Signals are one way messages. They carry input parameters, which are received by all objects which have registered for such a signal.
This function is similar to
dbus-call-method. The difference
is, that there are no returning output parameters.
The function emits signal on the D-Bus bus. bus is
either the keyword
:system or the keyword
doesn’t matter whether another object has registered for signal.
Signals can be unicast or broadcast messages. For broadcast messages,
service must be
nil. Otherwise, service is the
D-Bus service name the signal is sent to as a unicast
message.6 path is the D-Bus object path
signal is sent from. interface is an interface available
at path. It must provide signal.
The remaining arguments args are passed to signal as arguments. They are converted into D-Bus types as described in Mapping Lisp types and D-Bus types.. Example:
(dbus-send-signal :session nil dbus-path-emacs (concat dbus-interface-emacs ".FileManager") "FileModified" "/home/albinus/.emacs")
With this function, an application registers for a signal on the D-Bus bus.
bus is either the keyword
:system or the keyword
service is the D-Bus service name used by the sending D-Bus object. It can be either a known name or the unique name of the D-Bus object sending the signal. A known name will be mapped onto the unique name of the object, owning service at registration time. When the corresponding D-Bus object disappears, signals will no longer be received.
path is the corresponding D-Bus object path that service is registered at. interface is an interface offered by service. It must provide signal.
service, path, interface and signal can be
nil. This is interpreted as a wildcard for the respective
handler is a Lisp function to be called when the signal is received. It must accept as arguments the output parameters signal is sending.
The remaining arguments args can be keywords or keyword string pairs.7 Their meaning is as follows:
This stands for the nth argument of the signal.
:pathn arguments can be used for object path wildcard
matches as specified by D-Bus, while an
:argN argument requires
an exact match.
Register for those signals, whose first argument names a service or interface within the namespace string.
Register for the object path namespace string. All signals sent
from an object path, which has string as the preceding string,
are matched. This requires path to be
Register for unicast signals which are not directed to the D-Bus object Emacs is registered at D-Bus BUS, if the security policy of BUS allows this. Otherwise, this argument is ignored.
dbus-register-signal returns a Lisp object, which can be used
as argument in
dbus-unregister-object for removing the
registration for signal. Example:
(defun my-dbus-signal-handler (device) (message "Device %s added" device)) (dbus-register-signal :system "org.freedesktop.Hal" "/org/freedesktop/Hal/Manager" "org.freedesktop.Hal.Manager" "DeviceAdded" #'my-dbus-signal-handler) ⇒ ((:signal :system "org.freedesktop.Hal.Manager" "DeviceAdded") ("org.freedesktop.Hal" "/org/freedesktop/Hal/Manager" my-signal-handler))
As we know from the introspection data of interface
‘org.freedesktop.Hal.Manager’, the signal ‘DeviceAdded’
provides one single parameter, which is mapped into a Lisp string.
The callback function
my-dbus-signal-handler must therefore
define a single string argument. Plugging a USB device into your
machine, when registered for signal ‘DeviceAdded’, will show you
which objects the GNU/Linux
hal daemon adds.
Some of the match rules have been added to a later version of D-Bus. In order to test the availability of such features, you could register for a dummy signal, and check the result:
(dbus-ignore-errors (dbus-register-signal :system nil nil nil nil #'ignore :path-namespace "/invalid/path")) ⇒ nil
For backward compatibility, a broadcast message is also emitted if service is the known or unique name Emacs is registered at D-Bus bus.
For backward compatibility, the arguments args
can also be just strings. They stand for the respective arguments of
signal in their order, and are used for filtering as well. A
nil argument might be used to preserve the order.