[Swig-user] RE new to swig and callbacks wondering if swig can do this and any pointers

[Bronner, G. <gre...@le...>]

It can be done, and I've done it multiple times, but it isn't exactly
the easiest thing to do. I haven't seen a good discussion of this
anywhere:

My typical approach is to write a C++ class that implements the listener
interface(es). My C++ object has a captive python object delegate, and
the C++ callbacks simply use the python C-API to determine if the the
delegate object has a function of a specific name, and if it exists, the
callback calls the python function.

A further wrinkle is that event callbacks can be done on threads other
than the main python interpreter thread. In that case, every C++
callback that modifies python variables has to lock the global
interpreter lock using
  PyGILState_STATE gstate;
  gstate =3D PyGILState_Ensure();
=09....
 PyGILState_Release(gstate);


Likewise, the functions that start processing the event loop (or need to
block while other events are processed) have to release the global
interpreter lock using

Py_BEGIN_ALLOW_THREADS
=2E..
Py_END_ALLOW_THREADS


I use it through a swig exception declaration:
%define THREAD_WRAP(x)
%exception x=20
=7B
/* Generated by THREAD_WRAP x */
  Py_BEGIN_ALLOW_THREADS
  =24action
  Py_END_ALLOW_THREADS
=7D
%enddef


example:
THREAD_WRAP(A::b);



As a side effect of this, I never have to wrap the actual callback
functions -- I just wrap the constructor for the C++ class that
implements the callback functions, and the function that accepts a
python object delegate, as well as the  the functiont that starts the
event loop.


So I might have a script that does this:


import msgCallback

class MyCallback(object):
=09def onMsg(arg1, arg2):
=09=09print =22OnMsg called=21=22


cobj=3DmsgCallback.PythonCCallbackWrapper()
cobj.setDelegate(MyCallback())
msgCallback.startEventLoop()


To do this correctly requires a very good working knowledge of the
target API, SWIG, and the Python C-API, but when done properly, it gives
you the flexibility of writing extremely flexible event-driven programs.

------------------------------

Message: 5
Date: Wed, 12 Dec 2007 15:54:08 -0800 (PST)
From: mattdavis <someoneinjapan=40gmail.com>
Subject: =5BSwig-user=5D new to swig and callbacks wondering if swig can
=09do this and any pointers.
To: swig-user=40lists.sourceforge.net
Message-ID: <14306927.post=40talk.nabble.com>
Content-Type: text/plain; charset=3Dus-ascii


Hi everyone,

I am new to swig and have some questions about callbacks.  I would love
any
help, hints, or links to pages that might contain more information for
me.=20
So here is my current situation. I have 5 function that I am trying to
wrap
in swig. There are 2 RegisterForNotification function which accept an
EventID and an either an EventResponder or an EventListner.  The
EventListener and EventResponder are functions which get called when an
event is raised RaiseEvent notifies all the listeners first and then
calls
the event responder.  There may be multiple listeners, but only one
responder.  The listeners may do some processing, but do not provide a
result.  The responder handles the event and returns a result. the other
2
are UnregisterForNotification which takes an EventID and either an
EventListener or EventResponder.

The other question I have is do I need to wrap the EventListner and
EventResponder?  The following code is in the .h file

typedef CBFunctor3wRet<EventId, const String&, const Argument&, Result>
EventListener;
typedef CBFunctor3wRet<EventId, const String&, Argument&, Result>
EventResponder;

does that mean that it will be wrapped or do I have to specifically
include
it?  I have also already wrapped my String and Result classes but
haven't
yet wrapped the Argument class.  I am assuming that I will need to wrap
the
Argument class as well.

What I want to be able to do is call these functions from a python
script.=20
So for example if a REQUIREPASSWORD event pops up then I can send it a
password from the script and I get a result back.  I have done a little
digging around and from what I can understand to do call backs I have to
have the callback functions defined in C and I wrap them with swig.
Please
correct me if I am wrong.  For what I am trying to do that doesn't fit
very
well.  I want to be able to write function in python and be able to pass
them in.  Is this possible?  I have also seen some posts and links to
TypeMaps.  This was a little deeper into swig then I thought I would
have to
go.  I am not opposed to doing this, I was just hoping to know if that
is
the right approach to this problem or if there is a more simple approach
to
this.

Any help is greatly appreciated.
--=20
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