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SDL_Event(3) SDL API Reference SDL_Event(3)
SDL_Event - General event structure
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
The SDL_Event union is the core to all event handling is SDL, its probably the most impor-
tant 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
Event type Event Structure
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_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
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.
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_MOUSE-
MOTION 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:
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);
printf("Event queue empty.
It is also possible to push events onto the event queue and so use it as a two-way commu-
nication 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 appro-
priate member structure with information.
SDL_PollEvent, SDL_PushEvent, SDL_PeepEvents
SDL Tue 11 Sep 2001, 22:59 SDL_Event(3)
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