In order to ensure that multiple notifications can easily be displayed at once, and to provide a convenient implementation, all notifications are controlled by a single session-scoped service which exposes a D-BUS interface.
On startup, a conforming implementation should take the org.freedesktop.Notifications service on the session bus. This service will be referred to as the "notification server" or just "the server" in this document. It can optionally be activated automatically by the bus process, however this is not required and notification server clients must not assume that it is available.
The server should implement the org.freedesktop.Notifications interface on an object with the path "/org/freedesktop/Notifications". This is the only interface required by this version of the specification.
A notification has the following components:
Table 1. Notification Components
|Application Name||This is the optional name of the application sending the notification. This should be the application's formal name, rather than some sort of ID. An example would be "FredApp E-Mail Client," rather than "fredapp-email-client."|
|Replaces ID||An optional ID of an existing notification that this notification is intended to replace.|
|Notification Icon||The notification icon. This is represented either as a URI (file:// is the only URI schema supported right now) or a name in a freedesktop.org-compliant icon theme (not a GTK+ stock ID).|
|Summary||This is a single line overview of the notification. For instance, "You have mail" or "A friend has come online". It should generally not be longer than 40 characters, though this is not a requirement, and server implementations should word wrap if necessary. The summary must be encoded using UTF-8.|
This is a multi-line body of text. Each line is a paragraph, server implementations are free to word wrap them as they see fit.
The body may contain simple markup as specified in Markup. It must be encoded using UTF-8.
If the body is omitted, just the summary is displayed.
The actions send a request message back to the notification client when invoked. This functionality may not be implemented by the notification server, conforming clients should check if it is available before using it (see the GetCapabilities message in Protocol). An implementation is free to ignore any requested by the client. As an example one possible rendering of actions would be as buttons in the notification popup.
Actions are sent over as a list of pairs. Each even element in the list (starting at index 0) represents the identifier for the action. Each odd element in the list is the localized string that will be displayed to the user.
The default action (usually invoked my clicking the notification) should have a key named "default". The name can be anything, though implementations are free not to display it.
Beyond the core protocol is the hints table. A couple of core elements have been moved to hints mostly because in a huge number of cases their default values would be sufficent. The elements moved to hints are:
Elements Moved to Hints
Element: Category ID
Description: An optional ID representing the type of notification (the name has been changed from Notification Type ID in pervious versions). See Categories.
Element: Urgency Level
Description: The urgency of the notification. See Urgency Levels. (Defaults to 1 - Normal)
Element: Icon Data
Description: Instead of overloading the icon field we now have an icon_data field that is used when icon is blank.
The timeout time in milliseconds since the display of the notification at which the notification should automatically close.
If -1, the notification's expiration time is dependent on the notification server's settings, and may vary for the type of notification.
If 0, the notification never expires.
Each notification displayed is allocated a unique ID by the server. This is unique within the session. While the notification server is running, the ID will not be recycled unless the capacity of a uint32 is exceeded.
This can be used to hide the notification before the expiration timeout is reached. It can also be used to atomically replace the notification with another. This allows you to (for instance) modify the contents of a notification while it's on-screen.