Extending Twig
==============

Twig can be extended in many ways; you can add extra tags, filters, tests,
operators, global variables, and functions. You can even extend the parser
itself with node visitors.

.. note::

    This chapter describes how to extend Twig easily. If you want to reuse
    your changes in different projects or if you want to share them with
    others, you should then create an extension as described in the next
    chapter.

Before extending Twig, you must understand the differences between all the
different possible extension points and when to use them.

First, remember that Twig has two main language constructs:

* ``{{ }}``: used to print the result of an expression evaluation;

* ``{% %}``: used to execute statements.

To understand why Twig exposes so many extension points, let's see how to
implement a *Lorem ipsum* generator (it needs to know the number of words to
generate).

You can use a ``lipsum`` *tag*:

.. code-block:: jinja

    {% lipsum 40 %}

That works, but using a tag for ``lipsum`` is not a good idea for at least
three main reasons:

* ``lipsum`` is not a language construct;
* The tag outputs something;
* The tag is not flexible as you cannot use it in an expression:

  .. code-block:: jinja

      {{ 'some text' ~ {% lipsum 40 %} ~ 'some more text' }}

In fact, you rarely need to create tags; and that's good news because tags are
the most complex extension point of Twig.

Now, let's use a ``lipsum`` *filter*:

.. code-block:: jinja

    {{ 40|lipsum }}

Again, it works, but it looks weird. A filter transforms the passed value to
something else but here we use the value to indicate the number of words to
generate.

Next, let's use a ``lipsum`` *function*:

.. code-block:: jinja

    {{ lipsum(40) }}

Here we go. For this specific example, the creation of a function is the
extension point to use. And you can use it anywhere an expression is accepted:

.. code-block:: jinja

    {{ 'some text' ~ ipsum(40) ~ 'some more text' }}

    {% set ipsum = ipsum(40) %}

Last but not the least, you can also use a *global* object with a method able
to generate lorem ipsum text:

.. code-block:: jinja

    {{ text.lipsum(40) }}

As a rule of thumb, use functions for frequently used features and global
objects for everything else.

Keep in mind the following when you want to extend Twig:

========== ========================== ========== =========================
What?      Implementation difficulty? How often? When?
========== ========================== ========== =========================
*macro*    trivial                    frequent   Content generation
*global*   trivial                    frequent   Helper object
*function* trivial                    frequent   Content generation
*filter*   trivial                    frequent   Value transformation
*tag*      complex                    rare       DSL language construct
*test*     trivial                    rare       Boolean decision
*operator* trivial                    rare       Values transformation
========== ========================== ========== =========================

Globals
-------

A global variable is like any other template variable, except that it's
available in all templates and macros::

    $twig = new Twig_Environment($loader);
    $twig->addGlobal('text', new Text());

You can then use the ``text`` variable anywhere in a template:

.. code-block:: jinja

    {{ text.lipsum(40) }}

Filters
-------

A filter is a regular PHP function or an object method that takes the left
side of the filter (before the pipe ``|``) as first argument and the extra
arguments passed to the filter (within parentheses ``()``) as extra arguments.

Defining a filter is as easy as associating the filter name with a PHP
callable. For instance, let's say you have the following code in a template:

.. code-block:: jinja

    {{ 'TWIG'|lower }}

When compiling this template to PHP, Twig looks for the PHP callable
associated with the ``lower`` filter. The ``lower`` filter is a built-in Twig
filter, and it is simply mapped to the PHP ``strtolower()`` function. After
compilation, the generated PHP code is roughly equivalent to:

.. code-block:: html+php

    <?php echo strtolower('TWIG') ?>

As you can see, the ``'TWIG'`` string is passed as a first argument to the PHP
function.

A filter can also take extra arguments like in the following example:

.. code-block:: jinja

    {{ now|date('d/m/Y') }}

In this case, the extra arguments are passed to the function after the main
argument, and the compiled code is equivalent to:

.. code-block:: html+php

    <?php echo twig_date_format_filter($now, 'd/m/Y') ?>

Let's see how to create a new filter.

In this section, we will create a ``rot13`` filter, which should return the
`rot13`_ transformation of a string. Here is an example of its usage and the
expected output:

.. code-block:: jinja

    {{ "Twig"|rot13 }}

    {# should displays Gjvt #}

Adding a filter is as simple as calling the ``addFilter()`` method on the
``Twig_Environment`` instance::

    $twig = new Twig_Environment($loader);
    $twig->addFilter('rot13', new Twig_Filter_Function('str_rot13'));

The second argument of ``addFilter()`` is an instance of ``Twig_Filter``.
Here, we use ``Twig_Filter_Function`` as the filter is a PHP function. The
first argument passed to the ``Twig_Filter_Function`` constructor is the name
of the PHP function to call, here ``str_rot13``, a native PHP function.

Let's say I now want to be able to add a prefix before the converted string:

.. code-block:: jinja

    {{ "Twig"|rot13('prefix_') }}

    {# should displays prefix_Gjvt #}

As the PHP ``str_rot13()`` function does not support this requirement, let's
create a new PHP function::

    function project_compute_rot13($string, $prefix = '')
    {
        return $prefix.str_rot13($string);
    }

As you can see, the ``prefix`` argument of the filter is passed as an extra
argument to the ``project_compute_rot13()`` function.

Adding this filter is as easy as before::

    $twig->addFilter('rot13', new Twig_Filter_Function('project_compute_rot13'));

For better encapsulation, a filter can also be defined as a static method of a
class. The ``Twig_Filter_Function`` class can also be used to register such
static methods as filters::

    $twig->addFilter('rot13', new Twig_Filter_Function('SomeClass::rot13Filter'));

.. tip::

    In an extension, you can also define a filter as a static method of the
    extension class.

Environment aware Filters
~~~~~~~~~~~~~~~~~~~~~~~~~

The ``Twig_Filter`` classes take options as their last argument. For instance,
if you want access to the current environment instance in your filter, set the
``needs_environment`` option to ``true``::

    $filter = new Twig_Filter_Function('str_rot13', array('needs_environment' => true));

Twig will then pass the current environment as the first argument to the
filter call::

    function twig_compute_rot13(Twig_Environment $env, $string)
    {
        // get the current charset for instance
        $charset = $env->getCharset();

        return str_rot13($string);
    }

Automatic Escaping
~~~~~~~~~~~~~~~~~~

If automatic escaping is enabled, the output of the filter may be escaped
before printing. If your filter acts as an escaper (or explicitly outputs html
or javascript code), you will want the raw output to be printed. In such a
case, set the ``is_safe`` option::

    $filter = new Twig_Filter_Function('nl2br', array('is_safe' => array('html')));

Some filters may have to work on already escaped or safe values. In such a
case, set the ``pre_escape`` option::

    $filter = new Twig_Filter_Function('somefilter', array('pre_escape' => 'html', 'is_safe' => array('html')));

Dynamic Filters
~~~~~~~~~~~~~~~

.. versionadded:: 1.5
    Dynamic filters support was added in Twig 1.5.

A filter name containing the special ``*`` character is a dynamic filter as
the ``*`` can be any string::

    $twig->addFilter('*_path', new Twig_Filter_Function('twig_path'));

    function twig_path($name, $arguments)
    {
        // ...
    }

The following filters will be matched by the above defined dynamic filter:

* ``product_path``
* ``category_path``

A dynamic filter can define more than one dynamic parts::

    $twig->addFilter('*_path_*', new Twig_Filter_Function('twig_path'));

    function twig_path($name, $suffix, $arguments)
    {
        // ...
    }

The filter will receive all dynamic part values before the normal filters
arguments. For instance, a call to ``'foo'|a_path_b()`` will result in the
following PHP call: ``twig_path('a', 'b', 'foo')``.

Functions
---------

A function is a regular PHP function or an object method that can be called from
templates.

.. code-block:: jinja

    {{ constant("DATE_W3C") }}

When compiling this template to PHP, Twig looks for the PHP callable
associated with the ``constant`` function. The ``constant`` function is a built-in Twig
function, and it is simply mapped to the PHP ``constant()`` function. After
compilation, the generated PHP code is roughly equivalent to:

.. code-block:: html+php

    <?php echo constant('DATE_W3C') ?>

Adding a function is similar to adding a filter. This can be done by calling the
``addFunction()`` method on the ``Twig_Environment`` instance::

    $twig = new Twig_Environment($loader);
    $twig->addFunction('functionName', new Twig_Function_Function('someFunction'));

You can also expose extension methods as functions in your templates::

    // $this is an object that implements Twig_ExtensionInterface.
    $twig = new Twig_Environment($loader);
    $twig->addFunction('otherFunction', new Twig_Function_Method($this, 'someMethod'));

Functions also support ``needs_environment`` and ``is_safe`` parameters.

Dynamic Functions
~~~~~~~~~~~~~~~~~

.. versionadded:: 1.5
    Dynamic functions support was added in Twig 1.5.

A function name containing the special ``*`` character is a dynamic function
as the ``*`` can be any string::

    $twig->addFunction('*_path', new Twig_Function_Function('twig_path'));

    function twig_path($name, $arguments)
    {
        // ...
    }

The following functions will be matched by the above defined dynamic function:

* ``product_path``
* ``category_path``

A dynamic function can define more than one dynamic parts::

    $twig->addFilter('*_path_*', new Twig_Filter_Function('twig_path'));

    function twig_path($name, $suffix, $arguments)
    {
        // ...
    }

The function will receive all dynamic part values before the normal functions
arguments. For instance, a call to ``a_path_b('foo')`` will result in the
following PHP call: ``twig_path('a', 'b', 'foo')``.

Tags
----

One of the most exciting feature of a template engine like Twig is the
possibility to define new language constructs. This is also the most complex
feature as you need to understand how Twig's internals work.

Let's create a simple ``set`` tag that allows the definition of simple
variables from within a template. The tag can be used like follows:

.. code-block:: jinja

    {% set name = "value" %}

    {{ name }}

    {# should output value #}

.. note::

    The ``set`` tag is part of the Core extension and as such is always
    available. The built-in version is slightly more powerful and supports
    multiple assignments by default (cf. the template designers chapter for
    more information).

Three steps are needed to define a new tag:

* Defining a Token Parser class (responsible for parsing the template code);

* Defining a Node class (responsible for converting the parsed code to PHP);

* Registering the tag.

Registering a new tag
~~~~~~~~~~~~~~~~~~~~~

Adding a tag is as simple as calling the ``addTokenParser`` method on the
``Twig_Environment`` instance::

    $twig = new Twig_Environment($loader);
    $twig->addTokenParser(new Project_Set_TokenParser());

Defining a Token Parser
~~~~~~~~~~~~~~~~~~~~~~~

Now, let's see the actual code of this class::

    class Project_Set_TokenParser extends Twig_TokenParser
    {
        public function parse(Twig_Token $token)
        {
            $lineno = $token->getLine();
            $name = $this->parser->getStream()->expect(Twig_Token::NAME_TYPE)->getValue();
            $this->parser->getStream()->expect(Twig_Token::OPERATOR_TYPE, '=');
            $value = $this->parser->getExpressionParser()->parseExpression();

            $this->parser->getStream()->expect(Twig_Token::BLOCK_END_TYPE);

            return new Project_Set_Node($name, $value, $lineno, $this->getTag());
        }

        public function getTag()
        {
            return 'set';
        }
    }

The ``getTag()`` method must return the tag we want to parse, here ``set``.

The ``parse()`` method is invoked whenever the parser encounters a ``set``
tag. It should return a ``Twig_Node`` instance that represents the node (the
``Project_Set_Node`` calls creating is explained in the next section).

The parsing process is simplified thanks to a bunch of methods you can call
from the token stream (``$this->parser->getStream()``):

* ``getCurrent()``: Gets the current token in the stream.

* ``next()``: Moves to the next token in the stream, *but returns the old one*.

* ``test($type)``, ``test($value)`` or ``test($type, $value)``: Determines whether
  the current token is of a particular type or value (or both). The value may be an
  array of several possible values.

* ``expect($type[, $value[, $message]])``: If the current token isn't of the given
  type/value a syntax error is thrown. Otherwise, if the type and value are correct,
  the token is returned and the stream moves to the next token.

* ``look()``: Looks a the next token without consuming it.

Parsing expressions is done by calling the ``parseExpression()`` like we did for
the ``set`` tag.

.. tip::

    Reading the existing ``TokenParser`` classes is the best way to learn all
    the nitty-gritty details of the parsing process.

Defining a Node
~~~~~~~~~~~~~~~

The ``Project_Set_Node`` class itself is rather simple::

    class Project_Set_Node extends Twig_Node
    {
        public function __construct($name, Twig_Node_Expression $value, $lineno, $tag = null)
        {
            parent::__construct(array('value' => $value), array('name' => $name), $lineno, $tag);
        }

        public function compile(Twig_Compiler $compiler)
        {
            $compiler
                ->addDebugInfo($this)
                ->write('$context[\''.$this->getAttribute('name').'\'] = ')
                ->subcompile($this->getNode('value'))
                ->raw(";\n")
            ;
        }
    }

The compiler implements a fluid interface and provides methods that helps the
developer generate beautiful and readable PHP code:

* ``subcompile()``: Compiles a node.

* ``raw()``: Writes the given string as is.

* ``write()``: Writes the given string by adding indentation at the beginning
  of each line.

* ``string()``: Writes a quoted string.

* ``repr()``: Writes a PHP representation of a given value (see
  ``Twig_Node_For`` for a usage example).

* ``addDebugInfo()``: Adds the line of the original template file related to
  the current node as a comment.

* ``indent()``: Indents the generated code (see ``Twig_Node_Block`` for a
  usage example).

* ``outdent()``: Outdents the generated code (see ``Twig_Node_Block`` for a
  usage example).

.. _`rot13`: http://www.php.net/manual/en/function.str-rot13.php