advanced.rst

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('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')));

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'));
$twig->addFunction('otherFunction', new Twig_Function_Method($this, 'someMethod'));

Functions also support needs_environment and is_safe parameters.

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)
    {
        parent::__construct(array('value' => $value), array('name' => $name), $lineno);
        }

    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).