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User:Magick777/Adventures with D-Bus

Revision as of 01:01, 16 September 2010

This page started because I'm trying to figure out D-BUS, as a relative newbie to Python and Telepathy, and the documentation is, shall we say, "fragmented". All the D-BUS recipes on the Wiki don't seem to include a simple way to place a call via the sofiasip client. The Telepathy documentation speaks to a vague architecture, not to my N900's implementation of it, and seems better for someone who wants to write a whole new interface than someone who just wants to use an existing one. I've read more source code and more forum posts than you can shake a stick at and I'm still none the wiser. So, this is a generic HOWTO for newbies (written by one) on how to track down an existing application's behaviour on D-BUS and replicate it. Work in progress September 2010.

Step 1: dbus-monitor

After some playing around with dbus-monitor, I've established that when I make a SIP call via the inbuilt Phone application, the first thing that happens on the session bus is:

method call sender=:1.121 ->
dest=org.freedesktop.Telepathy.AccountManager
serial=439
path=/org/freedesktop/Telepathy/Account/sofiasip/sip/_31030137_40sipgate_2eco_2euk0;
interface=com.nokia.Account.Interface.ChannelRequests;
member=Create
   array [
      dict entry(
         string "org.freedesktop.Telepathy.Channel.TargetHandleType"
         variant             uint32 1
      )
      dict entry(
         string "org.freedesktop.Telepathy.Channel.TargetID"
         variant             string "sip:50000@sipgate.co.uk"
      )
      dict entry(
         string "org.freedesktop.Telepathy.Channel.ChannelType"
         variant             string "org.freedesktop.Telepathy.Channel.Type.StreamedMedia"
      )
   ]
   uint64 14701758
   string ""

and, for a call to a PSTN number, via a different SIP account:

method call sender=:1.191 ->
dest=org.freedesktop.Telepathy.AccountManager
serial=500
path=/org/freedesktop/Telepathy/Account/sofiasip/sip/_330147398_40sip_2evoipfone_2eco_2euk0; 
interface=com.nokia.Account.Interface.ChannelRequests; 
member=Create
   array [
      dict entry(
         string "org.freedesktop.Telepathy.Channel.TargetHandleType"
         variant             uint32 1
      )
      dict entry(
         string "org.freedesktop.Telepathy.Channel.ChannelType"
         variant             string "org.freedesktop.Telepathy.Channel.Type.StreamedMedia"
      )
      dict entry(
         string "org.freedesktop.Telepathy.Channel.TargetID"
         variant             string "08081703703"
      )
   ]
   uint64 1284595510
   string ""

From this, we can deduce that:

• we need to make a method call over D-BUS to initiate the SIP call
• we need to send it to dest=org.freedesktop.Telepathy.AccountManager
• our path is /org/freedesktop/Telepathy/Account/ plus SofiaSIP account path
  • which can be found via "mc-tool list" (package libmissioncontrol-utils)
  • or, no doubt, via D-BUS, but we'll worry about that later
• our interface is com.nokia.Account.Interface.ChannelRequests
• the action we must take on that interface is to Create (a new channel)
• for which we must supply an array of dicts, a number and an empty string
  • TargetHandleType is 1
  • ChannelType is StreamedMedia
  • Target ID contains our destination SIP address or phone number
  • not sure what that UInt64 is about, some kind of timestamp?

and we can hope that, if we can pack up that set of instructions and deliver it to telepathy-sofiasip, it will (hopefully) place a call for us.

Step 2: dbus-send (actually, a false step)

I spent a while playing with dbus-send and reading the forums, until a thread led me to the following statement in the (fine) manual page:

"dbus-send does not permit empty containers or nested containers (e.g. arrays of variants)"

We can assume that dbus-send isn't going to like an array of dicts any more than it likes an array of variants, ergo, that what we want to do cannot be done with dbus-send. That would explain why there's no simple recipe on the Wiki, then. Well, if we can't use dbus-send, I guess we'll have to resort to python. That's going to be fun; I'm not in the habit of conversing with reptiles.

Step 3: python

The only example I could find of a shell script making a D-BUS method call is the "athome" script on the DbusScripts page, which in theory should act as a starting point for what I want to do.

#!/usr/bin/python 

import dbus
import sys
import re
import telepathy

# This gets us a connnection to the session bus
bus = dbus.SessionBus()

# This would seem to set up a proxy object, referring to our target SIP  account
account = bus.get_object('org.freedesktop.Telepathy.AccountManager',
                         '/org/freedesktop/Telepathy/Account/sofiasip/sip/_309517129090')

# This would seem to make a method call on the object
#account.Set('org.freedesktop.Telepathy.Account', 'RequestedPresence', \
#               dbus.Struct(( dbus.UInt32( presence_const) ,  presence_text,  ""), signature='uss'),
#               dbus_interface='org.freedesktop.DBus.Properties')

# We need to make a method call to Create a new channel

account.Set('org.freedesktop.Telepathy.Account', 'RequestedPresence', \
               dbus.Struct(( dbus.UInt32( presence_const) ,  presence_text,  ""), signature='uss'),
               dbus_interface='org.freedesktop.DBus.Properties')