ObjectAdaptor::Continuation are for implementation of DBus calls which can not complete processing immediatly and have to wait for some external event.
For instance my application exposes some DBus interface for some DSP devices. It connects to these devices via TCPIP.
Thre is some protocol with Request Reply messages. Most of the time processing is like:
1) Linux client application issues DBus call
2) DBus-C++ server application translates it to TCP/IP messages and sends requests, it cannot answer immediatly and cannot block/wait - DBus-C++ server is supposed to be able handle many requests at the same time (altough it is single threaded - DBus-C++ is dispatching by means of select/pool)
3) To suspend the call DBus-C++ server application calls
return_later(&my_dsp_on_tag);
where my_dsp_on_tag is some unique object of DBus::Tag type, it is used at completion time to recover original context of client call
return_later internally calls exception, this is ugly and slow/inefficent way to transfer control but it is done that way and it works
object.cpp:
void ObjectAdaptor::return_later(const Tag *tag)
{
ReturnLaterError rle = { tag };
throw rle;
}
at some other place in this module:
catch(ReturnLaterError &rle)
{
_continuations[rle.tag] = new Continuation(conn(), cmsg, rle.tag);
}
4) when processing can be completed (external event occured, Reply is sent) the following code should executed
if( DBus::ObjectAdaptor::Continuation*c = find_continuation(&my_dsp_on_tag) ) {
uint32_t const ret=0;
c->writer() << ret;
return_now(c, my_dsp_on_tag);
}
So c->writer() stream is used to pass output parameters (in this case only one). Note that you can write anything to it and it can have nothing to do with original prototype of DBus method. So it is not type safe. For instance if you write
c->writer() << 0;
instead of
c->writer() << ret;
then it is wrong for strongly typed DBus client because you are returning int32_t instead of uint32_t (uint32_t is declared return type in xml interface).
-----Original Message-----
From: Andreas Volz [mailto:li...@br...]
Sent: Sun 12/5/2010 10:07 PM
To: dbu...@li...
Subject: Re: [dbus-cplusplus-devel] Setting DBus call return value in ObjectAdaptor::Continuation
Am Tue, 17 Aug 2010 09:42:52 +0200 schrieb Rafal Dzbek:
> Hello,
>
> I'm trying to use ObjectAdaptor::Continuation to handle DBus message
> asynchronously.
> Is there anywhere an example how to do it?
>
> It seems that I should call
> return_now( continuation );
> to finish DBus call, but how to setup return value of the call?
>
> I see there are structures like MessageIter and Message
> MessageIter &it = continuation->writer();
> Message &dbus_msg = it.msg();
> but I'm not sure how to use it.
Hello,
to be honest I'm not the original author of DBus-C++. I never used
ObjectAdaptor::Continuation and also don't know it's reason. Have you
found out more about it? Please tell us.
regards
Andreas
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