Creating Enlightenment Gadgets

The code blocks in this guide are not intended to be compiled separately or work individually. They are meant to be used as a visual reference of the changes that need to be made to make an EFL application a gadget. The final code block, along with the accompanying .desktop file and meson.build at the end of this page are the intended files to use if you would like to compile and install this example as a gadget.

Gadgets are standalone applications that Enlightenment can use as extensions to its desktop. They are similar to modules in that they can be placed in containers, called gadget sites; they differ in that they are not internal to Enlightenment and do not run in the same process.

This process separation occurs through Enlightenment placing gadgets into a sandbox. This sandbox is a gadget visual that manages the necessary gadget requirements, in place of the application handling this. Sandboxing is beneficial to the user desktop experience as it encourages the creation of gadgets through a much easier development path while also protecting the user desktop experience from being interrupted by faulty extensions.

This guide demonstrates how to develop these gadgets.

Prerequisites

Gadget Advantages and Limitations

Creating gadgets directly, rather than as modules, reduces the need to call upon additional application programming interfaces (APIs). Gadgets are also more robust: as gadgets run in a separate process from Enlightenment they will not crash the desktop when encountering an error, instead gracefully recovering from their own crashes.

There are, however, a few limitations for gadgets. It is not recommended to use forced sizes or size hints in the gadget application. The sandbox will manage sizing for the application; setting minimum or maximum sizes, resizing or moving objects will clash with how the sandbox is managing sizes. Simply setting the desired aspect for the application will tell the sandbox how it needs to prioritize the size of the application.

Running additional applications from within the gadget application will also bring unintended results. The gadget lives in a sandbox and is bound by the size and space that the sandbox provides. If the gadget tries to open further windows or applications on its own these windows and applications will be restricted to the gadget visual. Fortunately, the sandbox provides smart callbacks than can be called to run external applications and also provides a method for opening further windows or popups within the gadget application.

Gadget Basics

Enlightenment populates its list of gadgets by looking for .desktop files installed to the directory enlightenment/gadgets under Enlightenment's library directory, which can be found using pkg-config --variable=libdir enlightenment. This gadget list is then made available for use through each gadget site's "Add Gadgets" popup. These popups display either a live view of the gadget or a still image.

When Enlightenment executes the gadget application it sets the environment variable E_GADGET_ID. This environmental variable is how the gadget application will determine if it is being run as a gadget, if it is being displayed in the "Add Gadget" popup or if it has been added to a gadget site and has a unique gadget ID. The environment variable E_GADGET_ORIENTATION is also set. It will be equal to one of three values. Horizontal Vertical or None and will indicate whether the gadget is being placed in a container that will layout its children horizontally, vertically, or neither. For the purpose of this example, this environment variable is not used.

To demonstrate this, a modified version of the Hello World app referenced in the Prerequisites section of this guide is used. Note the following important change: in the original version the window is created using elm_win_util_standard_add(), but in order to achieve transparency on the window for use as a gadget this is changed to elm_win_add() in this guide.

In the elm_main() function the application can check for the E_GADGET_ID with the following code:

EAPI_MAIN int
elm_main(int argc, char **argv)
{
   Evas_Object *win, *btn, *icon;
   int gadget =0, id_num = 0;
   char buf[16];
 
   elm_policy_set(ELM_POLICY_QUIT, ELM_POLICY_QUIT_LAST_WINDOW_CLOSED);
 
   if (getenv("E_GADGET_ID"))
     {
        gadget = 1;
        snprintf(buf, sizeof(buf), "%s", getenv("E_GADGET_ID"));
        id_num = atoi(buf);
     }
 
   win = elm_win_add(NULL, "Main", ELM_WIN_BASIC);
   elm_win_title_set(win, "Hello, World!");
   elm_win_autodel_set(win, EINA_TRUE);
 
   if (gadget) elm_win_alpha_set(win, EINA_TRUE);
 
   if (gadget && id_num == -1)
     {
        icon = elm_icon_add(win);
        elm_icon_standard_set(icon, "start-here");
        evas_object_size_hint_weight_set(icon, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
        evas_object_size_hint_align_set(icon, EVAS_HINT_FILL, EVAS_HINT_FILL);
        elm_win_resize_object_add(win, icon);
        evas_object_show(icon);
     }
   else
     {
        btn = elm_button_add(win);
        elm_object_text_set(btn, "Goodbye Cruel World");
        elm_win_resize_object_add(win, btn);
        evas_object_smart_callback_add(btn, "clicked", on_click, win);
        evas_object_show(btn);
     }
   evas_object_show(win);
 
   elm_run();
 
   return 0;
}

The variable gadget is used to determine if the application is running as a gadget or as a standalone application. The variable id_num is used to identify the ID of the gadget, if the application is indeed running as a gadget. If the application is running as a gadget, the window is set to alpha so that the background of the window does not show up. This is for aesthetics purposes so that gadgets on gadget sites don't all have a background.

If the id of the gadget is -1 the application is being run in the "Add Gadget" popup. In this case the application displays an icon. If the ID of the gadget is not -1 the application is being run as a gadget in a gadget site and the ID will be unique to this gadget. This is important for complex gadgets where the application would like to save settings or configurations and have them loaded after restarts. This unique ID is how the application will identify which settings to load.

Gadget Events and Smart Callbacks

Gadget applications will receive the following smart callbacks on their main window using evas_object_smart_callback_add(application_window, smart_callback_type, function, data):

  • gadget_action - There was an error with the gadget action.
  • gadget_action_deleted - The gadget action has been deleted.
  • gadget_action_end - The gadget action has ended.
  • gadget_configure - The configure menu item of the gadget has been clicked.
  • gadget_site_anchor - The position of the gadget has changed.
  • gadget_site_gravity - The gravity of the gadget has changed.
  • gadget_site_orient - The orientation of the gadget has changed.

Gadget applications can call the following smart callbacks on their main window using evas_object_smart_callback_call(application_window, smart_callback_type, data):

  • gadget_action_request - This requests that the gadget sets up an action. The data on this call should be the action name.
  • gadget_open_uri - This requests the gadget to open an application that handles a uri. The data on this call should be the uri.

You can call actions you set up via the evas_object_smart_callback_call(application_window, action, action_data) function.

For more advanced purposes beyond the scope of this gadget introduction, it is worth noting that you can set an ecore event handler for ECORE_EVENT_SIGNAL_USER and the handler will be called when the user requests the gadget be removed. Inside of the handler, the user number will be equal to 2 if the user requested the gadget be removed.

For the purpose of the Hello World gadget, a label detailing the anchor position and orientation of the gadget can be displayed using the following code:

#include <Elementary.h>
#include <e_gadget_types.h>
 
static E_Gadget_Site_Orient gorient;
static E_Gadget_Site_Anchor ganchor;
 
static void
update_anchor_orient(void *data, E_Gadget_Site_Orient orient, E_Gadget_Site_Anchor anchor)
{
   char buf[4096];
   const char *s = "float";
   Evas_Object *lbl = data;
 
   if (anchor & E_GADGET_SITE_ANCHOR_LEFT)
     {
        if (anchor & E_GADGET_SITE_ANCHOR_TOP)
          {
             switch (orient)
               {
                case E_GADGET_SITE_ORIENT_HORIZONTAL:
                  s = "top_left";
                  break;
                case E_GADGET_SITE_ORIENT_VERTICAL:
                  s = "left_top";
                  break;
                case E_GADGET_SITE_ORIENT_NONE:
                  s = "left_top";
                  break;
               }
          }
        else if (anchor & E_GADGET_SITE_ANCHOR_BOTTOM)
          {
             switch (orient)
               {
                case E_GADGET_SITE_ORIENT_HORIZONTAL:
                  s = "bottom_left";
                  break;
                case E_GADGET_SITE_ORIENT_VERTICAL:
                  s = "left_bottom";
                  break;
                case E_GADGET_SITE_ORIENT_NONE:
                  s = "left_bottom";
                  break;
               }
          }
        else
          s = "left";
     }
   else if (anchor & E_GADGET_SITE_ANCHOR_RIGHT)
     {
        if (anchor & E_GADGET_SITE_ANCHOR_TOP)
          {
             switch (orient)
               {
                case E_GADGET_SITE_ORIENT_HORIZONTAL:
                  s = "top_right";
                  break;
                case E_GADGET_SITE_ORIENT_VERTICAL:
                  s = "right_top";
                  break;
                case E_GADGET_SITE_ORIENT_NONE:
                  s = "right_top";
                  break;
               }
          }
        else if (anchor & E_GADGET_SITE_ANCHOR_BOTTOM)
          {
             switch (orient)
               {
                case E_GADGET_SITE_ORIENT_HORIZONTAL:
                  s = "bottom_right";
                  break;
                case E_GADGET_SITE_ORIENT_VERTICAL:
                  s = "right_bottom";
                  break;
                case E_GADGET_SITE_ORIENT_NONE:
                  s = "right_bottom";
                  break;
               }
          }
        else
          s = "right";
     }
   else if (anchor & E_GADGET_SITE_ANCHOR_TOP)
     s = "top";
   else if (anchor & E_GADGET_SITE_ANCHOR_BOTTOM)
     s = "bottom";
   else
     {
        switch (orient)
          {
           case E_GADGET_SITE_ORIENT_HORIZONTAL:
             s = "horizontal";
             break;
           case E_GADGET_SITE_ORIENT_VERTICAL:
             s = "vertical";
             break;
           default: break;
          }
     }
   snprintf(buf, sizeof(buf), "e,state,orientation,%s", s);
   elm_object_text_set(lbl, buf);
}
 
static void
anchor_change(void *data, Evas_Object *obj EINA_UNUSED, void *event_info)
{
   ganchor = (uintptr_t)event_info;
   update_anchor_orient(data, gorient, ganchor);
}
 
static void
orient_change(void *data, Evas_Object *obj EINA_UNUSED, void *event_info)
{
   gorient = (uintptr_t)event_info;
   update_anchor_orient(data, gorient, ganchor);
}
 
static void 
on_click(void *data, Evas_Object *obj, void *event_info) 
{
    evas_object_del(data);
}
 
EAPI_MAIN int
elm_main(int argc EINA_UNUSED, char **argv EINA_UNUSED)
{
   Evas_Object *win, *box, *lbl, *icon, *btn;
   int gadget =0, id_num = 0;
   char buf[16];
 
   elm_policy_set(ELM_POLICY_QUIT, ELM_POLICY_QUIT_LAST_WINDOW_CLOSED);
 
   if (getenv("E_GADGET_ID"))
     {
        gadget = 1;
        snprintf(buf, sizeof(buf), "%s", getenv("E_GADGET_ID"));
        id_num = atoi(buf);
     }
 
   win = elm_win_add(NULL, "Main", ELM_WIN_BASIC);
   elm_win_title_set(win, "Hello, World!");
   elm_win_autodel_set(win, EINA_TRUE);
 
   if (gadget) elm_win_alpha_set(win, EINA_TRUE);
 
   box = elm_box_add(win);
   elm_box_horizontal_set(box, EINA_FALSE);
   evas_object_size_hint_weight_set(box, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   evas_object_size_hint_align_set(box, EVAS_HINT_FILL, EVAS_HINT_FILL);
   elm_win_resize_object_add(win, box);
   evas_object_show(box);
 
   if (gadget && id_num == -1)
     {
        icon = elm_icon_add(box);
        elm_icon_standard_set(icon, "start-here");
        evas_object_size_hint_weight_set(icon, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
        evas_object_size_hint_align_set(icon, EVAS_HINT_FILL, EVAS_HINT_FILL);
        elm_box_pack_end(box, icon);
        evas_object_show(icon);
     }
   else
     {
        lbl = elm_label_add(box);
        evas_object_size_hint_weight_set(lbl, EVAS_HINT_EXPAND, EVAS_HINT_FILL);
        elm_box_pack_end(box, lbl);
        evas_object_show(lbl);
 
        btn = elm_button_add(box);
        elm_object_text_set(btn, "Goodbye Cruel World");
        elm_box_pack_end(box, btn);
        evas_object_smart_callback_add(btn, "clicked", on_click, win);
        evas_object_show(btn);
     }
   if (gadget && id_num > -1)
     {
        evas_object_smart_callback_add(win, "gadget_site_anchor", anchor_change, lbl);
        evas_object_smart_callback_add(win, "gadget_site_orient", orient_change, lbl);
     }
   evas_object_show(win);
 
   elm_run();
 
   return 0;
}
ELM_MAIN()

Two smart callbacks are added if the application is in gadget mode, and a new #include was added for e_gadget_types.h. This header gives the anchor and orientation enumerations that are needed to describe the position of the gadget. These enumerations consist of E_GADGET_SITE_ANCHOR_* and E_GADGET_SITE_ORIENT_*. The anchor sites are TOP, BOTTOM, LEFT, and RIGHT and describe the position on the screen edge where the gadget is to be located.

The orientation of the site is HORIZONTAL, VERTICAL or NONE and describe the layout of the gadget site. Since gadget sites can have multiple gadgets this is important for noting that gadgets are going to be stacked horizontally or vertically. The NONE orientation is used when the gadget site is the desktop, which doesn't have a defined orientation.

Displaying Popups and Additional Windows in Gadgets

In the limitations section of this guide the problem with spawning popups and windows directly from the gadget were outlined. The sandbox, however, will take new windows that are spawned and show them as popups or ctxpopups based on the window type. If the window type is set to ELM_WIN_BASIC an elm_popup will be used. If the window type is set to ELM_WIN_POPUP_MENU a ctxpopup will be used. These windows will then be rendered outside of the gadget's visual.

In the original Hello World application a callback exists to close the window when the button is clicked. Gadgets, however, are designed to be persistent and not close in that manner. For the purposes of this example, the callback is changed to create a popup.

#include <Elementary.h>
#include <e_gadget_types.h>
 
static E_Gadget_Site_Orient gorient;
static E_Gadget_Site_Anchor ganchor;
static Evas_Object *popup;
 
static void
popup_del(void *data EINA_UNUSED, Evas *e EINA_UNUSED, Evas_Object *obj, void *event_info EINA_UNUSED)
{
   evas_object_del(obj);
}
 
static void
on_click(void *data, Evas_Object *obj EINA_UNUSED, void *event_info EINA_UNUSED)
{
   Evas_Object *win = data, *box, *lbl;
 
   if (popup)
     {
        evas_object_del(popup);
        popup = NULL;
        return;
     }
   popup = elm_win_add(win, "Popup", ELM_WIN_BASIC);
   elm_win_alpha_set(popup, 1);
   evas_object_event_callback_add(popup, EVAS_CALLBACK_HIDE, popup_del, NULL);
 
   box = elm_box_add(popup);
   elm_box_horizontal_set(box, EINA_FALSE);
   evas_object_size_hint_weight_set(box, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   evas_object_size_hint_align_set(box, EVAS_HINT_FILL, EVAS_HINT_FILL);
   elm_win_resize_object_add(popup, box);
   evas_object_show(box);
 
   lbl = elm_label_add(box);
   elm_object_text_set(lbl, "Goodbye Cruel World!");
   evas_object_size_hint_weight_set(lbl, EVAS_HINT_EXPAND, EVAS_HINT_FILL);
   elm_box_pack_end(box, lbl);
   evas_object_show(lbl);
 
   evas_object_show(popup);
}
 
static void
update_anchor_orient(void *data, E_Gadget_Site_Orient orient, E_Gadget_Site_Anchor anchor)
{
   char buf[4096];
   const char *s = "float";
   Evas_Object *lbl = data;
 
   if (anchor & E_GADGET_SITE_ANCHOR_LEFT)
     {
        if (anchor & E_GADGET_SITE_ANCHOR_TOP)
          {
             switch (orient)
               {
                case E_GADGET_SITE_ORIENT_HORIZONTAL:
                  s = "top_left";
                  break;
                case E_GADGET_SITE_ORIENT_VERTICAL:
                  s = "left_top";
                  break;
                case E_GADGET_SITE_ORIENT_NONE:
                  s = "left_top";
                  break;
               }
          }
        else if (anchor & E_GADGET_SITE_ANCHOR_BOTTOM)
          {
             switch (orient)
               {
                case E_GADGET_SITE_ORIENT_HORIZONTAL:
                  s = "bottom_left";
                  break;
                case E_GADGET_SITE_ORIENT_VERTICAL:
                  s = "left_bottom";
                  break;
                case E_GADGET_SITE_ORIENT_NONE:
                  s = "left_bottom";
                  break;
               }
          }
        else
          s = "left";
     }
   else if (anchor & E_GADGET_SITE_ANCHOR_RIGHT)
     {
        if (anchor & E_GADGET_SITE_ANCHOR_TOP)
          {
             switch (orient)
               {
                case E_GADGET_SITE_ORIENT_HORIZONTAL:
                  s = "top_right";
                  break;
                case E_GADGET_SITE_ORIENT_VERTICAL:
                  s = "right_top";
                  break;
                case E_GADGET_SITE_ORIENT_NONE:
                  s = "right_top";
                  break;
               }
          }
        else if (anchor & E_GADGET_SITE_ANCHOR_BOTTOM)
          {
             switch (orient)
               {
                case E_GADGET_SITE_ORIENT_HORIZONTAL:
                  s = "bottom_right";
                  break;
                case E_GADGET_SITE_ORIENT_VERTICAL:
                  s = "right_bottom";
                  break;
                case E_GADGET_SITE_ORIENT_NONE:
                  s = "right_bottom";
                  break;
               }          }
        else
          s = "right";
     }
   else if (anchor & E_GADGET_SITE_ANCHOR_TOP)
     s = "top";
   else if (anchor & E_GADGET_SITE_ANCHOR_BOTTOM)
     s = "bottom";
   else
     {
        switch (orient)
          {
           case E_GADGET_SITE_ORIENT_HORIZONTAL:
             s = "horizontal";
             break;
           case E_GADGET_SITE_ORIENT_VERTICAL:
             s = "vertical";
             break;
           default: break;
          }
     }
   snprintf(buf, sizeof(buf), "e,state,orientation,%s", s);
   elm_object_text_set(lbl, buf);
}
 
static void
anchor_change(void *data, Evas_Object *obj EINA_UNUSED, void *event_info)
{
   ganchor = (uintptr_t)event_info;
   update_anchor_orient(data, gorient, ganchor);
}
 
static void
orient_change(void *data, Evas_Object *obj EINA_UNUSED, void *event_info)
{
   gorient = (uintptr_t)event_info;
   update_anchor_orient(data, gorient, ganchor);
}
 
EAPI_MAIN int
elm_main(int argc EINA_UNUSED, char **argv EINA_UNUSED)
{
   Evas_Object *win, *box, *lbl, *icon, *btn;
   int gadget = 0, id_num = 0;
   char buf[16];
 
   elm_policy_set(ELM_POLICY_QUIT, ELM_POLICY_QUIT_LAST_WINDOW_CLOSED);
 
   if (getenv("E_GADGET_ID"))
     {
        gadget = 1;
        snprintf(buf, sizeof(buf), "%s", getenv("E_GADGET_ID"));
        id_num = atoi(buf);
     }
 
   win = elm_win_add(NULL, "Main", ELM_WIN_BASIC);
   elm_win_title_set(win, "Hello, World!");
   elm_win_autodel_set(win, EINA_TRUE);
 
   if (gadget) elm_win_alpha_set(win, EINA_TRUE);
 
   box = elm_box_add(win);
   elm_box_horizontal_set(box, EINA_FALSE);
   evas_object_size_hint_weight_set(box, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   evas_object_size_hint_align_set(box, EVAS_HINT_FILL, EVAS_HINT_FILL);
   elm_win_resize_object_add(win, box);
   evas_object_show(box);
 
   if (gadget && id_num == -1)
     {
        icon = elm_icon_add(box);
        elm_icon_standard_set(icon, "start-here");
        evas_object_size_hint_weight_set(icon, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
        evas_object_size_hint_align_set(icon, EVAS_HINT_FILL, EVAS_HINT_FILL);
        elm_box_pack_end(box, icon);
        evas_object_show(icon);
     }
   else
     {
        lbl = elm_label_add(box);
        evas_object_size_hint_weight_set(lbl, EVAS_HINT_EXPAND, EVAS_HINT_FILL);
        elm_box_pack_end(box, lbl);
        evas_object_show(lbl);
 
        btn = elm_button_add(box);
        elm_object_text_set(btn, "This is a popup");
        elm_box_pack_end(box, btn);
        evas_object_smart_callback_add(btn, "clicked", on_click, win);
        evas_object_show(btn);
     }
   if (gadget && id_num > -1)
     {
        evas_object_smart_callback_add(win, "gadget_site_anchor", anchor_change, lbl);
        evas_object_smart_callback_add(win, "gadget_site_orient", orient_change, lbl);
     }
   evas_object_show(win);
 
   elm_run();
 
   return 0;
}
ELM_MAIN()

The on_clicked function has been changed from the original Hello World application to show a popup that says "This is a popup". If the popup already exists, clicking the gadget closes the popup.

The popup_del function has been added to make sure when the gadget popup is hidden it is properly destroyed.

Installing Gadgets

Create the following three files, all in the same directory:

  • meson.build
  • helloworld.desktop.in
  • helloworld.c

"helloworld.c" must contain the example code provided earlier in this guide.

Create "helloworld.desktop.in" with the following contents:

[Desktop Entry]
Type=Application
Name=helloworld
Exec=@GADGET_DIR@/helloworld/e_gadget_helloworld
Icon=start-here
X-Gadget-Version=1.0
X-Gadget-Bugreport=https://www.enlightenment.org/contrib/report-bug

Note the required line X-Gadget-Version must be there for Enlightenment to recognize the gadget.

You can also specify if you want your gadget to only be accessible to horizontal, vertical, or desktop layouts by using the following line in the "helloworld.desktop.in" file:

X-Gadget-Orientation=None,Horizontal,Vertical

Create "meson.build" with the following contents:

project('helloworld', 'c',
        version: '1',
        license: 'BSD 2 clause',
        default_options: [ 'c_std=gnu99', 'warning_level=2' ],
        meson_version: '>= 0.40.0')
pkgconfig = import('pkgconfig')
cc = meson.get_compiler('c')
 
dep_e = dependency('enlightenment')
deps = [
        dep_e,
        dependency('elementary')
]
release = dep_e.get_pkgconfig_variable('release')
host_os = host_machine.system()
if host_os == 'linux'
        if cc.has_header_symbol('features.h', '__UCLIBC__')
                host_os = 'linux-uclibc'
        elif cc.has_header_symbol('features.h', '__dietlibc__')
                host_os = 'linux-dietlibc'
        else
                host_os = 'linux-gnu'
        endif
endif
module_arch = '@0@-@1@-@2@'.format(host_os, host_machine.cpu_family(), release)
dir_prefix = get_option('prefix')
dir_lib = join_paths(dir_prefix, get_option('libdir'))
dir_gadgets = join_paths([dir_lib, 'enlightenment/gadgets', module_arch])
build_files = [
        'helloworld.c'
]
executable('e_gadget_helloworld',
        build_files,
        dependencies: deps,
        c_args : ['-fPIE', '-fPIC'],
        link_args : ['-rdynamic', '-pie'],
        install_dir: join_paths([dir_gadgets, 'helloworld']),
        install: true)
 
desktop_data = configuration_data()
        desktop_data.set('GADGET_DIR', dir_gadgets)
configure_file(input: 'helloworld.desktop.in',
        output: 'helloworld.desktop',
        install: true,
        install_dir: join_paths([dir_gadgets, 'helloworld']),
        configuration: desktop_data)

This "meson.build" file determines the location Enlightenment will look for gadgets. This location is Enlightenment's library folder, followed by a sub directory string consisting of "gadgets/os-processor-enlightenmentversion". Once the location is determined, the meson file will install the "helloworld" binary and "helloworld.desktop" files to that location. Keep in mind when installing the ".desktop" file to Enlightenment's gadget directory that the file must have the same name as the directory to which it is being installed. In this case that is "helloworld/helloworld.desktop". Remember that the desktop is the crucial part of sandbox gadgets, as it is the means by which Enlightenment determines available gadgets. Enlightenment can only determine the location of gadgets by looking in its library directory. Note these two lines in the executable section of the meson.build are required:

        c_args : ['-fPIE', '-fPIC'],
        link_args : ['-rdynamic', '-pie'],

Run the following commands in order, from the directory where you placed the three files, to install the gadget:

meson build && cd build
ninja
sudo ninja install

Conclusion

The "helloworld" gadget is now installed and can be placed on a Gadget Bar. To place it on a Gadget Bar first add a Gadget Bar, left-click on the desktop, click "Desktop" then click "Add Gadget Bar" and follow the wizard. On an existing "Gadget Bar", hold the Alt key and right-click the Gadget Bar then click "Select Gadgets". To place it on the desktop, left-click the desktop, click "Desktop" then click "Add Gadgets to Desktop".

Remember that gadgets are standalone, so the "helloworld" gadget can be run as a standalone application simply by directly executing the installed binary.

The essentials for Enlightenment gadgets are follows:

  • Enlightenment sets an environment variable with a unique ID for gadgets.
  • Enlightenment determines available gadgets based on identified ".desktop" files in the Enlightenment gadget folder.
  • Gadget communication with the application is two-way and occurs through the use of smart callbacks.

Further Reading

Ephoto
Ephoto, an application used as both a standalone app and a gadget.
Wireless Gadget
Source code for the Enlightenment Wireless Gadget.
Meson
Meson documentation.
FDO Desktop Entry
Documentation for the ".desktop" file specification.