write DBUS Services (I)

Those days I had to make a D-Bus server and client in python and I thought to share with you guys some things about the D-Bus.

First of all, what is D-Bus? Well, D-Bus is a system that offers a communication channel for multiple programs.

Someone must also distinguish between: session and system bus.

The session bus is specific for each user, while the system bus is specially designed for the state of the whole system and can change this state (like adding a new storage device, the internet connection suffers some modification).

“Talk is cheap, show me the code”  Linus Torvalds

One example of a session bus server is the following

#!/usr/bin/env python3from gi.repository import GLibimport dbusimport dbus.servicefrom dbus.mainloop.glib import DBusGMainLoopclass Session_DBus(dbus.service.Object):    def __init__(self):        bus_name=dbus.service.BusName('org.me.test_session',bus=dbus.SessionBus())        dbus.service.Object.__init__(self,bus_name,'/org/me/test_session')    @dbus.service.method('org.me.test1')    def session_bus_message1(sefl):        return "Session Bus 1"    @dbus.service.method('org.me.test2')    def session_bus_message2(self):        return "Session Bus 2"    @dbus.service.method('org.me.test2')    def session_bus_strings(self,string1,string2):        return string1+" "+string2DBusGMainLoop(set_as_default=True)dbus_service=Session_DBus()try:    GLib.MainLoop().run()except KeyboardInterrupt:    print("\nThe MainLoop will close...")    GLib.MainLoop().quit()

And the client (2 versions):

Version 1:

#!/usr/bin/env python3import dbusbus = dbus.SessionBus()session = bus.get_object("org.me.test_session", "/org/me/test_session")method_message1 = session.get_dbus_method('session_bus_message1', 'org.me.test1')method_message2 = session.get_dbus_method('session_bus_message2', 'org.me.test2')method_message3 = session.get_dbus_method('session_bus_strings', 'org.me.test2')print(method_message1())#interface1 = dbus.Interface(session, "org.me.test1")#print(interface1.session_bus_message1())print(method_message2())print(method_message3("Hello", "World"))

Version 2:

#!/usr/bin/env python3import dbusbus = dbus.SessionBus()session = bus.get_object("org.me.test_session", "/org/me/test_session")interface1 = dbus.Interface(session, "org.me.test1")interface2 = dbus.Interface(session, "org.me.test2")print(interface1.session_bus_message1())print(interface2.session_bus_message2())print(interface2.session_bus_strings("hello", "world"))

For the client scripts to run, one must first run the script for the server and after that to run theclients. Also if you want to have to have the server initiated (to not manually run it) you could create a file with a .service extension  in/usr/share/dbus-1/service/; let’s say my-test-session.servicewith the following content:

[D-BUS Service]Name=org.me.test_sessionExec="/home/charles/my-test-session.py"

After this you can run only the clients and the system would know where server part is and how to handle it. Also, do not forget to make the server scriptexecutable.

we can verify this service using dbus tools:

$ dbus-send --session --type=method_call --print-reply --dest="org.me.test_session" /org/me/test_session org.freedesktop.DBus.Introspectable.Introspectmethod return time=1491700435.911482 sender=:1.133 -> destination=:1.134 serial=3 reply_serial=2   string "<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN""http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd"><node name="/org/me/test_session">  <interface name="org.freedesktop.DBus.Introspectable">    <method name="Introspect">      <arg direction="out" type="s" />    </method>  </interface>  <interface name="org.me.test1">    <method name="session_bus_message1">    </method>  </interface>  <interface name="org.me.test2">    <method name="session_bus_message2">    </method>    <method name="session_bus_strings">      <arg direction="in"  type="v" name="string1" />      <arg direction="in"  type="v" name="string2" />    </method>  </interface></node>"

dbus-send --session --type=method_call --print-reply --dest="org.me.test_session" /org/me/test_session org.me.test2.session_bus_strings string:"hello" string:"world"method return time=1491700611.464318 sender=:1.133 -> destination=:1.137 serial=4 reply_serial=2   string "hello world"

https://georgemuraruc.wordpress.com/2015/07/16/d-bus-tutorial-for-python/