=head1 NAME

Template::Context - runtime context in which templates are processed

=head1 SYNOPSIS

    use Template::Context;

    # constructor
    $context = Template::Context->new(\%config)
	|| die $Template::Context::ERROR;

    # fetch (load and compile) a template
    $template = $context->template($template_name);

    # fetch (load and instantiate) a plugin object
    $plugin = $context->plugin($name, \@args);

    # fetch (return or create) a filter subroutine
    $filter = $context->filter($name, \@args, $alias);

    # process/include a template, errors are thrown via die()
    $output = $context->process($template, \%vars);
    $output = $context->include($template, \%vars);

    # raise an exception via die()
    $context->throw($error_type, $error_message, \$output_buffer);

    # catch an exception, clean it up and fix output buffer
    $exception = $context->catch($exception, \$output_buffer);

    # save/restore the stash to effect variable localisation
    $new_stash = $context->localise(\%vars);
    $old_stash = $context->delocalise();

    # add new BLOCK or FILTER definitions
    $context->define_block($name, $block);
    $context->define_filter($name, \&filtersub, $is_dynamic);

    # reset context, clearing any imported BLOCK definitions
    $context->reset();

    # methods for accessing internal items
    $stash     = $context->stash();
    $tflag     = $context->trim();
    $epflag    = $context->eval_perl();
    $providers = $context->templates();
    $providers = $context->plugins();
    $providers = $context->filters();
    ...

=head1 DESCRIPTION

The Template::Context module defines an object class for representing
a a runtime context in which templates are processed.  It provides an
interface to the fundamental operations of the Template Toolkit
processing engine through which compiled templates (i.e. Perl code
constructed from the template source) can process templates, load
plugins and filters, raise exceptions and so on.

A default Template::Context object is created by the Template module.
Any Template::Context options may be passed to the Template new()
constructor method and will be forwarded to the Template::Context
constructor.

    use Template;
    
    my $template = Template->new({
	TRIM      => 1,
	EVAL_PERL => 1,
	BLOCKS    => {
	    header => 'This is the header',
	    footer => 'This is the footer',
	},
    });

Similarly, the Template::Context constructor will forward all configuration
parameters onto other default objects (e.g. Template::Provider, Template::Plugins,
Template::Filters, etc.) that it may need to instantiate.

    $context = Template::Context->new({
	INCLUDE_PATH => '/home/abw/templates', # provider option
	TAG_STYLE    => 'html',                # parser option
    });

A Template::Context object (or subclass/derivative) can be explicitly
instantiated and passed to the Template new() constructor method as 
the CONTEXT item.

    use Template;
    use Template::Context;

    my $context  = Template::Context->new({ TRIM => 1 });
    my $template = Template->new({ CONTEXT => $context });

The Template module uses the Template::Config context() factory method
to create a default context object when required.  The
$Template::Config::CONTEXT package variable may be set to specify an
alternate context module.  This will be loaded automatically and its
new() constructor method called by the context() factory method when
a default context object is required.

    use Template;

    $Template::Config::CONTEXT = 'MyOrg::Template::Context';

    my $template = Template->new({
	EVAL_PERL   => 1,
	EXTRA_MAGIC => 'red hot',  # your extra config items
	...
    });

=head1 METHODS

=head2 new(\%params) 

The new() constructor method is called to instantiate a Template::Context
object.  Configuration parameters may be specified as a HASH reference or
as a list of (name => value) pairs.

    my $context = Template::Context->new({
	INCLUDE_PATH => 'header',
	POST_PROCESS => 'footer',
    });

    my $context = Template::Context->new( EVAL_PERL => 1 );

The new() method returns a Template::Context object (or sub-class) or
undef on error.  In the latter case, a relevant error message can be
retrieved by the error() class method or directly from the
$Template::Context::ERROR package variable.

    my $context = Template::Context->new(\%config)
	|| die Template::Context->error();

    my $context = Template::Context->new(\%config)
	|| die $Template::Context::ERROR;

The following configuration items may be specified.

=over 4


=item VARIABLES, PRE_DEFINE

The VARIABLES option (or PRE_DEFINE - they're equivalent) can be used
to specify a hash array of template variables that should be used to
pre-initialise the stash when it is created.  These items are ignored
if the STASH item is defined.

    my $context = Template::Context->new({
	VARIABLES => {
	    title   => 'A Demo Page',
	    author  => 'Joe Random Hacker',
	    version => 3.14,
	},
    };

or

    my $context = Template::Context->new({
	PRE_DEFINE => {
	    title   => 'A Demo Page',
	    author  => 'Joe Random Hacker',
	    version => 3.14,
	},
    };

=item BLOCKS

The BLOCKS option can be used to pre-define a default set of template 
blocks.  These should be specified as a reference to a hash array 
mapping template names to template text, subroutines or Template::Document
objects.

    my $context = Template::Context->new({
	BLOCKS => {
	    header  => 'The Header.  [% title %]',
	    footer  => sub { return $some_output_text },
	    another => Template::Document->new({ ... }),
	},
    });

=item TRIM

The TRIM option can be set to have any leading and trailing whitespace 
automatically removed from the output of all template files and BLOCKs.

By example, the following BLOCK definition

    [% BLOCK foo %]
    Line 1 of foo
    [% END %]

will be processed is as "\nLine 1 of foo\n".  When INCLUDEd, the surrounding
newlines will also be introduced.

    before 
    [% INCLUDE foo %]
    after

output:
    before

    Line 1 of foo

    after

With the TRIM option set to any true value, the leading and trailing
newlines (which count as whitespace) will be removed from the output 
of the BLOCK.

    before
    Line 1 of foo
    after

The TRIM option is disabled (0) by default.

=item EVAL_PERL

This flag is used to indicate if PERL and/or RAWPERL blocks should be
evaluated.  By default, it is disabled and any PERL or RAWPERL blocks
encountered will raise exceptions of type 'perl' with the message
'EVAL_PERL not set'.  Note however that any RAWPERL blocks should
always contain valid Perl code, regardless of the EVAL_PERL flag.  The
parser will fail to compile templates that contain invalid Perl code
in RAWPERL blocks and will throw a 'file' exception.

When using compiled templates (see L<COMPILE_EXT> and L<COMPILE_DIR>),
the EVAL_PERL has an affect when the template is compiled, and again
when the templates is subsequently processed, possibly in a different
context to the one that compiled it.

If the EVAL_PERL is set when a template is compiled, then all PERL and
RAWPERL blocks will be included in the compiled template.  If the 
EVAL_PERL option isn't set, then Perl code will be generated which 
B<always> throws a 'perl' exception with the message 'EVAL_PERL not
set' B<whenever> the compiled template code is run.

Thus, you must have EVAL_PERL set if you want your compiled templates
to include PERL and RAWPERL blocks.

At some point in the future, using a different invocation of the
Template Toolkit, you may come to process such a pre-compiled
template.  Assuming the EVAL_PERL option was set at the time the
template was compiled, then the output of any RAWPERL blocks will be
included in the compiled template and will get executed when the
template is processed.  This will happen regardless of the runtime
EVAL_PERL status.

Regular PERL blocks are a little more cautious, however.  If the 
EVAL_PERL flag isn't set for the I<current> context, that is, the 
one which is trying to process it, then it will throw the familiar 'perl'
exception with the message, 'EVAL_PERL not set'.

Thus you can compile templates to include PERL blocks, but optionally
disable them when you process them later.  Note however that it is 
possible for a PERL block to contain a Perl "BEGIN { # some code }"
block which will always get run regardless of the runtime EVAL_PERL
status.  Thus, if you set EVAL_PERL when compiling templates, it is
assumed that you trust the templates to Do The Right Thing.  Otherwise
you must accept the fact that there's no bulletproof way to prevent 
any included code from trampling around in the living room of the 
runtime environment, making a real nuisance of itself if it really
wants to.  If you don't like the idea of such uninvited guests causing
a bother, then you can accept the default and keep EVAL_PERL disabled.

=item RECURSION

The template processor will raise a file exception if it detects
direct or indirect recursion into a template.  Setting this option to 
any true value will allow templates to include each other recursively.

=item LOAD_TEMPLATES

The LOAD_TEMPLATE option can be used to provide a reference to a list
of Template::Provider objects or sub-classes thereof which will take
responsibility for loading and compiling templates.

    my $context = Template::Context->new({
	LOAD_TEMPLATES => [
    	    MyOrg::Template::Provider->new({ ... }),
    	    Template::Provider->new({ ... }),
	],
    });

When a PROCESS, INCLUDE or WRAPPER directive is encountered, the named
template may refer to a locally defined BLOCK or a file relative to
the INCLUDE_PATH (or an absolute or relative path if the appropriate
ABSOLUTE or RELATIVE options are set).  If a BLOCK definition can't be
found (see the Template::Context template() method for a discussion of
BLOCK locality) then each of the LOAD_TEMPLATES provider objects is
queried in turn via the fetch() method to see if it can supply the
required template.  Each provider can return a compiled template, an
error, or decline to service the request in which case the
responsiblity is passed to the next provider.  If none of the
providers can service the request then a 'not found' error is
returned.  The same basic provider mechanism is also used for the 
INSERT directive but it bypasses any BLOCK definitions and doesn't
attempt is to parse or process the contents of the template file.

This is an implementation of the 'Chain of Responsiblity'
design pattern as described in 
"Design Patterns", Erich Gamma, Richard Helm, Ralph Johnson, John 
Vlissides), Addision-Wesley, ISBN 0-201-63361-2, page 223.

If LOAD_TEMPLATES is undefined, a single default provider will be
instantiated using the current configuration parameters.  For example,
the Template::Provider INCLUDE_PATH option can be specified in the Template::Context configuration and will be correctly passed to the provider's
constructor method.

    my $context = Template::Context->new({
	INCLUDE_PATH => '/here:/there',
    });

=item LOAD_PLUGINS

The LOAD_PLUGINS options can be used to specify a list of provider
objects (i.e. they implement the fetch() method) which are responsible
for loading and instantiating template plugin objects.  The
Template::Content plugin() method queries each provider in turn in a
"Chain of Responsibility" as per the template() and filter() methods.

    my $context = Template::Context->new({
	LOAD_PLUGINS => [
    	    MyOrg::Template::Plugins->new({ ... }),
    	    Template::Plugins->new({ ... }),
	],
    });

By default, a single Template::Plugins object is created using the 
current configuration hash.  Configuration items destined for the 
Template::Plugins constructor may be added to the Template::Context 
constructor.

    my $context = Template::Context->new({
	PLUGIN_BASE => 'MyOrg::Template::Plugins',
	LOAD_PERL   => 1,
    });

=item LOAD_FILTERS

The LOAD_FILTERS option can be used to specify a list of provider
objects (i.e. they implement the fetch() method) which are responsible
for returning and/or creating filter subroutines.  The
Template::Context filter() method queries each provider in turn in a
"Chain of Responsibility" as per the template() and plugin() methods.

    my $context = Template::Context->new({
	LOAD_FILTERS => [
    	    MyTemplate::Filters->new(),
    	    Template::Filters->new(),
	],
    });

By default, a single Template::Filters object is created for the
LOAD_FILTERS list.

=item STASH

A reference to a Template::Stash object or sub-class which will take
responsibility for managing template variables.  

    my $stash = MyOrg::Template::Stash->new({ ... });
    my $context = Template::Context->new({
	STASH => $stash,
    });

If unspecified, a default stash object is created using the VARIABLES
configuration item to initialise the stash variables.  These may also
be specified as the PRE_DEFINE option for backwards compatibility with 
version 1.

    my $context = Template::Context->new({
	VARIABLES => {
	    id    => 'abw',
	    name  => 'Andy Wardley',
	},
    };



=back

=head2 template($name) 

Returns a compiled template by querying each of the LOAD_TEMPLATES providers
(instances of Template::Provider, or sub-class) in turn.  

    $template = $context->template('header');

On error, a Template::Exception object of type 'file' is thrown via
die().  This can be caught by enclosing the call to template() in an
eval block and examining $@.

    eval {
	$template = $context->template('header');
    };
    if ($@) {
	print "failed to fetch template: $@\n";
    }

=head2 plugin($name, \@args)

Instantiates a plugin object by querying each of the LOAD_PLUGINS
providers.  The default LOAD_PLUGINS provider is a Template::Plugins
object which attempts to load plugin modules, according the various
configuration items such as PLUGIN_BASE, LOAD_PERL, etc., and then
instantiate an object via new().  A reference to a list of constructor
arguments may be passed as the second parameter.  These are forwarded
to the plugin constructor.

Returns a reference to a plugin (which is generally an object, but
doesn't have to be).  Errors are thrown as Template::Exception objects
of type 'plugin'.

    $plugin = $context->plugin('DBI', 'dbi:msql:mydbname');

=head2 filter($name, \@args, $alias)

Instantiates a filter subroutine by querying the LOAD_FILTERS providers.
The default LOAD_FILTERS providers is a Template::Filters object.
Additional arguments may be passed by list reference along with an
optional alias under which the filter will be cached for subsequent
use.  The filter is cached under its own $name if $alias is undefined.
Subsequent calls to filter($name) will return the cached entry, if
defined.  Specifying arguments bypasses the caching mechanism and
always creates a new filter.  Errors are thrown as Template::Exception
objects of typre 'filter'.

    # static filter (no args)
    $filter = $context->filter('html');

    # dynamic filter (args) aliased to 'padright'
    $filter = $context->filter('format', '%60s', 'padright');

    # retrieve previous filter via 'padright' alias
    $filter = $context->filter('padright');

=head2 process($template, \%vars)

Processes a template named or referenced by the first parameter and returns
the output generated.  An optional reference to a hash array may be passed
as the second parameter, containing variable definitions which will be set
before the template is processed.  The template is processed in the current
context, with no localisation of variables performed.   Errors are thrown
as Template::Exception objects via die().  

    $output = $context->process('header', { title => 'Hello World' });

=head2 include($template, \%vars)

Similar to process() above, but using localised variables.  Changes made to
any variables will only persist until the include() method completes.

    $output = $context->include('header', { title => 'Hello World' });

=head2 throw($error_type, $error_message, \$output)

Raises an exception in the form of a Template::Exception object by
calling die().  This method may be passed a reference to an existing
Template::Exception object; a single value containing an error message
which is used to instantiate a Template::Exception of type 'undef'; or
a pair of values representing the exception type and info from which a
Template::Exception object is instantiated.  e.g.

    $context->throw($exception);
    $context->throw("I'm sorry Dave, I can't do that");
    $context->throw('denied', "I'm sorry Dave, I can't do that");

The optional third parameter may be a reference to the current output
buffer.  This is then stored in the exception object when created,
allowing the catcher to examine and use the output up to the point at
which the exception was raised.

    $output .= 'blah blah blah';
    $output .= 'more rhubarb';
    $context->throw('yack', 'Too much yacking', \$output);

=head2 catch($exception, \$output)

Catches an exception thrown, either as a reference to a
Template::Exception object or some other value.  In the latter case,
the error string is promoted to a Template::Exception object of
'undef' type.  This method also accepts a reference to the current
output buffer which is passed to the Template::Exception constructor,
or is appended to the output buffer stored in an existing
Template::Exception object, if unique (i.e. not the same reference).
By this process, the correct state of the output buffer can be
reconstructed for simple or nested throws.

=head2 define_block($name, $block)

Adds a new block definition to the internal BLOCKS cache.  The first 
argument should contain the name of the block and the second a reference
to a Template::Document object or template sub-routine, or template text
which is automatically compiled into a template sub-routine.  Returns
a true value (the sub-routine or Template::Document reference) on 
success or undef on failure.  The relevant error message can be 
retrieved by calling the error() method.

=head2 define_filter($name, \&filter, $is_dynamic)

Adds a new filter definition by calling the store() method on each of
the LOAD_FILTERS providers until accepted (in the usual case, this is
accepted straight away by the one and only Template::Filters
provider).  The first argument should contain the name of the filter
and the second a reference to a filter subroutine.  The optional 
third argument can be set to any true value to indicate that the 
subroutine is a dynamic filter factory.  Returns a true value or
throws a 'filter' exception on error.

=head2 localise(\%vars)

Clones the stash to create a context with localised variables.  Returns a 
reference to the newly cloned Template::Stash object which is also stored
internally.

    $stash = $context->localise();

=head2 delocalise()

Restore the stash to its state prior to localisation.

    $stash = $context->delocalise();

=head2 visit(\%blocks)

This method is called by Template::Document objects immediately before
they process their content.  It is called to register any local BLOCK
definitions with the context object so that they may be subsequently
delivered on request.

=head2 leave()

Compliment to visit(), above.  Called by Template::Document objects 
immediately after they process their content.

=head2 reset()

Clears the local BLOCKS cache of any BLOCK definitions.  Any initial set of
BLOCKS specified as a configuration item to the constructor will be reinstated.

=head2 AUTOLOAD

An AUTOLOAD method provides access to context configuration items.

    $stash     = $context->stash();
    $tflag     = $context->trim();
    $epflag    = $context->eval_perl();
    ...

=head1 AUTHOR

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

    http://www.template-toolkit.org/
    http://www.kfs.org/~abw/

=head1 REVISION

$Revision: 1.4 $

=head1 COPYRIGHT

    Copyright (C) 1996-2000 Andy Wardley.  All Rights Reserved.
    Copyright (C) 1998-2000 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.


=head1 SEE ALSO


L<Template|Template>, 
L<Template::Document|Template::Document>, 
L<Template::Exception|Template::Exception>, 
L<Template::Filters|Template::Filters>, 
L<Template::Plugins|Template::Plugins>, 
L<Template::Provider|Template::Provider>, 
L<Template::Service|Template::Service>, 
L<Template::Stash|Template::Stash>