Event Examples

Filtering and Handling Events

uses
   SDL;

// This function may run in a separate event thread
function FilterEvents( const event : PSDL_Event ) : integer;
var
  boycott : integer;
begin
    boycott := 1;

    // This quit event signals the closing of the window
    if ( ( event.type_ = SDL_QUITEV ) and boycott ) then
    begin
      MessageBox( 0, 'Quit Event filtered out -- try again', 'Information', MB_OK or MB_ICONHAND );
      boycott := 0;
      result := 0;
      exit;
    end;
  
    if ( event.type_ = SDL_MOUSEMOTION ) then
    begin
      MessageBox( 0, PChar( Format( 'Mouse moved to ( x : %d, y : %d )', [ event.motion.x, event.motion.y ] ) ), 'Information', MB_OK or MB_ICONHAND );
      result := 0;    // Drop it, we've handled it 
      exit;
    end;
    result := 1;
end;

var
  event : TSDL_Event;
  keys : PKeyStateArr;
begin

    // Initialize the SDL library (starts the event loop) 
    if ( SDL_Init( SDL_INIT_VIDEO ) < 0 ) then
    begin
      MessageBox(0, PChar( Format( 'Couldn''t initialize SDL : %s', [SDL_GetError] ) ), 'Error', MB_OK or MB_ICONHAND );
      SQL_Quit;
      halt(1);
    end;

    // Ignore key events
    SDL_EventState( SDL_KEYDOWN, SDL_IGNORE );
    SDL_EventState( SDL_KEYUP, SDL_IGNORE );

    // Filter quit and mouse motion events
    SDL_SetEventFilter( FilterEvents );

    // The mouse isn't much use unless we have a display for reference
    if ( SDL_SetVideoMode(640, 480, 8, 0) = nil ) then
    begin
      MessageBox(0, PChar( Format( 'Couldn't set 640x480x8 video mode : %s', [SDL_GetError])), 'Error', MB_OK or MB_ICONHAND);
      halt(1);
    end;

    // Loop waiting for ESC+Mouse_Button 
    while ( SDL_WaitEvent( @event ) >= 0 ) do
    begin
        case event.type_ of
            SDL_ACTIVEEVENT: 
            begin
                if ( event.active.state and SDL_APPACTIVE ) then
                begin
                    if ( event.active.gain ) then
                    begin
                      MessageBox( 0, 'App Activated', 'Information', MB_OK or MB_ICONHAND );
                    end 
                    else 
                    begin
                      MessageBox( 0, 'App Iconified', 'Information', MB_OK or MB_ICONHAND );
                    end;
                end;
            end;
                    
            SDL_MOUSEBUTTONDOWN: 
            begin
                keys := PKeyStrateArr( SDL_GetKeyState( nil ) );
                if ( keys[SDLK_ESCAPE] = SDL_PRESSED ) then
                begin
                    MessageBox( 0, 'Bye bye...', 'Information', MB_OK or MB_ICONHAND );S
                    halt(0);
                end;
                MessageBox( 0, 'Moused Button Pressed', 'Information', MB_OK or MB_ICONHAND );
            end;

            SDL_QUITEV: 
            begin
                MessageBox( 0, 'Quit Requested, quitting.', 'Information', MB_OK or MB_ICONHAND );
                halt(0);
            end;
        end;
    end;
    // This should never happen
    MessageBox( 0, PChar( Format( 'SDL_WaitEvent error : %s', [SDL_GetError] ) ), 'Error', MB_OK or MB_ICONHAND ); 
    halt(1);
end;