Editing Adding multilang fields (section)

== Adding a new field type (VirtualWithValue)==
In order to create a multiple-language field we have to create an appropriate field type. EPrints' '''MetaField''' (see [[API:EPrints/MetaField|MetaField]] article for details) is a perfect candidate for this, and we need to extend it and override its '''get_value''' and '''set_value''' functions for the field to work properly with the rest of EPrints API, as well as its '''get_property_defaults''' function to sort out warnings for default values. The following code could be a rough implementation of such a field:

<source lang="perl">package EPrints::MetaField::Virtualwithvalue;

use strict;
use warnings;

use EPrints::MetaField;

our @ISA = qw( EPrints::MetaField );

use strict;

sub get_property_defaults
{
    my ( $self ) = @_;
    my %defaults = $self->SUPER::get_property_defaults;
    $defaults{get_value} = $EPrints::MetaField::UNDEF;
    $defaults{set_value} = $EPrints::MetaField::UNDEF;

    return %defaults;
}

sub get_value
{
    my( $self, $object ) = @_;
    if ( defined $self->get_property("get_value") )
    {
        return $self->call_property( "get_value", $object);
    }
    return undef; # This is important. Using just a 'return;' or no return statement may break your repository e.g. when an EPrint revision file tried to be created.
}

sub set_value
{
     my( $self, $object, $value ) = @_;
     if ( defined $self->get_property("set_value") )
     {
         return $self->call_property( "set_value", $object, $value);
     }
     return undef;
}

</source>

We could save this file in '''~eprints/lib/plugins/EPrints/MetaField/Virtualwithvalue.pm'''.