[DBUS] Observer pattern design

David Eriksson twogood at users.sourceforge.net
Mon May 23 11:37:05 PDT 2005

On Mon, 2005-05-23 at 15:06 -0300, Claudio Takahasi wrote:
> Hi folks,
> What is the correct design for a observer* pattern application using D-BUS?
> Suppose a wireless connection manager, when the signal is weak or
> the link is down, the connection manager must notify all connected clients.
> Implement the other side(clients requiresting  services) is trivial. There
> are a lot of sample addressing this context. However, for a observer
> pattern I was not able to figure out how implement it. I think use polling 
> is NOT a interesting approach.
> What are the possible implementations for this kind of D-BUS application? 

First a disclaimer:

1. I'm just a humble (ab)user of D-BUS and a Python newbie
2. I have currently only (ab)used D-BUS 0.2x 

Now I can share what I've done for an observer-like pattern:

I have an interface with a Subscribe method in a C++ daemon. In order to
know who subscribed, I save the value of dbus_message_get_sender() for
the message containing the Subscribe call. The Subscribe method returns
an object path.

When I have an event I want to share with my subscribers, I create and
send a method call for each subscriber with the same path as returned to
the subscriber from the Subscribe method.

In the Python client I created a special class for the base service:

class BaseService:

  def __init__(self, bus):
    self._service_name = bus.get_connection().get_base_service()
    self._bus = bus

  def get_bus(self):
    return self._bus

  def get_service_name(self):
    return self._service_name

An instance of this class is used as a parameter to
dbus.Object.__init__(), something like this:

class Subscriber:

  def __init__(self, name, type, path, base_service):
        path, base_service,

  def SomeEventISubscribeTo(self, message, param1, param2):
    print "event received!"

               -\- David Eriksson -/-

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