Extending Bolt / Twig Templating

There are four primary Twig components: tags, functions, filters, tests and operators. Bolt allows you to define custom components of all five types.

{# Example of a custom tag (setcontent) #}
{% setcontent pages = 'pages' %}

{# Example of a function (menu) #}
{% set variable = menu() %}

{# Example of a filter (length) #}
{% set len = 'one two three'|length %}

{# Example of a twig Test (odd) #}
{% if 5 is odd %}

You can create as many or as few filters, functions and tests within a single Twig extension class as you like. Crating tags and operators is rarely necessary and is a more complicated process. Bolt itself only uses one custom tag, namely setcontent. Read the official Twig documentation on creating tags and creating operators for more information.

Creating a custom Twig filter¶

To create a custom Twig filter, you need a class that extends Twig's AbstractExtension inside the src folder in the root of your project.

The AbstractExtension defines a number of functions that register all Twig functions, filters and tests that the class defines:

<?php

namespace App;

use Twig\Extension\AbstractExtension;
use Twig\TwigFilter;

class CustomExtension extends AbstractExtension
{
    /**
     * Returns a list of filters to add to the existing list.
     *
     * @return TwigFilter[]
     */
    public function getFilters()
    {
        return [
            new TwigFilter('substring', [$this, 'getSubstring']),
        ];
    }

    /**
     * In this function, $string is required.
     * $strat and $length are optional, and have default values of 0 and 5 respectively;
     */
    public function getSubstring(string $string, int $start=0, int $length=5): string
    {
        return substr($string, $start, $length);
    }
}

To verify that the substring Twig filter is correctly registered, check that it is shown after running php bin/console debug:twig.

You can use this filter like so:

{% set substr = 'one two three'|substring %}

{{ 'one two three'|substring(3) }}
{{ 'one two three'|substring(start=3) }} {# this and the example above produce the same result. #}

{{ dump('one two three'|substring(length=2)) }}

Creating a Twig function¶

If you want a Twig function in addition to the substring filter, the code chances just slightly with the new use clause and a new function called getFunctions:

<?php

namespace App;

use Twig\Extension\AbstractExtension;
use Twig\TwigFilter;
use Twig\TwigFunction;

class CustomExtension extends AbstractExtension
{
    /**
     * Returns a list of filters to add to the existing list.
     *
     * @return TwigFilter[]
     */
    public function getFilters()
    {
        return [
            new TwigFilter('substring', [$this, 'getSubstring']),
        ];
    }

    public function getFunctions()
    {
        return [
            new TwigFunction('substring', [$this, 'getSubstring']),
        ];
    }

    /**
     * In this function, $string is required.
     * $strat and $length are optional, and have default values of 0 and 5 respectively;
     */
    public function getSubstring(string $string, int $start=0, int $length=5): string
    {
        return substr($string, $start, $length);
    }
}

Then, we can use the substring function like so:

{% set substr = substring('one two three') %}

{{ substring('one two three', 3) }}
{{ substring('one two three', start=3) }} {# this and the example above produce the same result. #}

{{ dump(substring('one two three', length=2)) }}

Creating a Twig test¶

Twig tests are especially useful to use together with the if tag.

For example, let's create a negative test that works like this:

    {% if variable is negative %}
        {# handle a negative variable here...#}
    {% endif %}

Create a class in the src folder of your project that looks like this:

<?php

namespace App;

use Twig\Extension\AbstractExtension;
use Twig\TwigTest;

class CustomExtension extends AbstractExtension
{
    public function getTests()
    {
        return [
            new TwigTest('negative', [$this, 'isNegative']),
        ];
    }

    public function isNegative($variable): bool
    {
        if (isNan($variable)) {
            // Not a number is not negative.
            return false;
        }

        // true for -1, -5, -100
        // false for 0, 5, 100
        return $variable < 0;
    }
}

Read more about this topic in Symfony's official documentation: templating.