=encoding utf8

=head1 NAME

XML::Compile::Translate::Writer - translate HASH to XML

=head1 INHERITANCE

 XML::Compile::Translate::Writer
   is a XML::Compile::Translate

=head1 SYNOPSIS

 my $schema = XML::Compile::Schema->new(...);
 my $code   = $schema->compile(WRITER => ...);

=head1 DESCRIPTION

The translator understands schemas, but does not encode that into
actions.  This module implements those actions to translate from
a (nested) Perl HASH structure onto XML.

Extends L<"DESCRIPTION" in XML::Compile::Translate|XML::Compile::Translate/"DESCRIPTION">.
 
=head1 METHODS

Extends L<"METHODS" in XML::Compile::Translate|XML::Compile::Translate/"METHODS">.
 
=head1 DETAILS

Extends L<"DETAILS" in XML::Compile::Translate|XML::Compile::Translate/"DETAILS">.
 
=head2 Translator options

Extends L<"Translator options" in XML::Compile::Translate|XML::Compile::Translate/"Translator options">.
 
=head2 Processing Wildcards

Complex elements can define C<any> (element) and C<anyAttribute> components,
with unpredictable content.  In this case, you are quite on your own in
processing those constructs.  The use of both schema components should
be avoided: please specify your data-structures explicit by clean type
extensions.

The procedure for the WRITER is simple: add key-value pairs to your
hash, in which the value is a fully prepared XML::LibXML::Attr
or XML::LibXML::Element.  The keys have the form C<{namespace}type>.
The I<namespace> component is important, because only spec conformant
namespaces will be used. The elements and attributes are added in
random order.

=head2 Mixed elements

[0.79] ComplexType and ComplexContent components can be declared with the
C<<mixed="true">> attribute.

XML::Compile does not have a way to express these mixtures of information
and text as Perl data-structures; the only way you can use those to the
full extend, is by juggling with XML::LibXML nodes yourself.

You may provide a XML::LibXML::Element, which is complete, or a
HASH which contains attributes values and an XML node with key '_'.
When '_' contains a string, it will be translated into an XML text
node.

L<XML::Compile::Schema::compile(mixed_elements)|XML::Compile::Schema/"Compilers"> can be set to

=over 4

=item ATTRIBUTES (default)

Add attributes to the provided node.  When you provide a HASH, it is
taken as node content with attributes.  The content has to be stored
with key '_'.  When it is not a HASH, the data is node content.

There are various ways you can specify content.  Up to [1.51], you
could only pass a matching XML::LibXML::Element.  Release [1.51] added
strings to the spectrum.  If the string does not contain encoded entities
or E<lt> and E<gt>, then it is assumed to be a real perl string.  When
the string contains an XML fragment which has the same localname as to be
created, that will be used.  When the XML fragment is not wrapped in the
expected node, this is created for you.

In any case, attributes provided with the content will get added to the
content data.

=item STRUCTURAL

[0.89] behaves as if the attribute is not there: a data-structure can be
used or an XML node.

=back

=head2 Schema hooks

All writer hooks behave differently.  Be warned that the user values
can be a SCALAR or a HASH, dependent on the type.  You can intervene
on higher data-structure levels, to repair lower levels, if you want
to.

[1.48] The hooks get a long list of parameters.  The C<$fulltype>
indicates the type of object which is being processed, which is
especially useful with the 'extends' selector.

=head3 hooks executed before normal processing

The C<before> hook gives you the opportunity to fix the user
supplied data structure.  The XML generator will complain about
missing, superfluous, and erroneous values which you probably
want to avoid.

The C<before> hook returns new values.  Just must not interfere
with the user provided data.  When C<undef> is returned, the whole
node will be cancelled.

On the moment, the only predefined C<before> hook is C<PRINT_PATH>.

=head3 hooks replacing the usual XML node generation

Only one C<replace> hook can be defined.  It must return a
XML::LibXML::Node or C<undef>.  The hook must use the
C<XML::LibXML::Document> node (which is provided as first
argument) to create a node.

As parameters, the called replace function will receive the
document, user-provided values, location in the data tree (for
error messages), the tag of the node with prefix attached, and
a reference to the code which would be executed if the replace
hook had not been active.

On the moment, the only predefined C<replace> hook is C<SKIP>.

=head3 hooks executed after the node was created

The C<after> hooks, will each get a chance to modify the
produced XML node, for instance to encapsulate it.  Each time,
the new XML node has to be returned.

On the moment, the only predefined C<after> hook is C<PRINT_PATH>.

=head3 fixing bad schemas

When a schema makes a mess out of things, we can fix that with hooks.
Also, when you need things that XML::Compile does not support (yet).

=head2 Typemaps

In a typemap, a relation between an XML element type and a Perl class (or
object) is made.  Each translator back-end will implement this a little
differently.  This section is about how the writer handles typemaps.

=head3 Typemap to Class

Usually, an XML type will be mapped on a Perl class.  The Perl class
implements the C<toXML> method as serializer.  That method should
either return a data structure which fits that of the specific type,
or an XML::LibXML::Element.

When translating the data-structure to XML, the process may encounter
objects.  Only if these objects appear at locations where a typemap
is defined, they are treated smartly.  When some other data than an
objects is found on a location which has a typemap definition, it will
be used as such; objects are optional.

The object (if present) will be checked to be of the expected class.
It will be a compile-time error when the class does not implement the
C<toXML> method.

 $schema->typemap($sometype => 'My::Perl::Class');

 package My::Perl::Class;
 ...
 sub toXML
 {   my ($self, $xmltype, $doc) = @_;
     ...
     { a => { b => 42 }, c => 'aaa' };
 }

The C<$self> is the object found in the data-structure provided by the
user.  C<$doc> can be used to create your own XML::LibXML::Element.
It is possible to use the same object on locations for different types:
in this case, the toXML method can distiguisk what kind of data to return
based on the C<$xmltype>.

=head3 Typemap to Object

In this case, some helper object arranges the serialization of the
provided object.  This is especially useful when the provided object
does not have the toXML implemented, for instance because it is an
implementation not under your control.  The helper object works like
an interface.

 my $object = My::Perl::Class->new(...);
 $schema->typemap($sometype => $object);

 package My::Perl::Class;
 sub toXML
 {   my ($self, $object, $xmltype, $doc) = @_;
     ...
 }

The toXML will only be called then C<$object> is blessed.  If you wish
to have access to some data-type in any case, then use a simple "before"
hook.

=head3 Typemap to CODE

The light version of an interface object uses CODE references.  The CODE
reference is only called if a blessed value is found in the user provided
data.  It cannot be checked automatically whether it is blessed according
to the expectation.

 $schema->typemap($t1 => \&myhandler);

 sub myhandler
 {   my ($backend, $object, $xmltype, $doc) = @_;
     ...
 }

=head3 Typemap implementation

The typemap for the writer is implemented as a 'before' hook: just before
the writer wants to start.

Of course, it could have been implemented by accepting an object anywhere
in the input data.  However, this would mean that all the (many) internal
parser constructs would need to be extended.  That would slow-down the
writer considerably.

=head1 SEE ALSO

This module is part of XML-Compile distribution version 1.56,
built on March 30, 2017. Website: F<http://perl.overmeer.net/xml-compile/>

Please post questions or ideas to the mailinglist at
F<http://lists.scsys.co.uk/cgi-bin/mailman/listinfo/xml-compile> .
For live contact with other developers, visit the C<#xml-compile> channel
on C<irc.perl.org>.

=head1 LICENSE

Copyrights 2006-2017 by [Mark Overmeer]. For other contributors see ChangeLog.

This program is free software; you can redistribute it and/or modify it
under the Artistic license.
See F<http://dev.perl.org/licenses/artistic.html>