TSDL_Event

Structure Definition

  TSDL_Event = record
    case UInt8 of
      SDL_NOEVENT: (type_: byte);
      SDL_ACTIVEEVENT: (active: TSDL_ActiveEvent);
      SDL_KEYDOWN, SDL_KEYUP: (key: TSDL_KeyboardEvent);
      SDL_MOUSEMOTION: (motion: TSDL_MouseMotionEvent);
      SDL_MOUSEBUTTONDOWN, SDL_MOUSEBUTTONUP: (button: TSDL_MouseButtonEvent
        );
      SDL_QUITEV: (quit: TSDL_QuitEvent);
      SDL_SYSWMEVENT: (syswm: TSDL_SysWMEvent);
  end;

Structure Data

Description

The SDL_Event union is the core to all  event handling is SDL, its probably the most important structure after TSDL_Surface. TSDL_Event is a union  of all event structures used in SDL, using it is a simple matter of knowing  which union member relates to which event type   .

Use

The TSDL_Event structure has two uses

• Reading events on the event queue

• Placing events on the event queue

Reading events from the event queue is done with either SDL_PollEvent or SDL_PeepEvents. We'll use SDL_PollEvent and step through an example.

First off, we create an empty TSDL_Event   structure.

test_event : TSDL_Event;

SDL_PollEvent removes the next event from the event queue, if there are no events on the queue it returns 0 otherwise it returns 1. We use a while loop to process each event in turn.

while ( SDL_PollEvent( @test_event ) > 0 ) do

  The SDL_PollEvent function take a pointer to an TSDL_Event structure that is to be filled  with event information. We know that if SDL_PollEvent   removes an event from the queue then the event information will be placed  in our test_event structure, but we also know that the type of event will be placed in the type member of test_event. So to handle each event type   seperately we use a switch statement.

  case test_event.type_ of

We need to know what kind of events we're looking for and the event type's of those events. So lets assume we want to detect where the user is moving the mouse pointer within our application. We look through our event types and notice that SDL_MOUSEMOTION is, more than likely, the event we're looking for. A little more research tells use that SDL_MOUSEMOTION events are handled within the SDL_MouseMotionEvent structure which is the motion member of SDL_Event. We can check for the SDL_MOUSEMOTION event type within our switch statement like so:

    SDL_MOUSEMOTION:
    begin

All we need do now is read the information out of the motion member of test_event.

      printf("We got a motion event.\n");
      printf("Current mouse position is: (%d, %d)\n", test_event.motion.x, test_event.motion.y);
    end;

    default:
      printf("Unhandled Event!\n");
    end;
  end;
end;
printf("Event queue empty.\n");

It is also possible to push events onto the event queue and so use it as a two-way communication path. Both SDL_PushEvent and SDL_PeepEvents allow you to place events onto the event queue. This is usually used to place a SDL_USEREVENT on the event queue, however you could use it to post fake input events if you wished. Creating your own events is a simple matter of choosing the event type you want, setting the type member and filling the appropriate member structure with information.

user_event : TSDL_Event;

user_event.type := SDL_USEREVENT;
user_event.user.code := 2;
user_event.user.data1 := nil;
user_event.user.data2 := nil;
SDL_PushEvent( @user_event );

See Also

SDL_PollEvent , SDL_PushEvent , SDL_PeepEvents