
=head1 NAME

Mail::Message::Field::Attribute - one attribute of a full field

=head1 SYNOPSIS

 my $field    = $msg->head->get('Content-Disposition') or return;
 my $full     = $field->study;   # full understanding in unicode
 my $filename = $full->attribute(filename)             or return;

 print ref $filename;     # this class name
 print $filename;         # the attributes content in utf-8
 print $filename->value;  # same
 print $filename->string; # print string as was found in the file
 $filename->print(\*OUT); # print as was found in the file

=head1 DESCRIPTION

Attributes within MIME fields can be quite complex, and therefore be slow
and consumes a lot of memory.  The Mail::Message::Field::Fast and
Mail::Message::Field::Flex simplify them the attributes a lot, which
may result in erroneous behavior in rare cases.  With the increase of
non-western languages on Internet, the need for the complex headers
becomes more and more in demand.

A Mail::Message::Field::Attribute can be found in any structured
Mail::Message::Field::Full header field.

=head1 METHODS

=head2 Initiation

=over 4

=item B<addComponent> STRING

A component is a parameter as defined by RFC2045, optionally using
encoding or continuations as defined by RFC2231.  Components of an
attribute are found when a field is being parsed.  The RFCs are
very strict on valid characters, but we cannot be: you have to accept
what is coming in if you can.

Examples:

 my $param = Mail::Message::Field::Attribute->new('Content-Type');
 $param->addComponent("filename*=iso10646'nl-BE'%Re\47u");

=item B<charset> 

Returns the character set which is used for this parameter.  If any component
is added which contains character set information, this is directly
available.  Be warned that a character-set is case insensitive.

=item B<language> 

Returns the language which is defined in the argument.  If no language is
defined C<undef> is returned, which should be interpreted as "ANY"

=item B<name> 

Returns the name of this attribute.

=item B<new> (NAME, [VALUE] | STRING), OPTIONS

(Class method) 
Create a new attribute NAME with the optional VALUE.  If no VALUE is specified,
the first argument of this method is inspected for an equals sign C<'='>.
If that character is present, the argument is taken as STRING, containing
a preformatted attribute which is processed.  Otherwise, the argument is
taken as name without VALUE: set the value later with value().

Whether encoding takes place
depends on the OPTIONS and the existence of non-ascii characters in the VALUE.
The NAME can only contain ascii characters, hence is never encoded.

To speed things up, attributes are not derived from the Mail::Reporter
base-class.

 OPTION               DEFAULT
 charset              'us-ascii'
 language             C<undef>
 use_continuations    1

=over 4

=item B<charset> =E<gt> STRING

The VALUE is translated from utf-8 (Perl internal) to this character set,
and the resulting string is encoded if required.  C<us-ascii> is the normal
encoding for e-mail.  Valid character sets can be found with 
Encode::encodings(':all').

=item B<language> =E<gt> STRING

RFC2231 adds the possiblity to specify a language with the field.  When no
language is specified, none is included in the encoding.  Valid language names
are defined by RFC2130.  This module has only limited support for this feature.

=item B<use_continuations> =E<gt> BOOLEAN

Continuations are used to break-up long parameters into pieces which are no
longer than 76 characters. Encodings are specified in RFC2231, but not supported
by some Mail User Agents.

=back

Examples:

 my $fn    = Mail::Message::Field::Attribute->new(filename => 'xyz');

 my $fattr = 'Mail::Message::Field::Attribute';  # abbrev
 my $fn    = $fattr->new
     ( filename => "Re\xC7u"
     , charset  => 'iso-8859-15'
     , language => 'nl-BE'
     );
 print $fn;   # print  filename*=iso-8859-15'nl-BE'Re%C7u

=item B<string> 

Returns the parameter as reference to an array of lines.  When only one line
is returned, it may be short enough to fit on the same line with other components
of the header field.

=item B<value> [STRING]

Returns the value of this parameter, optionally after setting it first.

=back

=head2 Reading and Writing [internals]

=over 4

=item B<decode> 

Translate all known continuations into a value.  The produced value is
returned and may be utf-8 encoded or a plain string.

=item B<encode> 

=item B<mergeComponent> ATTRIBUTE

Merge the components from the specified attribute in this attribute.  This is
needed when components of the same attribute are created separately.  Merging
is required by the field parsing.

=back

=head1 SEE ALSO

A good start to read is Mail::Box-Overview.
More documentation and a mailinglist are available from the project's
website at L<http://perl.overmeer.net/mailbox/>.

=head1 AUTHOR

Written by Mark Overmeer (L<mark@overmeer.net|mailto:mark@overmeer.net>)
with the help of many.  See the ChangeLog for details.

=head1 VERSION

This code is beta, version 2.037.

Copyright (c) 2001-2003 by the authors. All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.
