Template Toolkit Modules: Template::Plugin

A "plugin" for the Template Toolkit is simply a Perl module which exists in a known package location (e.g. Template::Plugin::*) and conforms to a regular standard, allowing it to be loaded and used automatically.

The Template::Plugin module defines a base class from which other plugin modules can be derived. A plugin does not have to be derived from Template::Plugin but should at least conform to its object-oriented interface.

It is recommended that you create plugins in your own package namespace to avoid conflict with toolkit plugins. e.g.

    package MyOrg::Template::Plugin::FooBar;

Use the PLUGIN_BASE option to specify the namespace that you use. e.g.

    use Template;
    my $template = Template->new({ 
	PLUGIN_BASE => 'MyOrg::Template::Plugin',
    });

The following methods form the basic interface between the Template Toolkit and plugin modules.

load($context)

This method is called by the Template Toolkit when the plugin module is first loaded. It is called as a package method and thus implicitly receives the package name as the first parameter. A reference to the Template::Context object loading the plugin is also passed. The default behaviour for the load() method is to simply return the class name. The calling context then uses this class name to call the new() package method.
```
    package MyPlugin;
```
```
    sub load {               # called as MyPlugin->load($context)
	my ($class, $context) = @_;
	return $class;       # returns 'MyPlugin'
    }
```
new($context, @params)

This method is called to instantiate a new plugin object for the USE directive. It is called as a package method against the class name returned by load(). A reference to the Template::Context object creating the plugin is passed, along with any additional parameters specified in the USE directive.
```
    sub new {                # called as MyPlugin->new($context)
	my ($class, $context, @params) = @_;
	bless {
	    _CONTEXT => $context,
	}, $class;	     # returns blessed MyPlugin object
    }
```
error($error)

This method, inherited from the Template::Base module, is used for reporting and returning errors. It can be called as a package method to set/return the $ERROR package variable, or as an object method to set/return the object _ERROR member. When called with an argument, it sets the relevant variable and returns undef. When called without an argument, it returns the value of the variable.
```
    sub new {
	my ($class, $context, $dsn) = @_;
```
```
	return $class->error('No data source specified')
	    unless $dsn;
```
```
	bless {
	    _DSN => $dsn,
	}, $class;
    }
```
```
    ...
```
```
    my $something = MyModule->new()
	|| die MyModule->error(), "\n";
```
```
    $something->do_something()
	|| die $something->error(), "\n";
```

The Template::Context object that handles the loading and use of plugins calls the new() and error() methods against the package name returned by the load() method. In pseudo-code terms, it might look something like this:

    $class  = MyPlugin->load($context);       # returns 'MyPlugin'

    $object = $class->new($context, @params)  # MyPlugin->new(...)
	|| die $class->error();               # MyPlugin->error()

The load() method may alterately return a blessed reference to an object instance. In this case, new() and error() are then called as object methods against that prototype instance.

    package YourPlugin;

    sub load {
        my ($class, $context) = @_;
	bless {
	    _CONTEXT => $context,
	}, $class;
    }

    sub new {
	my ($self, $context, @params) = @_;
	return $self;
    }

In this example, we have implemented a 'Singleton' plugin. One object gets created when load() is called and this simply returns itself for each call to new().

Another implementation might require individual objects to be created for every call to new(), but with each object sharing a reference to some other object to maintain cached data, database handles, etc. This pseudo-code example demonstrates the principle.

    package MyServer;

    sub load {
        my ($class, $context) = @_;
	bless {
	    _CONTEXT => $context,
	    _CACHE   => { },
	}, $class;
    }

    sub new {
	my ($self, $context, @params) = @_;
	MyClient->new($self, @params);
    }

    sub add_to_cache   { ... }

    sub get_from_cache { ... }

    package MyClient;

    sub new {
	my ($class, $server, $blah) = @_;
	bless {
	    _SERVER => $server,
	    _BLAH   => $blah,
	}, $class;
    }

    sub get {
	my $self = shift;
	$self->{ _SERVER }->get_from_cache(@_);
    }

    sub put {
	my $self = shift;
	$self->{ _SERVER }->add_to_cache(@_);
    }

When the plugin is loaded, a MyServer instance is created. The new() method is called against this object which instantiates and returns a MyClient object, primed to communicate with the creating MyServer.

	http://www.template-toolkit.org/
	http://www.template-toolkit.org/

Template::Plugin

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