Kartik Thakore > SDL-2.511 > SDL::Event

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NAME ^

SDL::Event - General event structure

CATEGORY

Core, Events, Structure

SYNOPSIS ^

 use SDL::Event;  # for the event object itself
 use SDL::Events; # functions for event queue handling
 
 SDL::init(SDL_INIT_VIDEO);
 my $event = SDL::Event->new();
 
 while(1)
 {
     SDL::Events::pump_events();

     if(SDL::Events::poll_event($event))
     {
        if($event->type == SDL_MOUSEBUTTONDOWN)
        {
            # now you can handle the details
            $event->button_which;
            $event->button_button;
            $event->button_x;
            $event->button_y;
        }
        
        last if $event->type == SDL_QUIT;
     }

     # your screen drawing code will be here
 }

DESCRIPTION ^

Event handling allows your application to receive input from the user. Event handling is initalised (along with video) with a call to:

SDL::init(SDL_INIT_VIDEO);

Internally, SDL stores all the events waiting to be handled in an event queue. Using functions like SDL::Events::poll_event(), SDL::Events::peep_events and SDL::Events::wait_event() you can observe and handle waiting input events.

The key to event handling in SDL is the SDL::Event union. The event queue itself is composed of a series of SDL::Event unions, one for each waiting event. SDL::Event unions are read from the queue with the SDL::Events::poll_event() function and it is then up to the application to process the information stored with them.

METHODS ^

new

new creates an empty event-object, which can be used store information. Either by calling poll_event($event) that transferes one event from the queue into our object or by setting all the needed data manually in order to push the event to the queue.

 use SDL::Event;

 my $event = SDL::Event->new();

type

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.

 print 'heureka' if $event->type == SDL_MOUSEBUTTONDOWN;

Available type constants:

Event types are grouped by masks. SDL_EVENTMASK($type) will return the proper mask for the given type.

Available event mask constants:

This way you can check if a given type matches a mask:

 (SDL_JOYBUTTONDOWN   & SDL_MOUSEEVENTMASK) # is false
 (SDL_MOUSEBUTTONDOWN & SDL_MOUSEEVENTMASK) # is true
 (SDL_MOUSEBUTTONUP   & SDL_MOUSEEVENTMASK) # is true
 (SDL_MOUSEMOTION     & SDL_MOUSEEVENTMASK) # is true
 
 # and also true is:
 
 (SDL_MOUSEEVENTMASK == SDL_EVENTMASK(SDL_MOUSEBUTTONDOWN) 
                      | SDL_EVENTMASK(SDL_MOUSEBUTTONUP) 
                      | SDL_EVENTMASK(SDL_MOUSEMOTION))

Application visibility events

active is used when an event of type SDL_ACTIVEEVENT is reported.

When the mouse leaves or enters the window area a SDL_APPMOUSEFOCUS type activation event occurs, if the mouse entered the window then gain will be 1, otherwise gain will be 0.

A SDL_APPINPUTFOCUS type activation event occurs when the application loses or gains keyboard focus. This usually occurs when another application is made active.

Finally, a SDL_APPACTIVE type event occurs when the application is either minimised/iconified (gain=0) or restored.

A single event can have multiple values set in state.

Note: This event does not occur when an application window is first created.

A new ActiveEvent (to fake focus loss) will be created like this:

 my $event = SDL::Event->new();
    $event->type(SDL_ACTIVEEVENT);
    $event->active_gain(0);
    $event->active_state(SDL_APPMOUSEFOCUS);

 # I think this is wrong, ->active_type() should get SDL_APPMOUSEFOCUS, but what state gets?

active_gain

See active. 0 if the event is a loss or 1 if it is a gain.

active_state

A bitmask of the following values: SDL_APPMOUSEFOCUS if mouse focus was gained or lost, SDL_APPINPUTFOCUS if input focus was gained or lost, and SDL_APPACTIVE if the application was iconified (gain=0) or restored(gain=1).

Keyboard events

key is used when an event of type SDL_KEYDOWN or SDL_KEYUP is reported.

The type and state actually report the same information, they just use different values to do it. A keyboard event generally occurs when a key is released (type=SDL_KEYUP or key_state=SDL_RELEASED) and when a key is pressed (type=SDL_KEYDOWN or key_state=SDL_PRESSED).

The SDLK_CAPSLOCK and SDLK_NUMLOCK keys are special cases and report an SDL_KEYDOWN when first pressed, then an SDL_RELEASED when released and pressed again. For these keys KEYUP and KEYDOWN events are therefore analogous to the state of the caps lock and num lock LEDs rather than the keys themselves. These special cases are required for compatibility with Sun workstations.

Note: Repeating SDL_KEYDOWN events will occur if key repeat is enabled (see SDL_EnableKeyRepeat).

key_state

SDL_PRESSED or SDL_RELEASED

key_scancode

The scancode field should generally be left alone, it is the hardware-dependent scancode returned by the keyboard.

key_sym

The sym field is extremely useful. It is the SDL-defined value of the key (see the keysym definitions in SDLKey). This field is very useful when you are checking for certain key presses, like so:

 while(poll_event($event))
 {
     switch($event->type)
     {
         case SDL_KEYDOWN:
             move_left() if($event->key_sym == SDLK_LEFT);
             break;
         .
         .
         .
     }
 }

key_mod

mod stores the current state of the keyboard modifiers as explained in SDL_GetModState.

key_unicode

The unicode field is only used when UNICODE translation is enabled with SDL_EnableUNICODE. If unicode is non-zero then this is the UNICODE character corresponding to the keypress. If the high 9 bits of the character are 0, then this maps to the equivalent ASCII character:

 my $char;
 if(($event->key_unicode & 0xFF80) == 0)
 {
     $char = $event->key_unicode & 0x7F;
 }
 else
 {
     print("An International Character.\n");
 }

UNICODE translation does create a slight overhead so don't enable it unless its needed.

NOTE: Key release events (SDL_KEYUP) won't necessarily (ever?) contain unicode information. See http://lists.libsdl.org/pipermail/sdl-libsdl.org/2005-January/048355.html

Mouse motion events

Simply put, a SDL_MOUSEMOTION type event occurs when a user moves the mouse within the application window or when SDL_WarpMouse is called. Both the absolute (motion_x and motion_y) and relative (motion_xrel and motion_yrel) coordinates are reported along with the current button states (motion_state).

motion_state

The button state can be interpreted using the SDL_BUTTON macro (see SDL_GetMouseState).

motion_x, motion_y

The X/Y coordinates of the mouse

motion_xrel, motion_yrel

Relative motion in the X/Y direction.

If the cursor is hidden (SDL_ShowCursor(0)) and the input is grabbed (SDL_WM_GrabInput(SDL_GRAB_ON)), then the mouse will give relative motion events even when the cursor reaches the edge of the screen. This is currently only implemented on Windows and Linux/Unix-alikes.

Mouse button events

When a mouse button press or release is detected the number of the button pressed (from 1 to 255, with 1 usually being the left button and 2 the right) is placed into button_button, the position of the mouse when this event occured is stored in the button_x and the button_y fields. Like SDL_KeyboardEvent, information on whether the event was a press or a release event is stored in both the button_type and button_state fields, but this should be obvious.

Mouse wheel events are reported as buttons 4 (up) and 5 (down). Two events are generated i.e. you get a SDL_MOUSEBUTTONDOWN followed by a SDL_MOUSEBUTTONUP event.

button_which

The input device index

button_button

The mouse button index (SDL_BUTTON_LEFT, SDL_BUTTON_MIDDLE, SDL_BUTTON_RIGHT, SDL_BUTTON_WHEELUP, SDL_BUTTON_WHEELDOWN)

button_state

SDL_PRESSED or SDL_RELEASED

button_x, button_y

The X/Y coordinates of the mouse at press/release time

Joystick axis events

A SDL_JOYAXISMOTION event occurs whenever a user moves an axis on the joystick.

jaxis_which

The field jaxis_which is the index of the joystick that reported the event.

jaxis_axis

The jaxis_axis is the index of the axis (for a more detailed explaination see the Joystick section).

jaxis_value

jaxis_value is the current position of the axis (range: -32768 to 32767).

Joystick button events

A SDL_JOYBUTTONDOWN or SDL_JOYBUTTONUP event occurs when ever a user presses or releases a button on a joystick.

jbutton_which

The field jbutton_which is the index of the joystick that reported the event.

jbutton_button

The jbutton_button is the index of the button (for a more detailed explanation see the Joystick section).

jbutton_state

jbutton_state is the current state of the button which is either jbutton_SDL_PRESSED or jbutton_SDL_RELEASED.

Joystick hat events

A SDL_JOYHATMOTION event occurs when ever a user moves a hat on the joystick.

jhat_which

The field jhat_which is the index of the joystick that reported the event.

jhat_hat

jhat_hat is the index of the hat (for a more detailed explanation see the Joystick section).

jhat_value

jhat_value is the current position of the hat. It is a bitwise OR'd combination of the following values (whose meanings should be pretty obvious):

The following defines are also provided:

Joystrick trackball events

A SDL_JOYBALLMOTION event occurs when a user moves a trackball on the joystick.

jball_which

The field jball_which is the index of the joystick that reported the event.

jball_ball

jball_ball is the index of the trackball (for a more detailed explanation see the Joystick section).

jball_xrel, jball_yrel

Trackballs only return relative motion, this is the change in position on the ball since it was last polled (last cycle of the event loop) and it is stored in jball_xrel and jball_yrel.

Window resize events

resize_w, resize_h

When SDL_RESIZABLE is passed as a flag to SDL_SetVideoMode the user is allowed to resize the applications window. When the window is resized an SDL_VIDEORESIZE is reported, with the new window width and height values stored in the resize structure's resize_w and resize_h. When an SDL_VIDEORESIZE is received the window should be resized to the new dimensions using SDL_SetVideoMode.

Window expose events

A VIDEOEXPOSE event is triggered when the screen has been modified outside of the application, usually by the window manager and needs to be redrawn.

System window manager events

The system window manager event contains a system-specific information about unknown window manager events. If you enable this event using SDL_EventState, it will be generated whenever unhandled events are received from the window manager. This can be used, for example, to implement cut-and-paste in your application.

If you want to obtain system-specific information about the window manager, you can fill in the version member of a SDL_SysWMinfo structure (details can be found in SDL_syswm.h, which must be included) using the SDL_VERSION() macro found in SDL_version.h, and pass it to the function:

 int SDL_GetWMInfo(SDL_SysWMinfo *info);

See http://www.libsdl.org/cgi/docwiki.cgi/SDL_SysWMEvent

syswm_msg

User defined events

This event is unique, it is never created by SDL but only by the user. The event can be pushed onto the event queue using SDL::Events::push_event. The contents of the structure members are completely up to the programmer, the only requirement is that type is a value from SDL_USEREVENT to SDL_NUMEVENTS-1 (inclusive)

 my $event = SDL::Event->new();
    $event->type ( SDL_USEREVENT + 3 );
    $event->event_code(10);
    $event->data1('hello event');

 SDL::Events::push_event($event);

user_code

User defined event code (integer).

user_data1, user_data2

User defined data.

Quit event

As can be seen, the SDL_QuitEvent structure serves no useful purpose. The event itself, on the other hand, is very important. If you filter out or ignore a quit event then it is impossible for the user to close the window. On the other hand, if you do accept a quit event then the application window will be closed, and screen updates will still report success even though the application will no longer be visible.

Note: The macro SDL_QuitRequested will return non-zero if a quit event is pending

AUTHOR ^

SEE ALSO ^

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