How to render a tree in Twig

I would like to render a tree with an undetermined depth (children of children of children, etc.). I need to loop through the array recursively; how can I do this in Twig?

I played around with domi27's idea and came up with this. I made a nested array as my tree, ['link']['sublinks'] is null or another array of more of the same.

Templates

The sub-template file to recurse with:

<!--includes/menu-links.html-->
{% for link in links %}
    <li>
        <a href="{{ link.href }}">{{ link.name }}</a>
        {% if link.sublinks %}
            <ul>
                {% include "includes/menu-links.html" with {'links': link.sublinks} %}
            </ul>
        {% endif %}
    </li>
{% endfor %}

Then in the main template, call this (kind of redundant 'with' stuff there):

<ul class="main-menu">
    {% include "includes/menu-links.html" with {'links':links} only %}
</ul>

Macros

A similar effect can be achieved with macros:

<!--macros/menu-macros.html-->
{% macro menu_links(links) %}
    {% for link in links %}
        <li>
            <a href="{{ link.href }}">{{ link.name }}</a>
            {% if link.sublinks %}
                <ul>
                    {{ _self.menu_links(link.sublinks) }}
                </ul>
            {% endif %}
        </li>
    {% endfor %}
{% endmacro %}

In the main template, do this:

{% import "macros/menu-macros.html" as macros %}
<ul class="main-menu">
    {{ macros.menu_links(links) }}
</ul>

Twig 2.0 - 2.11

If you want to use a macro in the same template, you should use something like this to stay compatible with Twig 2.x:

{% macro menu_links(links) %}
    {% import _self as macros %}
    {% for link in links %}
        <li>
            <a href="{{ link.href }}">{{ link.name }}</a>
            {% if link.sublinks %}
                <ul>
                    {{ macros.menu_links(link.sublinks) }}
                </ul>
            {% endif %}
        </li>
    {% endfor %}
{% endmacro %}

{% import _self as macros %}

<ul class="main-menu">
    {{ macros.menu_links(links) }}
</ul>

This extends random-coder's answer and incorporates dr.scre's hint to the Twig documentation about macros to now use _self, but import locally.

Twig >= 2.11

As of Twig 2.11, you can omit the {% import _self as macros %}, as inlined macros are imported automatically under the _self namespace (see Twig announcement: Automatic macro import):

{# {% import _self as macros %} - Can be removed #}

<ul class="main-menu">
    {{ _self.menu_links(links) }} {# Use _self for inlined macros #}
</ul>

If you're running PHP 5.4 or higher, there is a wonderful new solution (as of May 2016) to this problem by Alain Tiemblo: https://github.com/ninsuo/jordan-tree.

It's a "tree" tag that serves this exact purpose. Markup would look like this:

{% tree link in links %}
    {% if treeloop.first %}<ul>{% endif %}
    
    <li>
        <a href="{{ link.href }}">{{ link.name }}</a>
        {% subtree link.sublinks %}
    </li>
    
    {% if treeloop.last %}</ul>{% endif %}
{% endtree %}

First I thought this may be solved in a straightforward way, but it isn't that easy.

You need to create logic, maybe with a PHP class method, when to include a Twig subtemplate and when not.

<!-- tpl.html.twig -->
<ul>
    {% for key, item in menu %}
        {# Pseudo Twig code #}
        {% if item|hassubitem %}
            {% include "subitem.html.tpl" %}
        {% else %}
            <li>{{ item }}</li>
        {% endif %}
    {% endfor %}
</ul>

So you could use the special Twig loop variable, which is available inside a Twig for loop. But I'm not sure about the scope of this loop variable.

This and other information are available on Twigs "for" Docu!

Took flu's answer and modified it a little:

{# Macro #}

{% macro tree(items) %}
    {% import _self as m %}
        {% if items %}
        <ul>
            {% for i in items %}
                <li>
                    <a href="{{ i.url }}">{{ i.title }}</a>
                    {{ m.tree(i.items) }}
                </li>
            {% endfor %}
        </ul>
    {% endif %}
{% endmacro %}

{# Usage #}

{% import 'macros.twig' as m %}

{{ m.tree(items) }}

The answers here lead my to my solution.

I have a category entity with a self-referencing many-to-one association (parent to children).

/**
 * @ORM\ManyToOne(targetEntity="Category", inversedBy="children")
 */
private $parent;

/**
 * @ORM\OneToMany(targetEntity="Category", mappedBy="parent")
 */
private $children;

In my Twig template I am rendering the tree view like this:

<ul>
{% for category in categories %}
    {% if category.parent == null %}
        <li>
            <a href="{{ category.id }}">{{ category.name }}</a>
            {% if category.children|length > 0 %}
            <ul>
            {% for category in category.children %}
                <li>
                    <a href="{{ category.id }}">{{ category.name }}</a>
                </li>
            {% endfor %}
            </ul>
            {% endif %}
        </li>
    {% endif %}
{% endfor %}
</ul>