User:Magick777/Adventures with D-Bus

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:

  1. we need to make a method call over D-BUS to initiate the SIP call
  2. we need to send it to dest=org.freedesktop.Telepathy.AccountManager
  3. our path is /org/freedesktop/Telepathy/Account/ plus SofiaSIP account path
    1. which can be found via "mc-tool list" (package libmissioncontrol-utils)
  4. our interface is com.nokia.Account.Interface.ChannelRequests
  5. the action we must take on that interface is to Create (a new channel)
  6. for which we must supply an array of dicts, a number and an empty string
    1. TargetHandleType is 1
    2. ChannelType is StreamedMedia
    3. Target ID contains our destination SIP address or phone number
    4. 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 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.

Step 3: python

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