User:Magick777/Adventures with D-Bus

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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 [[Nokia N900|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.
 
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== Step 1: dbus-monitor ==
 
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After some playing around with <code>dbus-monitor</code>, 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:
 
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<pre>
 
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method call sender=:1.121 ->
 
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dest=org.freedesktop.Telepathy.AccountManager
 
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serial=439
 
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path=/org/freedesktop/Telepathy/Account/sofiasip/sip/_31030137_40sipgate_2eco_2euk0;
 
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interface=com.nokia.Account.Interface.ChannelRequests;
 
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member=Create
 
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  array [
 
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      dict entry(
 
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        string "org.freedesktop.Telepathy.Channel.TargetHandleType"
 
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        variant            uint32 1
 
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      )
 
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      dict entry(
 
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        string "org.freedesktop.Telepathy.Channel.TargetID"
 
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        variant            string "sip:50000@sipgate.co.uk"
 
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      )
 
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      dict entry(
 
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        string "org.freedesktop.Telepathy.Channel.ChannelType"
 
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        variant            string "org.freedesktop.Telepathy.Channel.Type.StreamedMedia"
 
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      )
 
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  ]
 
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  uint64 14701758
 
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  string ""
 
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</pre>
 
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and, for a call to a PSTN number, via a different SIP account:
 
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<pre>
 
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method call sender=:1.191 ->
 
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dest=org.freedesktop.Telepathy.AccountManager
 
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serial=500
 
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path=/org/freedesktop/Telepathy/Account/sofiasip/sip/_330147398_40sip_2evoipfone_2eco_2euk0;
 
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interface=com.nokia.Account.Interface.ChannelRequests;
 
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member=Create
 
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  array [
 
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      dict entry(
 
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        string "org.freedesktop.Telepathy.Channel.TargetHandleType"
 
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        variant            uint32 1
 
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      )
 
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      dict entry(
 
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        string "org.freedesktop.Telepathy.Channel.ChannelType"
 
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        variant            string "org.freedesktop.Telepathy.Channel.Type.StreamedMedia"
 
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      )
 
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      dict entry(
 
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        string "org.freedesktop.Telepathy.Channel.TargetID"
 
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        variant            string "08081703703"
 
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      )
 
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  ]
 
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  uint64 1284595510
 
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  string ""
 
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</pre>
 
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From this, we can deduce that:
 
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* we need to make a method call over D-Bus to initiate the SIP call
 
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* we need to send it to <code>org.freedesktop.Telepathy.AccountManager</code>
 
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* our path is <code>/org/freedesktop/Telepathy/Account/</code> plus SofiaSIP account path
 
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** which can be found via "mc-tool list" (package libmissioncontrol-utils)
 
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** or, no doubt, via D-Bus, but we'll worry about that later
 
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* our interface is <code>com.nokia.Account.Interface.ChannelRequests</code>
 
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* the action we must take on that interface is to Create (a new channel)
 
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* for which we must supply an array of dicts, a number and an empty string
 
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** TargetHandleType is 1
 
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** ChannelType is StreamedMedia
 
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** Target ID contains our destination SIP address or phone number
 
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** not sure what that <code>uint64</code> is about, some kind of timestamp?
 
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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.
 
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== Step 2: dbus-send (actually, a false step) ==
 
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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:
 
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'''"dbus-send does not permit empty containers or nested containers (e.g. arrays of variants)"'''
 
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We can assume that <code>dbus-send</code> 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 <code>dbus-send</code>. That would explain why there's no simple recipe on the Wiki, then. Well, if we can't use <code>dbus-send</code>, 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.
 
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== Step 3: python ==
 
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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.
 
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<source lang="python">
 
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#!/usr/bin/python
 
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import dbus
 
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import sys
 
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import re
 
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import telepathy
 
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# This gets us a connnection to the session bus
 
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bus = dbus.SessionBus()
 
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# This would seem to set up a proxy object, referring to our target SIP  account
 
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account = bus.get_object('org.freedesktop.Telepathy.AccountManager',
 
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                        '/org/freedesktop/Telepathy/Account/sofiasip/sip/_309517129090')
 
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# This would seem to make a method call on the object
 
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#account.Set('org.freedesktop.Telepathy.Account', 'RequestedPresence', \
 
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#              dbus.Struct(( dbus.UInt32( presence_const) ,  presence_text,  ""), signature='uss'),
 
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#              dbus_interface='org.freedesktop.DBus.Properties')
 
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# We need to make a method call to Create a new channel
 
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account.Set('org.freedesktop.Telepathy.Account', 'RequestedPresence', \
 
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              dbus.Struct(( dbus.UInt32( presence_const) ,  presence_text,  ""), signature='uss'),
 
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              dbus_interface='org.freedesktop.DBus.Properties')
 
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</source>
 
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[[Category:Python]]
 

Revision as of 06:25, 10 May 2013

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