=head1 NAME

Mail::Message::Head::FieldGroup - a sub set of fields in a header

=head1 INHERITANCE

 Mail::Message::Head::FieldGroup
   is a Mail::Reporter

 Mail::Message::Head::FieldGroup is extended by
   Mail::Message::Head::ListGroup
   Mail::Message::Head::ResentGroup
   Mail::Message::Head::SpamGroup

=head1 SYNOPSIS

Never instantiated directly.

=head1 DESCRIPTION

Some fields have a combined meaning: a set of fields which represent
one intermediate step during the transport of the message (a
I<resent group>, implemented in L<Mail::Message::Head::ResentGroup|Mail::Message::Head::ResentGroup>), 
fields added by mailing list software (implemented in
L<Mail::Message::Head::ListGroup|Mail::Message::Head::ListGroup>), or fields added by Spam detection
related software (implemented by L<Mail::Message::Head::SpamGroup|Mail::Message::Head::SpamGroup>).
Each set of fields can be extracted or added as group with objects
which are based on the implementation in this class.

=head1 METHODS

=head2 Constructors

=over 4

=item $obj-E<gt>B<clone>()

Make a copy of this object.  The collected fieldnames are copied and the
list type information.  No deep copy is made for the header: this is
only copied as reference.

=item $obj-E<gt>B<from>(HEAD|MESSAGE)

Create a group of fields based on the specified MESSAGE or message HEAD.
This may return one or more of the objects, which depends on the
type of group.  Mailing list fields are all stored in one object,
where resent and spam groups can appear more than once.

=item $obj-E<gt>B<implementedTypes>()

=item Mail::Message::Head::FieldGroup-E<gt>B<implementedTypes>()

Returns a list of strings containing all possible return values for
L<type()|Mail::Message::Head::FieldGroup/"Access to the header">.

=item Mail::Message::Head::FieldGroup-E<gt>B<new>(FIELDS, OPTIONS)

Construct an object which maintains one set of header FIELDS.  The
FIELDS may be specified as C<Mail::Message::Field> objects or as key-value
pairs.  The OPTIONS and FIELDS (as key-value pair) can be mixed: they are
distinguished by their name, where the fields always start with a capital.
The field objects must aways lead the OPTIONS.

 -Option  --Defined in     --Default
  head                       undef
  log       Mail::Reporter   'WARNINGS'
  software                   undef
  trace     Mail::Reporter   'WARNINGS'
  type                       undef
  version                    undef

=over 2

=item head => HEAD

The header HEAD object is used to store the grouped fields in.
If no header is specified, a L<Mail::Message::Head::Partial|Mail::Message::Head::Partial> is created
for you.  If you wish to scan the existing fields in a header, then use
the L<from()|Mail::Message::Head::FieldGroup/"Constructors"> method.

=item log => LEVEL

=item software => STRING

Name of the software which produced the fields.

=item trace => LEVEL

=item type => STRING

Group name for the fields.  Often the same, or close
to the same STRING, as the C<software> option contains.

=item version => STRING

Version number for the software which produced the fields.

=back

=back

=head2 The header

=over 4

=item $obj-E<gt>B<add>((FIELD, VALUE) | OBJECT)

Add a field to the header, using the field group.  When the field group
is already attached to a real message header, it will appear in that
one as well as being registed in this set.  If no header is defined,
the field only appears internally.

example: adding a field to a detached list group

 my $this = Mail::Message::Head::ListGroup->new(...);
 $this->add('List-Id' => 'mailbox');
 $msg->addListGroup($this);
 $msg->send;

example: adding a field to an attached list group

 my $lg = Mail::Message::Head::ListGroup->from($msg);
 $lg->add('List-Id' => 'mailbox');

=item $obj-E<gt>B<addFields>([FIELDNAMES])

Add some FIELDNAMES to the set.

=item $obj-E<gt>B<attach>(HEAD)

Add a group of fields to a message HEAD.  The fields will be cloned(!)
into the header, so that the field group object can be used again.

example: attaching a list group to a message

 my $lg = Mail::Message::Head::ListGroup->new(...);
 $lg->attach($msg->head);
 $msg->head->addListGroup($lg);   # same

 $msg->head->addSpamGroup($sg);   # also implemented with attach

=item $obj-E<gt>B<delete>()

Remove all the header lines which are combined in this fields group,
from the header.

=item $obj-E<gt>B<fieldNames>()

Return the names of the fields which are used in this group.

=item $obj-E<gt>B<fields>()

Return the fields which are defined for this group.

=item $obj-E<gt>B<head>()

Returns the header object, which includes these fields.

=back

=head2 Access to the header

=over 4

=item $obj-E<gt>B<software>()

Returns the name of the software as is defined in the headers.  The may
be slightly different from the return value of L<type()|Mail::Message::Head::FieldGroup/"Access to the header">, but usually
not too different.

=item $obj-E<gt>B<type>()

Returns an abstract name for the field group; which software is
controling it.  C<undef> is returned in case the type is not known.
Valid names are group type dependent: see the applicable manual
pages.  A list of all types can be retreived with L<implementedTypes()|Mail::Message::Head::FieldGroup/"Constructors">.

=item $obj-E<gt>B<version>()

Returns the version number of the software used to produce the fields.
Some kinds of software do leave such a trace, other cases will return
C<undef>

=back

=head2 Internals

=over 4

=item $obj-E<gt>B<collectFields>([NAME])

Scan the header for fields which are usually contained in field group
with the specified NAME.  For mailinglist groups, you can not specify
a NAME: only one set of headers will be found (all headers are considered
to be produced by exactly one package of mailinglist software).

This method is automatically called when a field group is
constructed via L<from()|Mail::Message::Head::FieldGroup/"Constructors"> on an existing header or message.

Returned are the names of the list header fields found, in scalar
context the amount of fields.  An empty list/zero indicates that there
was no group to be found.

Please warn the author of MailBox if you see that to few
or too many fields are included.

=item $obj-E<gt>B<detected>(TYPE, SOFTWARE, VERSION)

Sets the values for the field group type, software, and version,
prossibly to C<undef>.

=back

=head2 Error handling

=over 4

=item $obj-E<gt>B<AUTOLOAD>()

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<addReport>(OBJECT)

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<defaultTrace>([LEVEL]|[LOGLEVEL, TRACELEVEL]|[LEVEL, CALLBACK])

=item Mail::Message::Head::FieldGroup-E<gt>B<defaultTrace>([LEVEL]|[LOGLEVEL, TRACELEVEL]|[LEVEL, CALLBACK])

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<details>()

Produce information about the detected/created field group, which may be
helpful during debugging.  A nicely formatted string is returned.

=item $obj-E<gt>B<errors>()

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<log>([LEVEL [,STRINGS]])

=item Mail::Message::Head::FieldGroup-E<gt>B<log>([LEVEL [,STRINGS]])

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<logPriority>(LEVEL)

=item Mail::Message::Head::FieldGroup-E<gt>B<logPriority>(LEVEL)

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<logSettings>()

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<notImplemented>()

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<print>([FILEHANDLE])

Print the group to the specified FILEHANDLE or GLOB.  This is probably only
useful for debugging purposed.  The output defaults to the selected file
handle.

=item $obj-E<gt>B<report>([LEVEL])

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<reportAll>([LEVEL])

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<trace>([LEVEL])

See L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<warnings>()

See L<Mail::Reporter/"Error handling">

=back

=head2 Cleanup

=over 4

=item $obj-E<gt>B<DESTROY>()

See L<Mail::Reporter/"Cleanup">

=item $obj-E<gt>B<inGlobalDestruction>()

See L<Mail::Reporter/"Cleanup">

=back

=head1 DIAGNOSTICS

=over 4

=item Error: Package $package does not implement $method.

Fatal error: the specific package (or one of its superclasses) does not
implement this method where it should. This message means that some other
related classes do implement this method however the class at hand does
not.  Probably you should investigate this and probably inform the author
of the package.

=back

=head1 SEE ALSO

This module is part of Mail-Box distribution version 2.106,
built on August 15, 2012. Website: F<http://perl.overmeer.net/mailbox/>

=head1 LICENSE

Copyrights 2001-2012 by [Mark Overmeer]. For other contributors see ChangeLog.

This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
See F<http://www.perl.com/perl/misc/Artistic.html>

