Handling the Keyboard

Examples:

Keyboard Related Structures

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.

SDLKey

SDLKey is an enumerated type defined in SDL/include/SDL_keysym.h. Each SDLKey symbol represents a key: SDLK_a corresponds to the 'a' key on a keyboard, SDLK_SPACE corresponds to the space bar, and so on.

SDLMod

SDLMod is an enumerated type, similar to SDLKey, however it enumerates keyboard modifiers (Control, Alt, Shift). SDLMod values can be AND'd together to represent several modifiers.

SDL_keysym

typedef struct{
  Uint8 scancode;
  SDLKey sym;
  SDLMod mod;
  Uint16 unicode;
} SDL_keysym;

The SDL_keysym structure describes a key press or a key release. The scancode field is hardware specific and should be ignored unless you know what you are doing. The sym field is the SDLKey value of the key being pressed or released. The mod field describes the state of the keyboard modifiers at the time the key press or release occurred. So a value of KMOD_NUM | KMOD_CAPS | KMOD_LSHIFT would mean that Numlock, Capslock and the left shift key were all press (or enabled in the case of the lock keys). Finally, the unicode field stores the 16-bit unicode value of the key.

SDL_KeyboardEvent

typedef struct{
  Uint8 type;
  Uint8 state;
  SDL_keysym keysym;
} SDL_KeyboardEvent;

The SDL_KeyboardEvent describes a keyboard event (obviously). The key member of the SDL_Event union is a SDL_KeyboardEvent structure. The type field specifies whether the event is a key release (SDL_KEYUP) or a key press (SDL_KEYDOWN) event. The state is largely redundant, it reports the same information as the type field but uses different values (SDL_RELEASED and SDL_PRESSED). The keysym contains information of the key press or release that this event represents (see above).

Reading Keyboard Events

Reading keyboard events from the event queue is quite simple (the event queue and using it is described here). We read events using SDL_PollEvent in a while() loop and check for SDL_KEYUP and SDL_KEYDOWN events using a switch statement, like so:

Example: Reading Keyboard Events

  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;
    }
  }
  .
  .

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.

A More Detailed Look

Before we can read events SDL must be initialised with SDL_Init and a video mode must be set using SDL_SetVideoMode. There are, however, two other functions we must use to obtain all the information required. We must enable unicode translation by calling SDL_EnableUNICODE(1) and we must convert SDLKey values into something printable, using SDL_GetKeyName.

Example: Interpreting Key Event Information

    #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 ) < 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( &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( "Modifiers: " );

        /* 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" );
    }

Game-type Input

I have found that people using keyboard events for games and other interactive applications don't always understand one fundemental point.

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.

    /* 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;
                }
            }
        }
    }
    .
    .

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.

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 separate section of code to move the alien. Something like this:

Example: Proper Game Movement

    /* 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 touch 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;

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.

See Also

Handling_Joysticks

Handling_the_Keyboard (last edited 2008-04-17 08:18:30 by localhost)