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

NAME

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

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();
    ...

DESCRIPTION

The Template::Context module defines an object class for representing 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
        ...
    });

METHODS

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.

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,
        },
    };

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({ ... }),
        },
    });

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.

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 COMPILE_EXT and 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 always throws a 'perl' exception with the message 'EVAL_PERL not set' 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 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.

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.

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 responsibility 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 Responsibility' 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',
    });

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,
    });

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.

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',
        },
    };

DEBUG
The DEBUG option can be used to enable various debugging features of the Template::Context module. 
    use Template::Constants qw( :debug );
    my $template = Template->new({
        DEBUG => DEBUG_CONTEXT | DEBUG_DIRS,
    });

The DEBUG value can include any of the following. Multiple values should be combined using the logical OR operator, '|'.

DEBUG_CONTEXT
Enables general debugging messages for the Template::Context module.

DEBUG_DIRS
This option causes the Template Toolkit to generate comments indicating the source file, line and original text of each directive in the template. These comments are embedded in the template output using the format defined in the DEBUG_FORMAT configuration item, or a simple default format if unspecified.

For example, the following template fragment:


    Hello World

would generate this output:

    ## input text line 1 :  ##
    Hello 
    ## input text line 2 : World ##
    World

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";
    }

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');

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');

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' });

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' });

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);

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.

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.

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.

localise(\%vars)

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

    $stash = $context->localise();

delocalise()

Restore the stash to its state prior to localisation.

    $stash = $context->delocalise();

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.

leave()

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

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.

AUTOLOAD

An AUTOLOAD method provides access to context configuration items.

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

AUTHOR

Andy Wardley <abw@andywardley.com>

http://www.andywardley.com/|http://www.andywardley.com/

VERSION

2.81, distributed as part of the Template Toolkit version 2.10, released on 24 July 2003.

COPYRIGHT

  Copyright (C) 1996-2003 Andy Wardley.  All Rights Reserved.
  Copyright (C) 1998-2002 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.

SEE ALSO

Template, Template::Document, Template::Exception, Template::Filters, Template::Plugins, Template::Provider, Template::Service, Template::Stash

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