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SDL_Event

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);
 

SEE ALSO

SDL_PollEvent, SDL_PushEvent, SDL_PeepEvents