How to add a keyboard modifier state applet to Unity panel?

This is an outstanding issue in Unity:

• lp#773078 Should display the StickyKeys status in some way (a11y)
• lp#1306584 No keyboard state applet in Unity (Thanks to @dotancohen)

The code below has been updated, now it can use icons to show state. But it may get slow sometime as I has to update icon file on hard drive then reload it again. (See notes about this issue/limitation in libappindicator)

A well packaged release was made available on webupd8 ppa (Thanks goes to Alin Andrei /Andrew/)

sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install indicator-xkbmod

Reference: Keyboard Modifiers State indicator For Ubuntu: Xkbmod Indicator

Original Answer:

This is not attended to be a canonical answer to the question. It could be counted as a work around. Hopping someone writes sophisticated solution for it.

This a simple prototype keyboard modifiers indicator for Unity.

Image starting from left: Icon, Shift, Locked Caps, Ctrl, Alt, Super, Locked AltGr (Small circle to indicate locked state)

Source file unity-xkbmod.c:

/*
 * unity-xkbmod.c
 *
 * Copyright 2014 Sneetsher <sneetsher@localhost>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02110-1301, USA.
 *
 *
 */

#include <string.h>

#include <X11/XKBlib.h>

#include <glib/gprintf.h>
#include <gtk/gtk.h>
#include <libappindicator/app-indicator.h>

//callback data structure
typedef struct _AppData {
  Display *_display;
  int *_deviceId;
  AppIndicator *indicator;
} AppData;

//menu ui
static GtkActionEntry entries[] = {
  { "Quit",     "application-exit", "_Quit", "<control>Q",
    "Exit the application", G_CALLBACK (gtk_main_quit) },
};

static guint n_entries = G_N_ELEMENTS (entries);

static const gchar *ui_info =
"<ui>"
"  <popup name='IndicatorPopup'>"
"    <menuitem action='Quit' />"
"  </popup>"
"</ui>";

//callback function, get xkb state, update indicator label (icon have limitation)
static gboolean update_xkb_state (gpointer data)
{
  //get xkb state
  XkbStateRec xkbState;
  XkbGetState(((AppData*) data)->_display, *(((AppData*) data)->_deviceId), &xkbState);

  //construct label
  GString *label = g_string_new("");

  register int i;
  unsigned bit;

  //loop taken from xkbwatch source
  for (i = XkbNumModifiers - 1, bit = 0x80; i >= 0; i--, bit >>= 1)
  {
    //printf("base%d %s  ", i, (xkbState.base_mods & bit) ? "on " : "off");
    //printf("latched%d %s  ", i, (xkbState.latched_mods & bit) ? "on " : "off");
    //printf("locked%d %s  ", i, (xkbState.locked_mods & bit) ? "on " : "off");
    //printf("effective%d %s  ", i, (xkbState.mods & bit) ? "on " : "off");
    //printf("compat%d %s\n", i, (xkbState.compat_state & bit) ? "on " : "off");

    //todo: change constant with xkb modifier constant (defined in the headers)
    // show effective modifier stat
    switch (i)
    {
      case 7:
        g_string_prepend (label,  ((xkbState.mods & bit) ? "⎇" : ""));
        break;
      case 6:
        g_string_prepend (label,  ((xkbState.mods & bit) ? "⌘" : ""));
        break;
      case 5:
        g_string_prepend (label,  ((xkbState.mods & bit) ? "5" : ""));
        break;
      case 4:
        g_string_prepend (label,  ((xkbState.mods & bit) ? "①" : ""));
        break;
      case 3:
        g_string_prepend (label,  ((xkbState.mods & bit) ? "⌥" : ""));
        break;
      case 2:
        g_string_prepend (label,  ((xkbState.mods & bit) ? "⋀" : ""));
        break;
      case 1:
        g_string_prepend (label,  ((xkbState.mods & bit) ? "⇬" : ""));
        break;
      case 0:
        g_string_prepend (label,  ((xkbState.mods & bit) ? "⇧" : ""));
        break;
      default:
        break;
    };

    // show if modifier is locked
    g_string_prepend (label,  ((xkbState.locked_mods & bit) ? " ˳" : " "));
  }

  //g_string_prepend (label,  "");
  app_indicator_set_label (((AppData*) data)->indicator, label->str, NULL);

  //g_free(label);
  return TRUE;
}


int main (int argc, char **argv)
{
  AppData appdata;
  Display *_display;
  int _deviceId;

  char* displayName = strdup("");
  int eventCode;
  int errorReturn;
  int major = XkbMajorVersion;
  int minor = XkbMinorVersion;;
  int reasonReturn;


  AppIndicator *indicator;
  GtkWidget *indicator_menu;
  GtkUIManager *uim;
  GtkActionGroup *action_group;
  GError *error = NULL;

  gtk_init (&argc, &argv);


  XkbIgnoreExtension(False);

  g_printf("Xkb client lib ver: %d.%d\n" , major , minor );
  _display = XkbOpenDisplay(displayName, &eventCode, &errorReturn,
                            &major, &minor, &reasonReturn);
  g_printf("Xkb server lib ver: %d.%d\n" , major , minor );

  if (reasonReturn != XkbOD_Success)
  {
    g_printf("Unable to open display!\n");
    return 1;
  }

  XkbDescRec* kbdDescPtr = XkbAllocKeyboard();
  if (kbdDescPtr == NULL)
  {
    g_printf ("Failed to get keyboard description.\n");
    return 2;
  }
  kbdDescPtr->dpy = _display;
  _deviceId = kbdDescPtr->device_spec;

  /*
  //no need for event listener, used gtk_timeout timer
  XkbSelectEventDetails(_display, XkbUseCoreKbd, XkbStateNotify,
                     XkbAllStateComponentsMask, XkbGroupStateMask);
  */


  action_group = gtk_action_group_new ("AppActions");
  gtk_action_group_add_actions (action_group, entries, n_entries, NULL);

  indicator = app_indicator_new_with_path (
                                        "Simple XKB Modifier Indicator",
                                        "icon",
                                        APP_INDICATOR_CATEGORY_HARDWARE,
                                        g_get_current_dir());

  uim = gtk_ui_manager_new ();
  gtk_ui_manager_insert_action_group (uim, action_group, 0);
  if (!gtk_ui_manager_add_ui_from_string (uim, ui_info, -1, &error))
  {
    g_printf ("Failed to build menus: %s\n", error->message);
    g_error_free (error);
    error = NULL;
    return 3;
  }

  indicator_menu = gtk_ui_manager_get_widget (uim, "/ui/IndicatorPopup");
  app_indicator_set_menu (indicator, GTK_MENU (indicator_menu));
  app_indicator_set_status (indicator, APP_INDICATOR_STATUS_ACTIVE);

  //app_indicator_set_label (indicator, " ⇧ ⋀ ⌥ ⎇  ⌘ ", NULL);
  //symbols: shift U21E7 ctrl U22C0 alt/altgr U2325 U2387  cmd U2318
  //from font: DejaVu Sans

  appdata._display = _display;
  appdata._deviceId = &_deviceId;
  appdata.indicator = indicator;
  gtk_timeout_add (120, (GtkFunction) update_xkb_state, &appdata);
  //nice for realtime tasks, to replace gtk_timeout
  //gtk_idle_add ((GtkFunction) idle_func, &appdata);

  gtk_main ();

  XCloseDisplay (_display);
  return 0;
}

1. Installing needed headers/libs: (Not sure if I miss any)

   sudo apt-get install libx11-dev libappindicator-dev libgtk2.0-dev
   

2. Compiling:

   gcc -Wall unity-xkbmod.c -o unity-xkbmod `pkg-config --cflags --libs appindicator-0.1` -lX11
   

3. Run:

   ./unity-xkbmod
   

Note:

• libappindicator used for Unity indicators lack an important feature which make easy to ports other desktops indicators. See Bug #812067 API needed: pixbuf icon setting support

  Without this feature, let's say we need (Shift, Ctrl, Alt, AltGr, Super) with sticky keys active; we have 3 main status for each (Off, On/Latched, Locked). So 3^5 combined icons should be generated. (Where normal case just 3x5 single icons)

  That's why I used indicator label with symbols from DejaVu Sans font.

• To put an icon, place it in same folder and name it icon.*. Accepted formats: png, svg, ico, xpm ...

  If you don't like any icon, just create a 1x1 px image instead.

References:

• The X Keyboard Extension
• Application Panel Indicators
• Source of xkbwatch & plasma-widget-kbstate

Another solution which is not perfect, but some may find it useful as it is possible to have full functionality as in KDE like activating a mod with a click.

1. install kbstate applet

   sudo apt-get install plasma-widget-kbstate kde-workspace-bin
   

2. Run it in plasma-windowed player

   • Regular window

     plasma-windowed kbstate
     

     

   • Borderless window

     plasma-windowed --noborder kbstate
     

     

I didn't have much time to play with, but wmctrl may help to position, resize and make it on top at launch.

Reference: What command to launch a kde plasmoid and the kickstart menu

I did a search for Ubuntu sticky keys monitor and found something which may be helpful here: http://code.google.com/p/key-mon/

Try running

key-mon --sticky for Support for sticky keys.

Reference: http://code.google.com/p/key-mon/

Note that the version available through the software center is 1.6-0ubuntu1. Released June of 2011 which does not support the --sticky switch. If the indicator looks exactly like the above you have the older version Try the latest version at http://code.google.com/p/key-mon/ as of this writing it is keymon_1.17-1_all.deb 229 KB Released Jan 3 2014. Support for --sticky switch tested and confirmed.