#============================================================= -*-perl-*-
#
# Template::Manual::Internals
#
# DESCRIPTION
#   This document provides an overview of the internal architecture of
#   the Template Toolkit. It is a work in progress and is far from
#   complete, currently providing little more than an overview of how
#   the major components fit together. Nevertheless, it's a good
#   starting point for anyone wishing to delve into the source code to
#   find out how it all works.
#
# AUTHOR
#   Andy Wardley  <abw@kfs.org>
#
# COPYRIGHT
#   Copyright (C) 1996-2001 Andy Wardley.  All Rights Reserved.
#   Copyright (C) 1998-2001 Canon Research Centre Europe Ltd.
#
#   This module is free software; you can redistribute it and/or
#   modify it under the same terms as Perl itself.
#
# REVISION
#   
#
#========================================================================


#------------------------------------------------------------------------
# IMPORTANT NOTE
#   This documentation is generated automatically from source
#   templates.  Any changes you make here may be lost.
# 
#   The 'docsrc' documentation source bundle is available for download
#   from http://www.template-toolkit.org/docs.html and contains all
#   the source templates, XML files, scripts, etc., from which the
#   documentation for the Template Toolkit is built.
#------------------------------------------------------------------------

=head1 NAME

Template::Manual::Internals - Template Toolkit internals

=head1 DESCRIPTION

This document provides an overview of the internal architecture of the
Template Toolkit. It is a work in progress and is far from complete,
currently providing little more than an overview of how the major
components fit together. Nevertheless, it's a good starting point for
anyone wishing to delve into the source code to find out how it all
works.

=head2 Outside Looking In

The B<Template> module is simply a front end module which creates and
uses a Template::Service and pipes the output wherever you want it to
go (STDOUT by default, or maybe a file, scalar, etc).  The
Apache::Template module (available separately from CPAN) is another
front end.  That creates a Template::Service::Apache object, calls on
it as required and sends the output back to the relevant
Apache::Request object.

These front-end modules are really only there to handle any specifics
of the environment in which they're being used.  The Apache::Template
front end, for example, handles Apache::Request specifics and
configuration via the httpd.conf.  The regular Template front-end
deals with STDOUT, variable refs, etc.  Otherwise it is
Template::Service (or subclass) which does all the work.

The B<Template::Service> module provides a high-quality template
delivery service, with bells, whistles, signed up service level
agreement and a 30-day no quibble money back guarantee.  "Have
a good time, all the time", that's our motto.

Within the lower levels of the Template Toolkit, there are lots of
messy details that we generally don't want to have to worry about most
of the time.  Things like templates not being found, or failing to
parse correctly, uncaught exceptions being thrown, missing plugin
modules or dependencies, and so on.  Template::Service hides that all
away and makes everything look simple to the outsider.  It provides
extra features, like PRE_PROCESS, PROCESS and POST_PROCESS, and also
provides the error recovery mechanism via ERROR.  You ask it to
process a template and it takes care of everything for you.  The 
Template::Service::Apache module goes a little bit further, adding 
some extra headers to the Apache::Request, setting a few extra template
variables, and so on.

For the most part, the job of a service is really just one of
scheduling and dispatching.  It receives a request in the form of a
call to its process() method and schedules the named template
specified as an argument, and possibly several other templates
(PRE_PROCESS, etc) to be processed in order.  It doesn't actually
process the templates itself, but instead makes a process() call
against a Template::Context object.

B<Template::Context> is the runtime engine for the Template Toolkit -
the module that hangs everything together in the lower levels of the
Template Toolkit and that one that does most of the real work, albeit
by crafty delegation to various other friendly helper modules.  

Given a template name (or perhaps a reference to a scalar or file
handle) the context process() method must load and compile, or fetch a
cached copy of a previously compiled template, corresponding to that
name.  It does this by calling on a list of one or more
Template::Provider objects (the LOAD_TEMPLATES posse) who themselves
might get involved with a Template::Parser to help turn source
templates into executable Perl code (but more on that later).  Thankfully,
all of this complexity is hidden away behind a simple template()
method.  You call it passing a template name as an argument, and it
returns a compiled template in the form of a Template::Document
object, or otherwise raises an exeception.

A B<Template::Document> is a thin object wrapper around a compiled 
template subroutine.  The object implements a process() method which
performs a little bit of housekeeping and then calls the template 
subroutine.  The object also defines template metadata (defined in 
C<[% META ... %]> directives) and has a block() method which returns
a hash of any additional C<[% BLOCK xxxx %]> definitions found in the 
template source.

So the context fetches a compiled document via its own template()
method and then gets ready to process it.  It first updates the stash
(the place where template variables get defined - more on that
shortly) to set any template variable definitions specified as the
second argument by reference to hash array.  Then, it calls the
document process() method, passing a reference to itself, the context
object, as an argument.  In doing this, it provides itself as an
object against which template code can make callbacks to access
runtime resources and Template Toolkit functionality.

What we're trying to say here is this:  not only does the Template::Context
object receive calls from the I<outside>, i.e. those originating in user
code calling the process() method on a Template object, but it also 
receives calls from the I<inside>, i.e. those originating in template
directives of the form C<[% PROCESS template %]>.

Before we move on to that, here's a simple structure diagram showing
the outer layers of the Template Toolkit heading inwards, with pseudo
code annotations showing a typical invocation sequence.

     ,--------.
     | Caller |	    use Template;
     `--------'     my $tt = Template->new( ... );
          |	    $tt->process($template, \%vars);
          |                                                     Outside
  - - - - | - - - - - - - - - - - - - - - - - - - - - - - - - - - - T T 
	  |         package Template;                            Inside
          V
    +----------+    sub process($template, \%vars) {
    | Template |	$out = $self->SERVICE->process($template, $vars);
    +----------+	print $out or send it to $self->OUTPUT;
          |	    }
          |
          |         package Template::Service;
          |
	  |	    sub process($template, \%vars) {
	  |		try {
    +----------+	    foreach $p in @self->PRE_PROCESS
    | Service  |	        $self->CONTEXT->process($p, $vars);
    +----------+
	  |		    $self->CONTEXT->process($template, $vars);
          |
	  |		    foreach $p @self->POST_PROCESS
	  |			$self->CONTEXT->process($p, $vars);
	  |		}
          |  		catch {
	  |		    $self->CONTEXT->process($self->ERROR);
	  |		}
	  |	    }
          |
          V         package Template::Context;
    +----------+    
    | Context  |    sub process($template, \%vars) {
    +----------+	# fetch compiled template
	  |		$template = $self->template($template)
          |             # update stash
          |	        $self->STASH->update($vars);
          |	        # process template
          |             $template->process($self)
          |         }
          V     
    +----------+    package Template::Document;
    | Document |    
    +----------+    sub process($context) {
			$output = &{ $self->BLOCK }($context);
		    }
        

=head2 Inside Looking Out

To understand more about what's going on in these lower levels, we
need to look at what a compiled template looks like.  In fact, a
compiled template is just a regular Perl sub-routine.  Here's a very
simple one.

    sub my_compiled_template {
	return "This is a compiled template.\n";
    }

You're unlikely to see a compiled template this simple unless you
wrote it yourself but it is entirely valid.  All a template subroutine
is obliged to do is return some output (which may be an empty of
course).  If it can't for some reason, then it should raise an error
via die().

    sub my_todo_template {
	die "This template not yet implemented\n";
    }

If it wants to get fancy, it can raise an error as a
Template::Exception object.  An exception object is really just a
convenient wrapper for the 'type' and 'info' fields.

    sub my_solilique_template {
	die Template::Exception->new('yorrick', 'Fellow of infinite jest');
    }

Templates generally need to do a lot more than just generate static
output or raise errors.  They may want to inspect variable values,
process another template, load a plugin, run a filter, and so on.
Whenever a template subroutine is called, it gets passed a reference
to a Template::Context object.  It is through this context object that
template code can access the features of the Template Toolkit.

We described earlier how the Template::Service object calls on
Template::Context to handle a process() request from the I<outside>.
We can make a similar request on a context to process a template, but
from within the code of another template.  This is a call from the
I<inside>.

    sub my_process_template {
	my $context = shift;

	my $output = $context->process('header', { title => 'Hello World' })
		   . "\nsome content\n"
		   . $context->process('footer');
    }

This is then roughly equivalent to a source template something
like this:

    [% PROCESS header
	title = 'Hello World'
    %]
    some content
    [% PROCESS footer %]

Template variables are stored in, and managed by a B<Template::Stash>
object.  This is a blessed hash array in which template variables are
defined.  The object wrapper provides get() and set() method which
implement all the magical.variable.features of the Template Toolkit.

Each context object has its own stash, a reference to which can be
returned by the appropriately named stash() method.  So to print the
value of some template variable, or for example, to represent the
following source template:

    <title>[% title %]</title>

we might have a subroutine definition something like this:

    sub {
	my $context = shift;
	my $stash = $context->stash();
	return '<title>' . $stash->get('title') . '</title>';
    }

The stash get() method hides the details of the underlying variable
types, automatically calling code references, checking return values,
and performing other such tricks.  If 'title' happens to be bound to a
subroutine then we can specify additional parameters as a list
reference passed as the second argument to get().

    [% title('The Cat Sat on the Mat') %]

This translates to the stash get() call:

    $stash->get('title', ['The Cat Sat on the Mat']);

Dotted compound variables can be requested by passing a single 
list reference to the get() method in place of the variable 
name.  Each pair of elements in the list should correspond to the
variable name and reference to a list of arguments for each 
dot-delimited element of the variable.

    [% foo(1, 2).bar(3, 4).baz(5) %]

is thus equivalent to

    $stash->get([ foo => [1,2], bar => [3,4], baz => [5] ]);

If there aren't any arguments for an element, you can specify an 
empty, zero or null argument list.

    [% foo.bar %]
    $stash->get([ 'foo', 0, 'bar', 0 ]);

The set() method works in a similar way.  It takes a variable 
name and a variable value which should be assigned to it.

    [% x = 10 %]         
    $stash->set('x', 10);

    [% x.y = 10 %]
    $stash->set([ 'x', 0, 'y', 0 ], 10);

So the stash gives us access to template variables and the context
provides the higher level functionality.  Alongside the process()
method lies the include() method.  Just as with the PROCESS / INCLUDE
directives, the key difference is in variable localisation.  Before
processing a template, the process() method simply updates the stash
to set any new variable definitions, overwriting any existing values.
In contrast, the include() method creates a copy of the existing
stash, in a process known as I<cloning> the stash, and then uses that
as a temporary variable store.  Any previously existing variables are
still defined, but any changes made to variables, including setting
the new variable values passed aas arguments will affect only the
local copy of the stash (although note that it's only a shallow copy,
so it's not foolproof).  When the template has been processed, the include()
method restores the previous variable state by I<decloning> the stash.

The context also provides an insert() method to implement the INSERT 
directive, but no wrapper() method.  This functionality can be implemented
by rewriting the Perl code and calling include().

    [% WRAPPER foo -%]
       blah blah [% x %]
    [%- END %]

    $context->include('foo', {
	content => 'blah blah ' . $stash->get('x'),
    });

Other than the template processing methods process(), include() and insert(),
the context defines methods for fetching plugin objects, plugin(), and 
filters, filter().

    [% USE foo = Bar(10) %]

    $stash->set('foo', $context->plugin('Bar', [10]));

    [% FILTER bar(20) %]
       blah blah blah
    [% END %]

    my $filter = $stash->filter('bar', [20]);
    &$filter('blah blah blah');

Pretty much everything else you might want to do in a template can be done
in Perl code.  Things like IF, UNLESS, FOREACH and so on all have direct
counterparts in Perl.

    [% IF msg %]
       Message: [% msg %]
    [% END %];

    if ($stash->get('msg')) {
	$output .=  'Message: ';
	$output .= $stash->get('msg');
    }

B<### WARNING: THIS IS THE ABRUPT END OF THE DOCUMENTATION ###>

=head1 AUTHOR

Andy Wardley E<lt>abw@kfs.orgE<gt>

L<http://www.andywardley.com/|http://www.andywardley.com/>

=head1 VERSION

Template Toolkit version 2.01, released on 30th March 2001.

=head1 COPYRIGHT

  Copyright (C) 1996-2001 Andy Wardley.  All Rights Reserved.
  Copyright (C) 1998-2001 Canon Research Centre Europe Ltd.

This module is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.



