Detect modifier key release in X11 root window - x11

Question

The overall goal is like Windows' Alt-Tab, so I will use these keys for explanation. I want:
Press Alt -> press Tab -> [ popup appears ] -> press Tab any time holding Alt -> release Alt -> [ popup disappears ].
I can't detect final Alt release.
Trivial approach: grabbing Alt-Tab:
XGrabKey (dpy,
XKeysymToKeycode(dpy,XK_Tab), Mod1Mask,
root, True, GrabModeAsync, GrabModeAsync);
(full code: http://pastebin.com/K2P65KJn)
Result:
[ Alt pressed ]
[ Tab pressed ]
Pressing Alt-Tab reported
[ Tab released ]
Releasing Alt-Tab reported
[ Alt released ] -> nothing reported
Grabbing both Alt-Tab and Any-Alt:
XGrabKey (dpy,
XKeysymToKeycode(dpy,XK_Tab), Mod1Mask,
root, True, GrabModeAsync, GrabModeAsync);
XGrabKey (dpy,
XKeysymToKeycode(dpy,XK_Alt_L), AnyModifier,
root, True, GrabModeAsync, GrabModeAsync);
(full code: http://pastebin.com/75mD1tjA)
It works!
[ Alt pressed ]
Pressing Alt reported
[ Tab pressed ]
Pressing Alt-Tab reported
[ Tab released ]
Releasing Alt-Tab reported
[ Alt released ]
Releasing Alt-Alt reported
But this hides any Alt combination from any running program.
I can't find the way to push back events which don't belong to us (tried XSendEvent), and overally grabbing Alt from the start looks too invasive.
Grabbing Alt after first Alt-Tab press, then ungrab Alt after its release.
Unfortunately, first Alt release is still not reported:
[ Alt pressed ]
[ Tab pressed ]
Pressing Alt-Tab reported, Alt grabbed here
[ Tab released ]
Releasing Alt-Tab reported
[ Alt released ] -> nothing reported! subsequent Alt press/release are reported, though not useful:
[ Alt pressed ]
Pressing Alt reported
...
Do I need to mess with low-level xinput or there is another way to achieve the goal?

Answer 1

It looks like you won't get a KeyRelease event if you have registered interest in it after the key was pressed.
I can think of two different ways around this.
Select KeyReleaseMask for all windows (and keep track of appearing and disappearing windows); or
Once you know Alt is pressed, poll the keyboard state with XQueryKeyboard every 0.1 second or so until it's released.
I have tested the first method and it seems to be working:
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <stdbool.h>
#include <stdio.h>
void dowin (Display* dpy, Window win, int reg)
{
Window root, parent;
Window* children;
int nchildren, i;
XSelectInput (dpy, win, reg ? KeyReleaseMask|SubstructureNotifyMask : 0);
XQueryTree (dpy, win, &root, &parent, &children, &nchildren);
for (i = 0; i < nchildren; ++i)
{
dowin (dpy, children[i], reg);
}
XFree(children);
}
int main()
{
Display* dpy = XOpenDisplay(0);
Window win = DefaultRootWindow(dpy);
XEvent ev;
unsigned int alt_modmask = Mod1Mask;
unsigned int ignored_modmask = 0; // stub
KeyCode tab_keycode = XKeysymToKeycode(dpy,XK_Tab);
KeyCode alt_keycode = XKeysymToKeycode(dpy,XK_Alt_L);
dowin (dpy, win, True);
XGrabKey (dpy,
tab_keycode,
alt_modmask | ignored_modmask,
win,
True,
GrabModeAsync, GrabModeAsync);
while(true)
{
ev.xkey.keycode = 0;
ev.xkey.state = 0;
ev.xkey.type = 0;
XNextEvent(dpy, &ev);
switch(ev.type)
{
case KeyPress:
printf ("Press %x: d-%d\n", ev.xkey.window, ev.xkey.state, ev.xkey.keycode);
break;
case KeyRelease:
printf ("Release %x: %d-%d\n", ev.xkey.window, ev.xkey.state, ev.xkey.keycode);
break;
case MapNotify:
printf ("Mapped %x\n", ev.xmap.window);
dowin (dpy, ev.xmap.window, True);
break;
case UnmapNotify:
printf ("Unmapped %x\n", ev.xunmap.window);
dowin (dpy, ev.xunmap.window, False);
break;
default:
printf ("Event type %d\n", ev.type);
break;
}
}
XCloseDisplay(dpy);
return 0;
}