Using Pico’s page tree

Recursion isn’t that hard…


Did you ever wanted to use a dropdown for your page navigation? It never was that easy! Starting with Pico 2.0 you can access a page’s tree node using the tree_node element of the page data array (e.g. {{ current_page.tree_node }}). But first you’ve to understand how Pico’s page tree is built up. A picture is worth a thousand words, so before we start explaining the details here’s a simple example of how a page tree looks in practice. Just think of the following Markdown files in your content directory:

content/
├── shop/
│   ├── flowers/
│   │   ├── daisies.md
│   │   ├── red-roses.md
│   │   └── tiger-lilies.md
│   ├── index.md
│   └── shipping.md
├── index.md
└── contact.md

As you can see you’ll end up having a website with the pages index, contact, shop/index, shop/shipping, shop/flowers/daisies, shop/flowers/red-roses and shop/flowers/tiger-lilies. The corresponding page tree looks like the following:

root node `/`
├── inner node `shop`
│   ├── inner node `flowers`
│   │   ├── leaf node `daisies`
│   │   ├── leaf node `red-roses`
│   │   └── leaf node `tiger-lilies`
│   └── leaf node `shipping`
└── leaf node `contact`

Let’s take a look at this. The first thing we’ve to explain is the terminology: Nodes with children are called “inner nodes”, whereas nodes without children are called “leaf nodes”. The second thing you might notice is that the pages index and shop/index seem to be gone. But they aren’t gone, index is rather represented by the root node /, and shop/index is represented by the inner node shop. Did you notice that there seems to be no difference between the inner nodes shop and flowers, even though there’s no shop/flowers/index page? That’s because nodes aren’t pages. While all leaf nodes do represent a page (e.g. the leaf node contact represents the page contact), inner nodes may or may not represent a page. So, if there’s a shop/flowers/daisies.md, but neither a shop/flowers/index.md nor a shop/flowers.md, the inner node flowers represents no page.

You should always keep this in mind when working with Pico’s page tree. If you access a tree node, you get an array with the keys id, page and children. The id key contains a string with the node’s name. If the node represents a page, the page key is a reference to the page’s data array. If the node is a inner node, the children key is a list of the node’s child nodes. The order of a node’s children matches the order in Pico’s pages array.

You can access a page’s tree node using the tree_node key in the page’s data array. So, to access the root node, use {{ pages["index"].tree_node }}. To access the current page’s tree node, use {{ current_page.tree_node }}.

But what can I do with Pico’s page tree? Probably the most common task is to build a recursive page navigation like the following:

  • Home
  • Shop
    • Flowers
      • Daisies
      • Red Roses
      • Tiger Lilies
    • Shipping
  • Contact

Implementing this using a Twig macro is pretty straight forward:

{% macro tree(parent) %}
    {% import _self as utils %}
    {% for child in parent.children %}
        <li>
            {% if child.page %}
                <div>{{ child.page.title }}</div>
            {% endif %}
            {% if child.children %}
                <ul>
                    {{ utils.tree(child) }}
                </ul>
            {% endif %}
        </li>
    {% endfor %}
{% endmacro %}

<ul>
    <li>
        <div>{{ pages["index"].title }}</div>
    </li>

    {% import _self as utils %}
    {{ utils.tree(pages["index"].tree_node) }}
</ul>

Although Pico’s page tree primarily targets theme developers, plugin developers can benefit from the page tree, too. As for Twig you can access a page’s tree node using the tree_node key in the page’s data array (e.g. $pageData['tree_node']). As always, there’s an event to access (and modify) Pico’s page tree: the onPageTreeBuilt(array &$pageTree) event is triggered right after the onCurrentPageDiscovered event. You can later access the page tree using the Pico::getPageTree() method, but don’t forget about execution order: You can’t access the page tree during the onPagesDiscovered event because it wasn’t built yet. However, this also means that you can add pages to Pico’s pages array during the onPagesDiscovered event and they will automatically appear in the page tree later. But how does the $pageTree array look like? It’s a list of all the tree’s branches (no matter the depth). Refer to the phpDoc class docs of the Pico::buildPageTree() method for details.

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Pico was made by Gilbert Pellegrom and is maintained by The Pico Community. Released under the MIT license.