SDL_Event(3) SDL API Reference SDL_Event(3)

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

+o Reading events on the event queue

+o 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);

SEE ALSO

SDL_PollEvent, SDL_PushEvent, SDL_PeepEvents

SDL Tue 11 Sep 2001, 22:59 SDL_Event(3)