (see above).</P
282	></DIV
283	></DIV
284	><DIV
285	CLASS="SECT2"
286	><H2
287	CLASS="SECT2"
288	><A
289	NAME="AEN324"
290	></A
291	>Reading Keyboard Events</H2
292	><P
293	>Reading keybaord events from the event queue is quite simple (the event queue and using it is described <A
294	HREF="sdlevent.html"
295	>here</A
296	>). We read events using <A
297	HREF="sdlpollevent.html"
298	><TT
299	CLASS="FUNCTION"
300	>SDL_PollEvent</TT
301	></A
302	> in a <TT
303	CLASS="LITERAL"
304	>while()</TT
305	> loop and check for <TT
306	CLASS="LITERAL"
307	>SDL_KEYUP</TT
308	> and <TT
309	CLASS="LITERAL"
310	>SDL_KEYDOWN</TT
311	> events using a <TT
312	CLASS="LITERAL"
313	>switch</TT
314	> statement, like so:</P
315	><DIV
316	CLASS="EXAMPLE"
317	><A
318	NAME="AEN334"
319	></A
320	><P
321	><B
322	>Example 3-10. Reading Keyboard Events</B
323	></P
324	><PRE
325	CLASS="PROGRAMLISTING"
326	> SDL_Event event;
327	.
328	.
329	/* Poll for events. SDL_PollEvent() returns 0 when there are no */
330	/* more events on the event queue, our while loop will exit when */
331	/* that occurs. */
332	while( SDL_PollEvent( &event ) ){
333	/* We are only worried about SDL_KEYDOWN and SDL_KEYUP events */
334	switch( event.type ){
335	case SDL_KEYDOWN:
336	printf( "Key press detected\n" );
337	break;
338
339	case SDL_KEYUP:
340	printf( "Key release detected\n" );
341	break;
342
343	default:
344	break;
345	}
346	}
347	.
348	.</PRE
349	></DIV
350	><P
351	>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
352	></DIV
353	><DIV
354	CLASS="SECT2"
355	><H2
356	CLASS="SECT2"
357	><A
358	NAME="AEN338"
359	></A
360	>A More Detailed Look</H2
361	><P
362	>Before we can read events SDL must be initialised with <A
363	HREF="sdlinit.html"
364	><TT
365	CLASS="FUNCTION"
366	>SDL_Init</TT
367	></A
368	> and a video mode must be set using <A
369	HREF="sdlsetvideomode.html"
370	><TT
371	CLASS="FUNCTION"
372	>SDL_SetVideoMode</TT
373	></A
374	>. There are, however, two other functions we must use to obtain all the information required. We must enable unicode translation by calling <TT
375	CLASS="FUNCTION"
376	>SDL_EnableUNICODE(1)</TT
377	> and we must convert <SPAN
378	CLASS="STRUCTNAME"
379	>SDLKey</SPAN
380	> values into something printable, using <A
381	HREF="sdlgetkeyname.html"
382	><TT
383	CLASS="FUNCTION"
384	>SDL_GetKeyName</TT
385	></A
386	></P
387	><DIV
388	CLASS="NOTE"
389	><BLOCKQUOTE
390	CLASS="NOTE"
391	><P
392	><B
393	>Note: </B
394	>It is useful to note that unicode values < 0x80 translate directly a characters ASCII value. THis is used in the example below</P
395	></BLOCKQUOTE
396	></DIV
397	><DIV
398	CLASS="EXAMPLE"
399	><A
400	NAME="AEN351"
401	></A
402	><P
403	><B
404	>Example 3-11. Interpreting Key Event Information</B
405	></P
406	><PRE
407	CLASS="PROGRAMLISTING"
408	> #include "SDL.h"
409
410	/* Function Prototypes */
411	void PrintKeyInfo( SDL_KeyboardEvent *key );
412	void PrintModifiers( SDLMod mod );
413
414	/* main */
415	int main( int argc, char *argv[] ){
416
417	SDL_Event event;
418	int quit = 0;
419
420	/* Initialise SDL */
421	if( SDL_Init( SDL_INIT_VIDEO ) < 0){
422	fprintf( stderr, "Could not initialise SDL: %s\n", SDL_GetError() );
423	exit( -1 );
424	}
425
426	/* Set a video mode */
427	if( !SDL_SetVideoMode( 320, 200, 0, 0 ) ){
428	fprintf( stderr, "Could not set video mode: %s\n", SDL_GetError() );
429	SDL_Quit();
430	exit( -1 );
431	}
432
433	/* Enable Unicode translation */
434	SDL_EnableUNICODE( 1 );
435
436	/* Loop until an SDL_QUIT event is found */
437	while( !quit ){
438
439	/* Poll for events */
440	while( SDL_PollEvent( &event ) ){
441
442	switch( event.type ){
443	/* Keyboard event */
444	/* Pass the event data onto PrintKeyInfo() */
445	case SDL_KEYDOWN:
446	case SDL_KEYUP:
447	PrintKeyInfo( &event.key );
448	break;
449
450	/* SDL_QUIT event (window close) */
451	case SDL_QUIT:
452	quit = 1;
453	break;
454
455	default:
456	break;
457	}
458
459	}
460
461	}
462
463	/* Clean up */
464	SDL_Quit();
465	exit( 0 );
466	}
467
468	/* Print all information about a key event */
469	void PrintKeyInfo( SDL_KeyboardEvent *key ){
470	/* Is it a release or a press? */
471	if( key->type == SDL_KEYUP )
472	printf( "Release:- " );
473	else
474	printf( "Press:- " );
475
476	/* Print the hardware scancode first */
477	printf( "Scancode: 0x%02X", key->keysym.scancode );
478	/* Print the name of the key */
479	printf( ", Name: %s", SDL_GetKeyName( key->keysym.sym ) );
480	/* We want to print the unicode info, but we need to make */
481	/* sure its a press event first (remember, release events */
482	/* don't have unicode info */
483	if( key->type == SDL_KEYDOWN ){
484	/* If the Unicode value is less than 0x80 then the */
485	/* unicode value can be used to get a printable */
486	/* representation of the key, using (char)unicode. */
487	printf(", Unicode: " );
488	if( key->keysym.unicode < 0x80 && key->keysym.unicode > 0 ){
489	printf( "%c (0x%04X)", (char)key->keysym.unicode,
490	key->keysym.unicode );
491	}
492	else{
493	printf( "? (0x%04X)", key->keysym.unicode );
494	}
495	}
496	printf( "\n" );
497	/* Print modifier info */
498	PrintModifiers( key->keysym.mod );
499	}
500
501	/* Print modifier info */
502	void PrintModifiers( SDLMod mod ){
503	printf( "Modifers: " );
504
505	/* If there are none then say so and return */
506	if( mod == KMOD_NONE ){
507	printf( "None\n" );
508	return;
509	}
510
511	/* Check for the presence of each SDLMod value */
512	/* This looks messy, but there really isn't */
513	/* a clearer way. */
514	if( mod & KMOD_NUM ) printf( "NUMLOCK " );
515	if( mod & KMOD_CAPS ) printf( "CAPSLOCK " );
516	if( mod & KMOD_LCTRL ) printf( "LCTRL " );
517	if( mod & KMOD_RCTRL ) printf( "RCTRL " );
518	if( mod & KMOD_RSHIFT ) printf( "RSHIFT " );
519	if( mod & KMOD_LSHIFT ) printf( "LSHIFT " );
520	if( mod & KMOD_RALT ) printf( "RALT " );
521	if( mod & KMOD_LALT ) printf( "LALT " );
522	if( mod & KMOD_CTRL ) printf( "CTRL " );
523	if( mod & KMOD_SHIFT ) printf( "SHIFT " );
524	if( mod & KMOD_ALT ) printf( "ALT " );
525	printf( "\n" );
526	}</PRE
527	></DIV
528	></DIV
529	><DIV
530	CLASS="SECT2"
531	><H2
532	CLASS="SECT2"
533	><A
534	NAME="AEN354"
535	></A
536	>Game-type Input</H2
537	><P
538	>I have found that people using keyboard events for games and other interactive applications don't always understand one fundemental point.</P
539	><A
540	NAME="AEN357"
541	></A
542	><BLOCKQUOTE
543	CLASS="BLOCKQUOTE"
544	><P
545	>Keyboard events <SPAN
546	CLASS="emphasis"
547	><I
548	CLASS="EMPHASIS"
549	>only</I
550	></SPAN
551	> take place when a keys state changes from being unpressed to pressed, and vice versa.</P
552	></BLOCKQUOTE
553	><P
554	>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, and when you press the down key you want him to slide down the screen. Examine the following code; it highlights an error that many people have made.
555	<PRE
556	CLASS="PROGRAMLISTING"
557	> /* Alien screen coordinates */
558	int alien_x=0, alien_y=0;
559	.
560	.
561	/* Initialise SDL and video modes and all that */
562	.
563	/* Main game loop */
564	/* Check for events */
565	while( SDL_PollEvent( &event ) ){
566	switch( event.type ){
567	/* Look for a keypress */
568	case SDL_KEYDOWN:
569	/* Check the SDLKey values and move change the coords */
570	switch( event.key.keysym.sym ){
571	case SDLK_LEFT:
572	alien_x -= 1;
573	break;
574	case SDLK_RIGHT:
575	alien_x += 1;
576	break;
577	case SDLK_UP:
578	alien_y -= 1;
579	break;
580	case SDLK_DOWN:
581	alien_y += 1;
582	break;
583	default:
584	break;
585	}
586	}
587	}
588	}
589	.
590	.</PRE
591	>
592	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
593	><P
594	>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:</P
595	><DIV
596	CLASS="EXAMPLE"
597	><A
598	NAME="AEN363"
599	></A
600	><P
601	><B
602	>Example 3-12. Proper Game Movement</B
603	></P
604	><PRE
605	CLASS="PROGRAMLISTING"
606	> /* Alien screen coordinates */
607	int alien_x=0, alien_y=0;
608	int alien_xvel=0, alien_yvel=0;
609	.
610	.
611	/* Initialise SDL and video modes and all that */
612	.
613	/* Main game loop */
614	/* Check for events */
615	while( SDL_PollEvent( &event ) ){
616	switch( event.type ){
617	/* Look for a keypress */
618	case SDL_KEYDOWN:
619	/* Check the SDLKey values and move change the coords */
620	switch( event.key.keysym.sym ){
621	case SDLK_LEFT:
622	alien_xvel = -1;
623	break;
624	case SDLK_RIGHT:
625	alien_xvel = 1;
626	break;
627	case SDLK_UP:
628	alien_yvel = -1;
629	break;
630	case SDLK_DOWN:
631	alien_yvel = 1;
632	break;
633	default:
634	break;
635	}
636	break;
637	/* We must also use the SDL_KEYUP events to zero the x */
638	/* and y velocity variables. But we must also be */
639	/* careful not to zero the velocities when we shouldn't*/
640	case SDL_KEYUP:
641	switch( event.key.keysym.sym ){
642	case SDLK_LEFT:
643	/* We check to make sure the alien is moving */
644	/* to the left. If it is then we zero the */
645	/* velocity. If the alien is moving to the */
646	/* right then the right key is still press */
647	/* so we don't tocuh the velocity */
648	if( alien_xvel < 0 )
649	alien_xvel = 0;
650	break;
651	case SDLK_RIGHT:
652	if( alien_xvel > 0 )
653	alien_xvel = 0;
654	break;
655	case SDLK_UP:
656	if( alien_yvel < 0 )
657	alien_yvel = 0;
658	break;
659	case SDLK_DOWN:
660	if( alien_yvel > 0 )
661	alien_yvel = 0;
662	break;
663	default:
664	break;
665	}
666	break;
667
668	default:
669	break;
670	}
671	}
672	.
673	.
674	/* Update the alien position */
675	alien_x += alien_xvel;
676	alien_y += alien_yvel;</PRE
677	></DIV
678	><P
679	>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
680	></DIV
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