At system runtime, it may be necessary for applications to start other applications on demand. Such actions can be executed in reaction to a user request, or they may be needed to perform a specific task.

In order to do so, running applications and services need an established way of discovering installed applications and executing those. The Desktop Entry specification defines how applications can be discovered by using .desktop files, but there's no simple reference implementation for this function.

In order to provide a language-independent interface for applications and service to use, AGL includes applaunchd, a user service part of the default session.

Note: as mentioned previously, services are managed using systemd and are therefore not in the scope of this document.

Application launcher service

The purpose of applaunchd is to enumerate applications available on the system and provide a way for other applications to query this list and start those on demand. It is also able to notify clients of the startup and termination of applications it manages.

To that effect, applaunchd provides a D-Bus interface other applications can use in order to execute those actions.

Note: applaunchd will only send notifications for applications it started; it isn't aware of applications started by other means (systemd, direct executable call...), and therefore can't send notifications for those.

Application discovery

On startup, applaunchd inspects all .desktop files present under the applications/ subfolder of any element of the XDG_DATA_DIRS environment variable, ignoring all entries containing either the NoDisplay=true or Hidden=true lines.

It then looks for the following keys: - Terminal - DBusActivatable

If the desktop entry file contains the Terminal key set to true, then the application is marked as a non-graphical one. As such, it won't be included in the applications list if the client requests only graphical applications.

If DBusActivatable is set to true, then the application is marked as D-Bus activated. Additionally, applaunchd will search for a corresponding D-Bus service file in case this line is missing. This is a workaround allowing D-Bus activated applications providing an incomplete desktop entry file (i.e missing the DBusActivatable key) to be identified as such.

Requirements for D-Bus activation

applaunchd will always start D-Bus activatable applications using D-Bus activation instead of executing the command line stated in the desktop entry file.

This is handled by calling the Activate method of the org.freedesktop.Application interface with an empty argument.

As a consequence, all D-Bus activatable applications must implement this D-Bus interface.

Application identifiers

Each application is identified by a unique Application ID. Although this ID can be any valid string, it is highly recommended to use the "reverse DNS" convention in order to avoid potential name collisions and ease D-Bus integration.

The application ID is set in the desktop entry file itself for graphical applications: it is the value of the StartupWMClass field, which must be identical to the app-id advertised through the Wayland XDG toplevel protocol. In case this field is missing (as is usually the case for non-graphical application), the application ID will be the desktop entry file name, stripped from its .desktop extension.

D-Bus interface

The applaunchd D-Bus interface is named org.automotivelinux.AppLaunch. The object path for applaunchd is /org/automotivelinux/AppLaunch. The interface provides methods for the following actions: - retrieve the list of available applications; the client can choose to retrieve all available applications, or only those suitable for a graphical environment - request an application to be started

Moreover, signals are available for clients to be notified when an application has successfully started or its execution terminated.

Applications list

The listApplications method allows clients to retrieve the list of available applications. It takes one boolean argument named graphical: - if set to true, only applications suitable for graphical environments are returned - otherwise, the list contains all applications

This method returns an array of variants (type av), each element being a structure made up of 3 strings (type (sss)): - the application ID - the application's displayed name - the full path to the application icon file (or an empty string if no icon was specified in the application's desktop entry file)

Application startup request

Applications can be started by using the start method, passing the corresponding application ID as the only argument. This method doesn't return any data.

If the application is already running, applaunchd won't start another instance, but instead emit a started signal to notify clients the application is ready.

Status notifications

The org.automotivelinux.AppLaunch interface provides 2 signals clients can connect to: - started indicates an application has started - for D-Bus activated applications, it is emitted upon successful completion of the call to the Activate method of the org.freedesktop.Application interface - for other applications, this signal is emitted as soon as the child process has been successfully created - terminated is emitted when an application quits

Both signals have an additional argument named appid, containing the ID of the application affected by the event.

As mentioned above, the started signal is also emitted if applaunchd receives a request to start an already running application. This can be useful, for example, when switching between graphical applications: - the application switcher doesn't need to track the state of each application; instead, it can simply send a start request to applaunchd every time the user requests to switch to another application - the desktop environment then receives the started signal, indicating it should activate the window with the corresponding app-id


applaunchd can be manually tested using the gdbus command-line tool:

  1. Query the application list (graphical applications only):
$ gdbus call --session --dest "org.automotivelinux.AppLaunch" \
                       --object-path "/org/automotivelinux/AppLaunch" \
                       --method "org.automotivelinux.AppLaunch.listApplications" \

This command will output something similar to what follows:

([<('navigation', 'Navigation', '/usr/share/icons/hicolor/scalable/navigation.svg')>,
 <('settings', 'Settings', '/usr/share/icons/hicolor/scalable/settings.svg')>,
 <('dashboard', 'Dashboard', '/usr/share/icons/hicolor/scalable/dashboard.svg')>,
 <('hvac', 'HVAC', '/usr/share/icons/hicolor/scalable/hvac.svg')>,
 <('org.freedesktop.weston.wayland-terminal', 'Weston Terminal', '/usr/share/icons/Adwaita/scalable/apps/utilities-terminal-symbolic.svg')>],)
  1. Request startup of the org.freedesktop.weston.wayland-terminal application:
$ gdbus call --session --dest "org.automotivelinux.AppLaunch" \
                       --object-path "/org/automotivelinux/AppLaunch" \
                       --method "org.automotivelinux.AppLaunch.start" \
  1. Monitor signals emitted by applaunchd:
$ gdbus monitor --session --dest "org.automotivelinux.AppLaunch"

This results in the following output when starting, then exiting, the org.freedesktop.weston.wayland-terminal application:

Monitoring signals from all objects owned by org.automotivelinux.AppLaunch
The name org.automotivelinux.AppLaunch is owned by :1.4
/org/automotivelinux/AppLaunch: org.automotivelinux.AppLaunch.started ('org.freedesktop.weston.wayland-terminal',)
/org/automotivelinux/AppLaunch: org.automotivelinux.AppLaunch.terminated ('org.freedesktop.weston.wayland-terminal',)