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.

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' ~ {{ '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 (so, 40 is an argument of the filter, not the value we want to transform).

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' ~ lipsum(40) ~ 'some more text' }}

{% set lipsum = lipsum(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

Creating a filter is as simple as associating a name with a PHP callable:

// an anonymous function
$filter = new Twig_Filter('rot13', function ($string) {
    return str_rot13($string);
});

// or a simple PHP function
$filter = new Twig_Filter('rot13', 'str_rot13');

// or a class static method
$filter = new Twig_Filter('rot13', array('SomeClass', 'rot13Filter'));
$filter = new Twig_Filter('rot13', 'SomeClass::rot13Filter');

// or a class method
$filter = new Twig_Filter('rot13', array($this, 'rot13Filter'));
// the one below needs a runtime implementation (see below for more information)
$filter = new Twig_Filter('rot13', array('SomeClass', 'rot13Filter'));

The first argument passed to the Twig_Filter constructor is the name of the filter you will use in templates and the second one is the PHP callable to associate with it.

Then, add the filter to your Twig environment:

$twig = new Twig_Environment($loader);
$twig->addFilter($filter);

And here is how to use it in a template:

{{ 'Twig'|rot13 }}

{# will output Gjvt #}

When called by Twig, the PHP callable receives the left side of the filter (before the pipe |) as the first argument and the extra arguments passed to the filter (within parentheses ()) as extra arguments.

For instance, the following code:

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

is compiled to something like the following:

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

The Twig_Filter class takes an array of options as its last argument:

$filter = new Twig_Filter('rot13', 'str_rot13', $options);

Environment-aware Filters

If you want to access the current environment instance in your filter, set the needs_environment option to true; Twig will pass the current environment as the first argument to the filter call:

$filter = new Twig_Filter('rot13', function (Twig_Environment $env, $string) {
    // get the current charset for instance
    $charset = $env->getCharset();

    return str_rot13($string);
}, array('needs_environment' => true));

Context-aware Filters

If you want to access the current context in your filter, set the needs_context option to true; Twig will pass the current context as the first argument to the filter call (or the second one if needs_environment is also set to true):

$filter = new Twig_Filter('rot13', function ($context, $string) {
    // ...
}, array('needs_context' => true));

$filter = new Twig_Filter('rot13', function (Twig_Environment $env, $context, $string) {
    // ...
}, array('needs_context' => true, 'needs_environment' => true));

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('nl2br', 'nl2br', array('is_safe' => array('html')));

Some filters may need to work on input that is already escaped or safe, for example when adding (safe) HTML tags to originally unsafe output. In such a case, set the pre_escape option to escape the input data before it is run through your filter:

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

Variadic Filters

When a filter should accept an arbitrary number of arguments, set the is_variadic option to true; Twig will pass the extra arguments as the last argument to the filter call as an array:

$filter = new Twig_Filter('thumbnail', function ($file, array $options = array()) {
    // ...
}, array('is_variadic' => true));

Be warned that named arguments passed to a variadic filter cannot be checked for validity as they will automatically end up in the option array.

Dynamic Filters

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

$filter = new Twig_Filter('*_path', function ($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:

$filter = new Twig_Filter('*_path_*', function ($name, $suffix, $arguments) {
    // ...
});

The filter will receive all dynamic part values before the normal filter arguments, but after the environment and the context. For instance, a call to 'foo'|a_path_b() will result in the following arguments to be passed to the filter: ('a', 'b', 'foo').

Deprecated Filters

You can mark a filter as being deprecated by setting the deprecated option to true. You can also give an alternative filter that replaces the deprecated one when that makes sense:

$filter = new Twig_Filter('obsolete', function () {
    // ...
}, array('deprecated' => true, 'alternative' => 'new_one'));

When a filter is deprecated, Twig emits a deprecation notice when compiling a template using it. See Displaying Deprecation Notices for more information.

Functions

Functions are defined in the exact same way as filters, but you need to create an instance of Twig_Function:

$twig = new Twig_Environment($loader);
$function = new Twig_Function('function_name', function () {
    // ...
});
$twig->addFunction($function);

Functions support the same features as filters, except for the pre_escape and preserves_safety options.

Tests

Tests are defined in the exact same way as filters and functions, but you need to create an instance of Twig_Test:

$twig = new Twig_Environment($loader);
$test = new Twig_Test('test_name', function () {
    // ...
});
$twig->addTest($test);

Tests allow you to create custom application specific logic for evaluating boolean conditions. As a simple example, let's create a Twig test that checks if objects are 'red':

$twig = new Twig_Environment($loader);
$test = new Twig_Test('red', function ($value) {
    if (isset($value->color) && $value->color == 'red') {
        return true;
    }
    if (isset($value->paint) && $value->paint == 'red') {
        return true;
    }
    return false;
});
$twig->addTest($test);

Test functions should always return true/false.

When creating tests you can use the node_class option to provide custom test compilation. This is useful if your test can be compiled into PHP primitives. This is used by many of the tests built into Twig:

$twig = new Twig_Environment($loader);
$test = new Twig_Test(
    'odd',
    null,
    array('node_class' => 'Twig_Node_Expression_Test_Odd'));
$twig->addTest($test);

class Twig_Node_Expression_Test_Odd extends Twig_Node_Expression_Test
{
    public function compile(Twig_Compiler $compiler)
    {
        $compiler
            ->raw('(')
            ->subcompile($this->getNode('node'))
            ->raw(' % 2 == 1')
            ->raw(')')
        ;
    }
}

The above example shows how you can create tests that use a node class. The node class has access to one sub-node called 'node'. This sub-node contains the value that is being tested. When the odd filter is used in code such as:

{% if my_value is odd %}

The node sub-node will contain an expression of my_value. Node-based tests also have access to the arguments node. This node will contain the various other arguments that have been provided to your test.

If you want to pass a variable number of positional or named arguments to the test, set the is_variadic option to true. Tests also support dynamic name feature as filters and functions.

Tags

One of the most exciting features 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 #}

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)
    {
        $parser = $this->parser;
        $stream = $parser->getStream();

        $name = $stream->expect(Twig_Token::NAME_TYPE)->getValue();
        $stream->expect(Twig_Token::OPERATOR_TYPE, '=');
        $value = $parser->getExpressionParser()->parseExpression();
        $stream->expect(Twig_Token::BLOCK_END_TYPE);

        return new Project_Set_Node($name, $value, $token->getLine(), $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 at the next token without consuming it.

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

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, $line, $tag = null)
    {
        parent::__construct(array('value' => $value), array('name' => $name), $line, $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.

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.

An extension is a class that implements the following interface:

interface Twig_ExtensionInterface
{
    /**
     * Returns the token parser instances to add to the existing list.
     *
     * @return Twig_TokenParserInterface[]
     */
    public function getTokenParsers();

    /**
     * Returns the node visitor instances to add to the existing list.
     *
     * @return Twig_NodeVisitorInterface[]
     */
    public function getNodeVisitors();

    /**
     * Returns a list of filters to add to the existing list.
     *
     * @return Twig_Filter[]
     */
    public function getFilters();

    /**
     * Returns a list of tests to add to the existing list.
     *
     * @return Twig_Test[]
     */
    public function getTests();

    /**
     * Returns a list of functions to add to the existing list.
     *
     * @return Twig_Function[]
     */
    public function getFunctions();

    /**
     * Returns a list of operators to add to the existing list.
     *
     * @return array<array> First array of unary operators, second array of binary operators
     */
    public function getOperators();
}

To keep your extension class clean and lean, inherit from the built-in Twig_Extension class instead of implementing the interface as it provides empty implementations for all methods:

class Project_Twig_Extension extends Twig_Extension { }

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());

Globals

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

class Project_Twig_Extension extends Twig_Extension implements Twig_Extension_GlobalsInterface
{
    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(
            new Twig_Function('lipsum', '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(
            new Twig_Filter('rot13', 'str_rot13'),
        );
    }

    // ...
}

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 lets you 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() method lets you add new test functions:

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

    // ...
}

Definition vs Runtime

Twig filters, functions, and tests runtime implementations can be defined as any valid PHP callable:

• functions/static methods: Simple to implement and fast (used by all Twig core extensions); but it is hard for the runtime to depend on external objects;
• closures: Simple to implement;
• object methods: More flexible and required if your runtime code depends on external objects.

The simplest way to use methods is to define them on the extension itself:

class Project_Twig_Extension extends Twig_Extension
{
    private $rot13Provider;

    public function __construct($rot13Provider)
    {
        $this->rot13Provider = $rot13Provider;
    }

    public function getFunctions()
    {
        return array(
            new Twig_Function('rot13', array($this, 'rot13')),
        );
    }

    public function rot13($value)
    {
        return $rot13Provider->rot13($value);
    }
}

This is very convenient but not recommended as it makes template compilation depend on runtime dependencies even if they are not needed (think for instance as a dependency that connects to a database engine).

You can easily decouple the extension definitions from their runtime implementations by registering a Twig_RuntimeLoaderInterface instance on the environment that knows how to instantiate such runtime classes (runtime classes must be autoload-able):

class RuntimeLoader implements Twig_RuntimeLoaderInterface
{
    public function load($class)
    {
        // implement the logic to create an instance of $class
        // and inject its dependencies
        // most of the time, it means using your dependency injection container
        if ('Project_Twig_RuntimeExtension' === $class) {
            return new $class(new Rot13Provider());
        } else {
            // ...
        }
    }
}

$twig->addRuntimeLoader(new RuntimeLoader());

It is now possible to move the runtime logic to a new Project_Twig_RuntimeExtension class and use it directly in the extension:

class Project_Twig_RuntimeExtension
{
    private $rot13Provider;

    public function __construct($rot13Provider)
    {
        $this->rot13Provider = $rot13Provider;
    }

    public function rot13($value)
    {
        return $rot13Provider->rot13($value);
    }
}

class Project_Twig_Extension extends Twig_Extension
{
    public function getFunctions()
    {
        return array(
            new Twig_Function('rot13', array('Project_Twig_RuntimeExtension', 'rot13')),
            // or
            new Twig_Function('rot13', 'Project_Twig_RuntimeExtension::rot13'),
        );
    }
}

Overloading

To overload an already defined filter, test, operator, global variable, or function, re-define it in an extension and register it as late as possible (order matters):

class MyCoreExtension extends Twig_Extension
{
    public function getFilters()
    {
        return array(
            new Twig_Filter('date', array($this, 'dateFilter')),
        );
    }

    public function dateFilter($timestamp, $format = 'F j, Y H:i')
    {
        // do something different from the built-in date filter
    }
}

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

Here, we have overloaded the built-in date filter with a custom one.

If you do the same on the Twig_Environment itself, beware that it takes precedence over any other registered extensions:

$twig = new Twig_Environment($loader);
$twig->addFilter(new Twig_Filter('date', function ($timestamp, $format = 'F j, Y H:i') {
    // do something different from the built-in date filter
}));
// the date filter will come from the above registration, not
// from the registered extension below
$twig->addExtension(new MyCoreExtension());

Testing an Extension

Functional Tests

You can create functional tests for extensions simply by creating the following file structure in your test directory:

Fixtures/
    filters/
        foo.test
        bar.test
    functions/
        foo.test
        bar.test
    tags/
        foo.test
        bar.test
IntegrationTest.php

The IntegrationTest.php file should look like this:

class Project_Tests_IntegrationTest extends Twig_Test_IntegrationTestCase
{
    public function getExtensions()
    {
        return array(
            new Project_Twig_Extension1(),
            new Project_Twig_Extension2(),
        );
    }

    public function getFixturesDir()
    {
        return dirname(__FILE__).'/Fixtures/';
    }
}

Fixtures examples can be found within the Twig repository tests/Twig/Fixtures directory.

Node Tests

Testing the node visitors can be complex, so extend your test cases from Twig_Test_NodeTestCase. Examples can be found in the Twig repository tests/Twig/Node directory.