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Tutorial: Accessing X window system from GNUstep, part I

Posted on 1 September 2006 by Yen-Ju Chen

While GNUstep is a portable development environment, some applications do need to access the underneath X window system. This tutorial illustrates a simple way to do so.

Note: the codes may not be executable. It is only used as demostration

First, application delegate needs to register itself for X window event:

- (void) applicationWillFinishLaunching:(NSNotification *)aNotification
{
  Display *dpy = (Display *)[GSCurrentServer() serverDevice];
  int screen = [[NSScreen mainScreen] screenNumber];
  Window root_win = RootWindow(dpy, screen);
/ Listen event /
NSRunLoop     *loop = [NSRunLoop currentRunLoop];
int xEventQueueFd = XConnectionNumber(dpy);
[loop addEvent: (void*)(gsaddr)xEventQueueFd
type: ET_RDESC
watcher: (id)self
forMode: NSDefaultRunLoopMode];
}

Then application delegate can listen to the event:

- (void)receivedEvent:(void *)data
                 type:(RunLoopEventType)type
                extra:(void *)extra
              forMode:(NSString *)mode
{
  XEvent event;
while (XPending(dpy))
{
XNextEvent (dpy, &event);
/ Intercept event here /
switch (event.type) {
case Expose:
case DestroyNotify:
case PropertyNotify:
case FocusIn:
default:
/ Go back to GNUstep /
[server processEvent: &event];
}
}
}

After [NSApp run], X window events will go into -receivedEvent:type:extra:forMode: and application delegate can make use of it.

If applications need listen to root window, use XSelectInput() in the end of -applicationWillFinishLaunching:. For example:

  /* Listen to root window for window closing and opening */
  XSelectInput(dpy, root_win, PropertyChangeMask);

By default, applications only receive events acting on their windows. If they listen to other windows, such as root window, do not pass the events belonging to other windows into applications ([server processEvent: &event]). Otherwise, it will behaves weird.