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SDL_Event

NAME
STRUCTURE DEFINITION
STRUCTURE DATA
DESCRIPTION
USE
SEE ALSO

NAME

SDL_Event − General event structure

STRUCTURE DEFINITION

typedef union{
  Uint8 type;
  SDL_ActiveEvent active;
  SDL_KeyboardEvent key;
  SDL_MouseMotionEvent motion;
  SDL_MouseButtonEvent button;
  SDL_JoyAxisEvent jaxis;
  SDL_JoyBallEvent jball;
  SDL_JoyHatEvent jhat;
  SDL_JoyButtonEvent jbutton;
  SDL_ResizeEvent resize;
  SDL_ExposeEvent expose;
  SDL_QuitEvent quit;
  SDL_UserEvent user;
  SDL_SysWMEvent syswm;
} SDL_Event;

STRUCTURE DATA

	type		The type of event
	active		Activation event
	key		Keyboard event
	motion		Mouse motion event
	button		Mouse button event
	jaxis		Joystick axis motion event
	jball		Joystick trackball motion event
	jhat		Joystick hat motion event
	jbutton		Joystick button event
	resize		Application window resize event
	expose		Application window expose event
	quit		Application quit request event
	user		User defined event
	syswm		Undefined window manager event

DESCRIPTION

The SDL_Event union is the core to all event handling is SDL, its probably the most important structure after SDL_Surface. SDL_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.

	Event type		Event Structure
	SDL_ACTIVEEVENT		SDL_ActiveEvent
	SDL_KEYDOWN/UP		SDL_KeyboardEvent
	SDL_MOUSEMOTION		SDL_MouseMotionEvent

SDL_MOUSEBUTTONDOWN/UP

SDL_MouseButtonEvent

	SDL_JOYAXISMOTION		SDL_JoyAxisEvent
	SDL_JOYBALLMOTION		SDL_JoyBallEvent
	SDL_JOYHATMOTION		SDL_JoyHatEvent

SDL_JOYBUTTONDOWN/UP

SDL_JoyButtonEvent

	SDL_QUIT		SDL_QuitEvent
	SDL_SYSWMEVENT		SDL_SysWMEvent
	SDL_VIDEORESIZE		SDL_ResizeEvent
	SDL_VIDEOEXPOSE		SDL_ExposeEvent
	SDL_USEREVENT		SDL_UserEvent

USE

The SDL_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 SDL_Event structure.

SDL_Event test_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)) {

The SDL_PollEvent function take a pointer to an SDL_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.

switch(test_event.type) {

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:

case SDL_MOUSEMOTION:

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

      printf("We got a motion event.
");
      printf("Current mouse position is: (%d, %d)
", test_event.motion.x, test_event.motion.y);
      break;
    default:
      printf("Unhandled Event!
");
      break;
  }
}
printf("Event queue empty.
");

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.

SDL_Event user_event;

user_event.type=SDL_USEREVENT;
user_event.user.code=2;
user_event.user.data1=NULL;
user_event.user.data2=NULL;
SDL_PushEvent(&user_event);

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