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

", 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

" , major , minor );

  _display = XkbOpenDisplay(displayName, &eventCode, &errorReturn,

                            &major, &minor, &reasonReturn);

  g_printf("Xkb server lib ver: %d.%d

" , major , minor );



  if (reasonReturn != XkbOD_Success)

  {

    g_printf("Unable to open display!

");

    return 1;

  }



  XkbDescRec* kbdDescPtr = XkbAllocKeyboard();

  if (kbdDescPtr == NULL)

  {

    g_printf ("Failed to get keyboard description.

");

    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

", 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