--- /dev/null
+---
+title: Application Startup with D-Bus
+---
+
+*Note: The that the D-Bus interface is in deprecation phase and for the time
+being only available for application & services that still rely on them. Once
+we migrate everything to gRPC, we will remove D-Bus IPC support. Please see
+[Application Startup with gRPC](02_Application_Startup.md) for the latest
+information*
+
+# Introduction
+
+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](https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html)
+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](01_Introduction.md), 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](https://specifications.freedesktop.org/desktop-entry-spec/1.1/ar01s07.html)
+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](04_Creating_a_New_Application_Dbus.md#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`
+
+## Testing
+
+`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" \
+ true
+```
+
+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')>],)
+```
+
+2. 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" \
+ "org.freedesktop.weston.wayland-terminal"
+```
+
+3. 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',)
+```
Code Review / AGL / documentation.git / blobdiff