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diff --git a/docs/06_Component_Documentation/Application_Framework/02_Application_Startup_Dbus.md b/docs/06_Component_Documentation/Application_Framework/02_Application_Startup_Dbus.md new file mode 100644 index 0000000..f951e0e --- /dev/null +++ b/docs/06_Component_Documentation/Application_Framework/02_Application_Startup_Dbus.md @@ -0,0 +1,190 @@ +--- +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',) +``` |