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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:

{% 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:

    {{ '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:

{{ 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:

{{ 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:

{{ '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:

{{ 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:

{{ 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:

{{ '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:

<?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:

{{ 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:

<?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:

{{ "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:

{{ "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.

{{ 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:

<?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:

{% 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).

Creating an Extension

The main motivation for writing an extension is to move often used code into a reusable class like adding support for internationalization. An extension can define tags, filters, tests, operators, global variables, functions, and node visitors.

Creating an extension also makes for a better separation of code that is executed at compilation time and code needed at runtime. As such, it makes your code faster.

Most of the time, it is useful to create a single extension for your project, to host all the specific tags and filters you want to add to Twig.

Note

Before writing your own extensions, have a look at the Twig official extension repository: http://github.com/fabpot/Twig-extensions.

An extension is a class that implements the following interface:

interface Twig_ExtensionInterface
{
    /**
     * Initializes the runtime environment.
     *
     * This is where you can load some file that contains filter functions for instance.
     *
     * @param Twig_Environment $environment The current Twig_Environment instance
     */
    function initRuntime(Twig_Environment $environment);

    /**
     * Returns the token parser instances to add to the existing list.
     *
     * @return array An array of Twig_TokenParserInterface or Twig_TokenParserBrokerInterface instances
     */
    function getTokenParsers();

    /**
     * Returns the node visitor instances to add to the existing list.
     *
     * @return array An array of Twig_NodeVisitorInterface instances
     */
    function getNodeVisitors();

    /**
     * Returns a list of filters to add to the existing list.
     *
     * @return array An array of filters
     */
    function getFilters();

    /**
     * Returns a list of tests to add to the existing list.
     *
     * @return array An array of tests
     */
    function getTests();

    /**
     * Returns a list of functions to add to the existing list.
     *
     * @return array An array of functions
     */
    function getFunctions();

    /**
     * Returns a list of operators to add to the existing list.
     *
     * @return array An array of operators
     */
    function getOperators();

    /**
     * Returns a list of global variables to add to the existing list.
     *
     * @return array An array of global variables
     */
    function getGlobals();

    /**
     * Returns the name of the extension.
     *
     * @return string The extension name
     */
    function getName();
}

To keep your extension class clean and lean, it can inherit from the built-in Twig_Extension class instead of implementing the whole interface. That way, you just need to implement the getName() method as the Twig_Extension provides empty implementations for all other methods.

The getName() method must return a unique identifier for your extension.

Now, with this information in mind, let's create the most basic extension possible:

class Project_Twig_Extension extends Twig_Extension
{
    public function getName()
    {
        return 'project';
    }
}

Note

Of course, this extension does nothing for now. We will customize it in the next sections.

Twig does not care where you save your extension on the filesystem, as all extensions must be registered explicitly to be available in your templates.

You can register an extension by using the addExtension() method on your main Environment object:

$twig = new Twig_Environment($loader);
$twig->addExtension(new Project_Twig_Extension());

Of course, you need to first load the extension file by either using require_once() or by using an autoloader (see spl_autoload_register()).

Tip

The bundled extensions are great examples of how extensions work.

Globals

Global variables can be registered in an extension via the getGlobals() method:

class Project_Twig_Extension extends Twig_Extension
{
    public function getGlobals()
    {
        return array(
            'text' => new Text(),
        );
    }

    // ...
}

Functions

Functions can be registered in an extension via the getFunctions() method:

class Project_Twig_Extension extends Twig_Extension
{
    public function getFunctions()
    {
        return array(
            'lipsum' => new Twig_Function_Function('generate_lipsum'),
        );
    }

    // ...
}

Filters

To add a filter to an extension, you need to override the getFilters() method. This method must return an array of filters to add to the Twig environment:

class Project_Twig_Extension extends Twig_Extension
{
    public function getFilters()
    {
        return array(
            'rot13' => new Twig_Filter_Function('str_rot13'),
        );
    }

    // ...
}

As you can see in the above code, the getFilters() method returns an array where keys are the name of the filters (rot13) and the values the definition of the filter (new Twig_Filter_Function('str_rot13')).

As seen in the previous chapter, you can also define filters as static methods on the extension class:

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

You can also use Twig_Filter_Method instead of Twig_Filter_Function when defining a filter to use a method:

class Project_Twig_Extension extends Twig_Extension
{
    public function getFilters()
    {
        return array(
            'rot13' => new Twig_Filter_Method($this, 'rot13Filter'),
        );
    }

    public function rot13Filter($string)
    {
        return str_rot13($string);
    }

    // ...
}

The first argument of the Twig_Filter_Method constructor is always $this, the current extension object. The second one is the name of the method to call.

Using methods for filters is a great way to package your filter without polluting the global namespace. This also gives the developer more flexibility at the cost of a small overhead.

Overriding default Filters

If some default core filters do not suit your needs, you can easily override them by creating your own core extension. Of course, you don't need to copy and paste the whole core extension code of Twig. Instead, you can just extends it and override the filter(s) you want by overriding the getFilters() method:

class MyCoreExtension extends Twig_Extension_Core
{
    public function getFilters()
    {
        return array_merge(parent::getFilters(), array(
            'date' => new Twig_Filter_Method($this, 'dateFilter'),
            // ...
        ));
    }

    public function dateFilter($timestamp, $format = 'F j, Y H:i')
    {
        return '...'.twig_date_format_filter($timestamp, $format);
    }

    // ...
}

Here, we override the date filter with a custom one. Using this new core extension is as simple as registering the MyCoreExtension extension by calling the addExtension() method on the environment instance:

$twig = new Twig_Environment($loader);
$twig->addExtension(new MyCoreExtension());

But I can already hear some people wondering how it can work as the Core extension is loaded by default. That's true, but the trick is that both extensions share the same unique identifier (core - defined in the getName() method). By registering an extension with the same name as an existing one, you have actually overridden the default one, even if it is already registered:

$twig->addExtension(new Twig_Extension_Core());
$twig->addExtension(new MyCoreExtension());

Tags

Adding a tag in an extension can be done by overriding the getTokenParsers() method. This method must return an array of tags to add to the Twig environment:

class Project_Twig_Extension extends Twig_Extension
{
    public function getTokenParsers()
    {
        return array(new Project_Set_TokenParser());
    }

    // ...
}

In the above code, we have added a single new tag, defined by the Project_Set_TokenParser class. The Project_Set_TokenParser class is responsible for parsing the tag and compiling it to PHP.

Operators

The getOperators() methods allows to add new operators. Here is how to add !, ||, and && operators:

class Project_Twig_Extension extends Twig_Extension
{
    public function getOperators()
    {
        return array(
            array(
                '!' => array('precedence' => 50, 'class' => 'Twig_Node_Expression_Unary_Not'),
            ),
            array(
                '||' => array('precedence' => 10, 'class' => 'Twig_Node_Expression_Binary_Or', 'associativity' => Twig_ExpressionParser::OPERATOR_LEFT),
                '&&' => array('precedence' => 15, 'class' => 'Twig_Node_Expression_Binary_And', 'associativity' => Twig_ExpressionParser::OPERATOR_LEFT),
            ),
        );
    }

    // ...
}

Tests

The getTests() methods allows to add new test functions:

class Project_Twig_Extension extends Twig_Extension
{
    public function getTests()
    {
        return array(
            'even' => new Twig_Test_Function('twig_test_even'),
        );
    }

    // ...
}