Provided by: libnet-dbus-perl_1.1.0-4build2_amd64 
NAME
Net::DBus::Tutorial::ExportingObjects - tutorials on providing a DBus service
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
Music::Player distribution, which is a simple DBus service providing a music track player.
CREATING AN OBJECT
The first step in creating an object is to create a new package which inherits from
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,
Net::DBus::Object expects to be given to parameters, a handle to the 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;
}
1;
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 $self;
}
And now a method to register a new backend. This takes a Perl module name and uses it to
instantiate a backend. Since the backends are also going to be DBus objects, we need to
pass in a reference to the service we are attached to, along with a path under which to
register the backend. We use the "get_service" method to retreieve a reference to the
service the manager is attached to, and attach the player backend to this same service:
When a method on DBus object is invoked, the first parameter is the object reference
($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
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 "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 "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 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");
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 defintions for exported method is known in the DBus world as
"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 "org.cpan" as the
first component, followed by the package name of the module (replacing :: with .), eg
"org.cpan.music.player.manager". If it is not planned to host the module on CPAN, a
personal/project domain might be used eg "com.berrange.music.player.manager". The
interface for an object is defined by loading the 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 Net::DBus::Exporter module provides a method "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 preferrable to associate calls to "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...
}
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 Net::DBus::Service module provides the means to register a service. Its constructor
expects a reference to the bus object (an instance of 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 "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 Net::DBus::Service automatically provides one special object to all services, under
the path "/org/freedesktop/DBus/Exporter". This object implements the
"org.freedesktop.DBus.Exporter" interface which has a method "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.
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 "session" or <system> methods on Net::DBus to get a handle to the
desired bus, while in the latter case, the "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 "/usr/bin/music-player.pl", coding is complete and the
service ready for use by clients on the bus.
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 "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 directoy
"/usr/share/dbus-1/services". The file should be named to match the service name, with the
file extension ".service" appended. eg,
"/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
SEE ALSO
Net::DBus::Tutorial for details of other tutorials, and Net::DBus for API documentation
AUTHORS
Daniel Berrange <dan@berrange.com>
COPYRIGHT
Copyright (C) 2005 Daniel P. Berrange