JezK

Edit File: ExportingObjects.pod

# -*- perl -*-
#
# Copyright (C) 2004-2005 Daniel P. Berrange
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
#
# $Id: ExportingObjects.pod,v 1.1 2006/01/27 14:02:35 dan Exp $

=pod

=head1 NAME

Net::DBus::Tutorial::ExportingObjects - tutorials on providing a DBus service

=head1 DESCRIPTION

This document provides a tutorial on providing a DBus service using the
Perl Net::DBus application bindings. This examples in this document
will be based on the code from the L<Music::Player> distribution, which
is a simple DBus service providing a music track player.

=head1 CREATING AN OBJECT

The first step in creating an object is to create a new package
which inherits from L<Net::DBus::Object>. The Music::Player::Manager
object provides an API for managing the collection of music player
backends for different track types. To start with, lets create the
skeleton of the package & its constructor. The constructor of the
super type, L<Net::DBus::Object> expects to be given to parameters,
a handle to the L<Net::DBus::Service> owning the object, and a path
under which the object shall be exported. Since the manager class is
intended to be a singleton object, we can hard code the path to it
within the constructor:

package Music::Player::Manager;

use base qw(Net::DBus::Object);

sub new {
      my $class = shift;
      my $service = shift;
      my $self = $class->SUPER::new($service, "/music/player/manager");

bless $self, $class;

return $self;
  }

Now, as mentioned, the manager with handle a number of different
player backends. So we need to provide methods for registering
new backends, and querying for backends capable of playing a
particular file type. So modifying the above code we add a hash
table in the constructor, to store the backends:

sub new {
      my $class = shift;
      my $service = shift;
      my $self = $class->SUPER::new($service, "/music/player/manager");

$self->{backends} = {};

bless $self, $class;

return $self;
  }

And now a method to register a new backend. This takes a Perl
module name and uses it to instantiate a backend. Since the
backends are also going to be DBus objects, we need to pass
in a reference to the service we are attached to, along with
a path under which to register the backend. We use the C<get_service>
method to retreieve a reference to the service the manager is
attached to, and attach the player backend to this same service:
When a method on DBus object is invoked, the first parameter is
the object reference (C<$self>), and the remainder are the
parameters provided to the method call. Thus writing a method
implementation on a DBUs is really no different to normal object
oriented Perl (cf L<perltoot>):

sub register_backend {
      my $self = shift;
      my $name = shift;
      my $module = shift;

eval "use $module";
      if ($@) {
          die "cannot load backend $module: $@" ;
      }

$self->{backends}->{$name} = $module->new($self->get_service,
                                                "/music/player/backend/$name");
  }

Looking at this one might wonder what happens if the C<die>
method is triggered. In such a scenario, rather than terminating
the service process, the error will be caught and propagated back
to the remote caller to deal with.

The player backends provide a method C<get_track_types> which returns
an array reference of the music track types they support. We can use
this method to provide an API to allow easy retrieval of a backend
for a particular track type. This method will return a path with which
the backend object can be accessed

sub find_backend {
      my $self = shift;
      my $extension = shift;

foreach my $name (keys %{$self->{backends}}) {
         my $backend = $self->{backends}->{$name};
         foreach my $type (@{$backend->get_track_types}) {
            if ($type eq $extension) {
                return $backend->get_object_path;
            }
         }
      }

die "no backend for type $extension";
  }

Lets take a quick moment to consider how this method would be used to
play a music track. If you've not already done so, refresh your memory
from L<Net::DBus::Tutorial::UsingObjects>. Now, we have an MP3 file
which we wish to play, so we search for the path to a backend, then
retrieve the object for it, and play the track:

...get the music player service...
  # Ask for a path to a player for mp3 files
  my $path = $service->find_backend("mp3");
  # $path now contains '/music/player/backend/mpg123'
  # and we can get the backend object
  my $backend = $service->get_object($path);
  # and finally play the track
  $backend->play("/vol/music/beck/guero/09-scarecrow.mp3");

=head1 PROVIDING INTROSPECTION DATA

The code above is a complete working object, ready to be registered with
a service, and since the parameters and return values for the two methods
are both simple strings we could stop there. In some cases, however, one
might want to be more specific about data types expected for parameters,
for example signed vs unsigned integers. Adding explicit data typing also
makes interaction with other programming languages more reliable. Providing
explicit data type definitions for exported method is known in the DBus world
as C<Introspection>, and it makes life much more reliable for users of one's
service whom may be using a strongly typed language such as C.

The first step in providing introspection data for a DBus object in Perl, is
to specify the name of the interface provided by the object. This is typically
a period separated string, by convention containing the domain name of the
application as its first component. Since most Perl modules end up living on
CPAN, one might use C<org.cpan> as the first component, followed by the package
name of the module (replacing :: with .), eg C<org.cpan.music.player.manager>. If it is
not planned to host the module on CPAN, a personal/project domain might be
used eg C<com.berrange.music.player.manager>. The interface for an object is defined
by loading the L<Net::DBus::Exporter> module, providing the interface as its
first parameter. So the earlier code example would be modified to look like:

package Music::Player::Manager;

use base qw(Net::DBus);
  use Net::DBus::Exporter qw(com.berrange.music.player.manager)

Next up, it is necessary to provide data types for the parameters and return
values of the methods. The L<Net::DBus::Exporter> module provides a method
C<dbus_method> for this purpose, which takes three parameter, the name of the
method being exported, an array reference of parameter types, and an array
reference of return types (the latter can be omitted if there are no return
values). This can be called at any point in the module's code, but by convention
it is preferable to associate calls to C<dbus_method> with the actual method
implementation, thus:

dbus_method("register_backend", ["string", "string"]);
  sub register_backend {
      my $self = shift;
      my $name = shift;
      my $module = shift;

.. snipped rest of method body ...
  }

And, thus:

dbus_method("find_backend", ["string"], ["string"])
  sub find_backend {
      my $self = shift;
      my $extension = shift;
      ... snip method body...
  }

=head1 DEFINING A SERVICE

Now that the objects have been written, it is time to define
a service. A service is nothing more than a well known name
for a given API contract. A contract can be thought of as a
definition of a list of object paths, and the corresponding
interfaces they provide. So, someone else could come along a
provide an alternate music player implementation using the
Python or QT bindings for DBus, and if they provided the same
set of object paths & interfaces, they could justifiably register
the same service on the bus.

The L<Net::DBus::Service> module provides the means to register
a service. Its constructor expects a reference to the bus object
(an instance of L<Net::DBus>), along with the name of the service.
As with interface names, the first component of a service name is
usually derived from a domain name, and then suffixed with the
name of the application, in our example forming C<org.cpan.Music.Player>.
While some objects will be created on the fly during execution
of the application, others are created upon initial startup. The
music player manager object created earlier in this tutorial is
an example of the latter. It is typical to instantiate and register
these objects in the constructor for the service. Thus a service
object for the music player application would look like:

package Music::Player;

use base qw(Net::DBus::Service);

sub new {
        my $class = shift;
        my $bus = shift;
        my $self = $class->SUPER::new($bus, "org.cpan.music.player");

bless $self, $class;

$self->{manager} = Music::Player::Manager->new($self);

return $self;
    }

The L<Net::DBus::Service> automatically provides one special
object to all services, under the path C</org/freedesktop/DBus/Exporter>.
This object implements the C<org.freedesktop.DBus.Exporter> interface
which has a method C<ListObject>. This enables clients to determine
a list of all objects exported within a service. While not functionally
necessary for most applications, it is none-the-less a useful tool for
developers debugging applications, or wondering what a service provides.

=head1 CONNECTING TO THE BUS

The final step in getting our service up and running is to connect it
to the bus. This brings up an interesting conundrum, does one export
the service on the system bus (shared by all users & processes on the
machine), or the session bus (one per user logged into a machine). In
some cases the answer, with only one of the two buses conceptually making
sense. In other cases, however, both the session & system bus are valid.
In the former one would use the C<session> or <system> methods on L<Net::DBus>
to get a handle to the desired bus, while in the latter case, the C<find>
method would be used. This applies a heuristic to determine the correct
bus based on execution environment. In the case of the music player, either
bus is relevant, so the code to connect the service to the bus would look
like:

use Net::DBus;

my $bus = Net::DBus->find;
   my $player = Music::Player->new($bus);

With the service attached to the bus, it is merely necessary to run
the main event processing loop to listen out for & handle incoming
DBus messages. So the above code is modified to start a simple reactor:

use Net::DBus;
   use Net::DBus::Reactor;

my $bus = Net::DBus->find;
   my $player = Music::Player->new($bus);

Net::DBus::Reactor->main->run;

exit 0;

Saving this code into a script C</usr/bin/music-player.pl>, coding
is complete and the service ready for use by clients on the bus.

=head1 SERVICE ACTIVATION

One might now wonder how best to start the service, particularly
if it is a service capable of running on
both the system and session buses. DBus has the answer in the
concept of C<activation>. What happens is that when a client
on the bus attempts to call a method, or register a signal
handler against, a service not currently running, it will first
try and start the service. Service's which wish to participate
in this process merely need stick a simple service definition
file into the directory C</usr/share/dbus-1/services>. The file
should be named to match the service name, with the file extension
C<.service> appended. eg, C</usr/share/dbus-1/services/org.cpan.music.player.service>
The file contains two keys, first the name of the service, and
second the name of the executable used to run the service, or in
this case the Perl script. So, for our simple service the data
file would contain:

[D-BUS Service]
  Name=org.cpan.music.player
  Exec=/usr/bin/music-player.pl

=head1 SEE ALSO

L<Net::DBus::Tutorial> for details of other tutorials, and
L<Net::DBus> for API documentation

=head1 AUTHORS

Daniel Berrange <dan@berrange.com>

=head1 COPYRIGHT

=cut

Anmeldung Newsletter | Fairnetzt-Hochrhein