Starting from version 4.4 the Xfce Panel support two types of plugins: internal and external. The internal plugins are loadable modules, using the GModule interface. External plugins are separate programs that are embedded into the panel using the GtkPlug and GtkSocket mechanism.

In order to handle these plugins the plugin system was entirely rewritten, along with the rest of the panel framework. This file describes the way plugin writers should interact with this system.

The API documentation is installed with the panel and also available from http://www.xfce.org/documentation/api/

New in version 4.4 is the requirement for a so called .desktop file, that is pluginname.desktop, to be installed. It should look like this for an external plugin:

[Xfce Panel]
Type=X-XFCE-PanelPlugin
Encoding=UTF-8
_Name=Plugin name
_Comment=Plugin description
Icon=gtk-icon-name
X-XFCE-Exec=<prefix>/libexec/xfce4/panel-plugins/<plugin_name>

For the internal plugin you would use:

X-XFCE-Module=<plugin_name>
X-XFCE-Module-Path=<prefix>/lib/xfce4/panel-plugins

For instance,

X-XFCE-Module=quicklauncher
X-XFCE-Module-Path=/usr/lib/xfce4/panel-plugins

If the module should have no more than 1 instance running at the same time, you add this line:

X-XFCE-Unique=true

If the plugin is compatible with GTK+ 3, you need to add this line:

X-XFCE-API=2.0

The necessary widgets are provided by libxfce4panel. In your configure.ac you should add a line like this:

XDT_CHECK_PACKAGE ([LIBXFCE4PANEL], [libxfce4panel-2.0], [4.12.0])

The above assumes that you are using the xfce4-dev-tools package, which you really should, because it will make your life easier. Otherwise, you'd have to adjust it to include the relevant PKG_CONFIG macro.

There is only one header file that needs to be included, which will take care of including other required headers (gtk and libxfce4util):

#include <libxfce4panel/libxfce4panel.h>

To register a plugin with the plugin system there is one macro available that should be used, instead of using the library functions directly.

XFCE_PANEL_PLUGIN_REGISTER(construct);

Older versions use the following two deprecated macros, one for internal plugins and one for external plugins.

XFCE_PANEL_PLUGIN_REGISTER_EXTERNAL(construct);

XFCE_PANEL_PLUGIN_REGISTER_INTERNAL(construct);

The 'construct' argument is the name of a function that may be cast to XfcePanelPluginFunc, i.e. it takes a single XfcePanelPlugin pointer as argument. In the function all widgets should be created and callbacks connected to the appropriate plugin signals (see below).

static void plugin_construct (XfcePanelPlugin *plugin);

XFCE_PANEL_PLUGIN_REGISTER_EXTERNAL (plugin_construct);

/* implement functions */
...

There are several signals that plugins may be interested in:

void
user_function (XfcePanelPlugin *plugin, 
               GtkOrientation   orientation,
               gpointer         user_data);

void
user_function (XfcePanelPlugin    *plugin,
	       XfceScreenPosition *position,
	       gpointer            user_data);

The XfceScreenPosition describes the position of the panel on the screen. There are 12 positions, 3 on each side, plus two floating positions.

typedef enum
{
    XFCE_SCREEN_POSITION_NONE,
  
    /* top */
    XFCE_SCREEN_POSITION_NW_H,          /* North West Horizontal */
    XFCE_SCREEN_POSITION_N,             /* North                 */
    XFCE_SCREEN_POSITION_NE_H,          /* North East Horizontal */
 
    /* left */
    XFCE_SCREEN_POSITION_NW_V,          /* North West Vertical   */
    XFCE_SCREEN_POSITION_W,             /* West                  */
    XFCE_SCREEN_POSITION_SW_V,          /* South West Vertical   */
    
    /* right */
    XFCE_SCREEN_POSITION_NE_V,          /* North East Vertical   */
    XFCE_SCREEN_POSITION_E,             /* East                  */
    XFCE_SCREEN_POSITION_SE_V,          /* South East Vertical   */

    /* bottom */
    XFCE_SCREEN_POSITION_SW_H,          /* South West Horizontal */
    XFCE_SCREEN_POSITION_S,             /* South                 */
    XFCE_SCREEN_POSITION_SE_H,          /* South East Horizontal */

    /* floating */
    XFCE_SCREEN_POSITION_FLOATING_H,    /* Floating Horizontal */
    XFCE_SCREEN_POSITION_FLOATING_V,    /* Floating Vertical */
}
XfceScreenPosition;

Several macros are defined to make it easier to work with screen positions:

xfce_screen_position_is_horizontal(position);
xfce_screen_position_get_orientation(position);
xfce_screen_position_is_floating(position);
xfce_screen_position_is_top(position);
xfce_screen_position_is_left(position);
xfce_screen_position_is_right(position);
xfce_screen_position_is_bottom(position);

This function will return TRUE when you handle the size change

gboolean
user_function (XfcePanelPlugin *plugin, 
               gint             size,
               gpointer         user_data);

void
user_function (XfcePanelPlugin *plugin,
	       gpointer         user_data);

void
user_function (XfcePanelPlugin *plugin,
	       gpointer         user_data);

void
user_function (XfcePanelPlugin *plugin,
	       gpointer         user_data);

To show the menu item the plugin writer should also call:

void
xfce_panel_plugin_menu_show_about (XfcePanelPlugin *plugin);

void
user_function (XfcePanelPlugin *plugin,
	       gpointer         user_data);

To show the menu item the plugin writer should also call:

void
xfce_panel_plugin_menu_show_configure (XfcePanelPlugin *plugin);

Several functions are available to get more information about the plugin (and the panel it is part of). Only one property can also be changed, the 'expand' behavior. The plugin API also provides convenience functions to store and retrieve a pointer to user data.

/* identification */
const gchar *
xfce_panel_plugin_get_name (XfcePanelPlugin *plugin);

const gchar *
xfce_panel_plugin_get_id (XfcePanelPlugin *plugin);

const gchar *
xfce_panel_plugin_get_display_name (XfcePanelPlugin *plugin);

/* getting properties */
gint
xfce_panel_plugin_get_size (XfcePanelPlugin *plugin);

XfceScreenPosition 
xfce_panel_plugin_get_screen_position (XfcePanelPlugin *plugin);

gboolean
xfce_panel_plugin_get_expand (XfcePanelPlugin *plugin);

GtkOrientation
xfce_panel_plugin_get_orientation (XfcePanelPlugin *plugin);

/* settings properties */
void xfce_panel_plugin_set_expand (XfcePanelPlugin *plugin, 
                                   gboolean         expand);

The plugin has a right-click mouse menu connected to it that allows the user to show the about or settings dialog, to remove the plugin, or to show the panel settings dialog. Plugin writers have to make sure all widgets in the plugin that receive mouse events are connected to the menu by using the xfce_panel_plugin_add_action_widget() function. A plugin can also add additional, custom menu items.

IMPORTANT: If your custom menu item allows changes to the plugin, make sure it is safe when running in Kiosk mode!

void
xfce_panel_plugin_add_action_widget (XfcePanelPlugin *plugin, 
                                     GtkWidget       *widget);

void
xfce_panel_plugin_menu_insert_item (XfcePanelPlugin *plugin,
                                    GtkMenuItem     *item);

If your plugin has a configuration dialog you need to make that menu item visible and connect to the “configure-plugin” signal. The same for an about dialog and the “about” signal.

void xfce_panel_plugin_menu_show_about (XfcePanelPlugin *plugin);

void xfce_panel_plugin_menu_show_configure (XfcePanelPlugin *plugin);

Plugins can save and retrieve their configuration, using a unique file name. There's one function for looking up the config file for reading and one for the file to save.

gchar *
xfce_panel_plugin_lookup_rc_file (XfcePanelPlugin *plugin);

gchar *
xfce_panel_plugin_save_location (XfcePanelPlugin *plugin,
                                 gboolean         create);

When you test your new plugin, use the -Wall -Werror CFLAGS (and a recent version of GCC) to detect code problems. Also make sure the plugins is linked (plugin_name_LDADD in Makefile.am) to all the needed libraries, so there will be no problems when building with LDFLAGS=“-Wl,–as-needed”.

For creating a release you need to run ./autogen.sh && make distcheck. Fix all warnings and errors in make distcheck before distributing the package.

To help development, you can also test plugins without installing them on panel's prefix (e.g. /usr). As for 4.19.4, you can install the plugin in a directory of XDG_DATA_DIRS or in XDG_HOME_DIR, and the panel should find it. In particular, a default plugin installation in /usr/local with a panel in /usr should work on its own (or a plugin installation in ~/.local without root permissions).

You can also directly run plugins from the source so you don't need to overwrite a plugin version from your distribution. To accomplish this, build the plugin with make and then follow these steps:

• Create a symbolic link from your plugin binary to the panel's prefix installation, e.g. ln -s /home/johndoe/where/your/src/is/panel-plugin/.libs/libyourplugin.so /usr/lib/xfce4/panel-plugins/libyourplugin-git.so
• Copy /usr/share/xfce4/panel/plugins/yourplugin.desktop to /usr/share/xfce4/panel/plugins/yourplugin-git.desktop
• Edit the latter to point at 'yourplugin-git' instead of 'yourplugin': modify the Name entry to distinguish your instance and X-XFCE-Module to match the symbolic link, i.e. yourplugin-git.
• If you're porting a plugin to GTK+ 3, make sure to add X-XFCE-API=2.0 to the desktop-git file.

An example:

$cat /usr/share/xfce4/panel/plugins/diskperf-git.desktop
[Xfce Panel]
Type=X-XFCE-PanelPlugin
Encoding=UTF-8
Name=Disk Performance Monitor (git)
Comment=Show disk performance
Icon=drive-harddisk
X-XFCE-Internal=FALSE
X-XFCE-Module=diskperf-git
X-XFCE-API=2.0

Now the 'git' version of the panel plugin should be available in the 'add plugin' panel dialog. Whenever you make changes and compile the plugin, all you need to do is to add the plugin to panel again or simply refresh the panel xfce4-panel -r.

One can use GDB and Valgrind to debug external plugins, using the technique described here.