Magento 2 has no ability to order an event's observers execution

[apiuser]

Feature request from mage2pro, posted on GitHub Nov 13, 2015

https://mage2.pro/t/200

For example, if there are mutiple observers for the catalog_block_product_status_display event then the Magento 2 behavior becomes unpredictable because we can not set the observer's ordering.

[apiuser]

Comment from rhamoudi, posted on GitHub Dec 02, 2015

Have you tested that the ordering isn't done by reading the xml from top to bottom?

[apiuser]

Comment from Yonn-Trimoreau, posted on GitHub Feb 17, 2016

+1

It will be really useful for ordering menu items when observing on page_block_html_topmenu_gethtml_before event.

[apiuser]

Comment from ntzz, posted on GitHub May 17, 2016

@Yonn-Trimoreau Is possible to order top menu yet?

[apiuser]

Comment from Yonn-Trimoreau, posted on GitHub May 20, 2016

Yes, you can plug an observer to the event "page_block_html_topmenu_gethtml_before" and recreate your own nodes. That's the only way I found.

In file Vendor/ModuleName/etc/frontend/events.xml :

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="page_block_html_topmenu_gethtml_before">
        <observer name="add_items_to_menu" instance="Vendor\ModuleName\Observer\AddItemsToMenu"/>
    </event>
</config>

In file Vendor/ModuleName/Observer/AddItemsToMenu.php :

<?php
namespace Vendor\ModuleName\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;

class AddItemsToMenu implements ObserverInterface
{
    /**
     * @var \Magento\Store\Model\StoreManagerInterface
     */
    protected $_storeManager;

    /**
     * @var \Magento\Framework\UrlInterface
     */
    protected $_urlInterface;

    /**
     * AddItemsToMenu Observer constructor.
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param \Magento\Framework\UrlInterface $urlInterface
     */
    public function __construct(
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Framework\UrlInterface $urlInterface
    ) {
        $this->_storeManager = $storeManager;
        $this->_urlInterface = $urlInterface;
    }

    /**
     * @param Observer $observer
     * @return void
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        /** @var \Magento\Framework\Data\Tree\Node $parentCategoryNode */
        $parentCategoryNode = $observer->getMenu();
        $block = $observer->getEvent()->getBlock();
        $tree = $parentCategoryNode->getTree();
        $currentUrl = $this->_urlInterface->getCurrentUrl();

        // Create menu array
        $nodesData = array(
            array(
                "name" => "Home",
                "id" => "home",
                "url" => $url = $block->getUrl(''),
                "has_active" => false,
                "is_active" => $currentUrl == $url ? true : false,
                "extra_class" => "home",
                "order" => 1,
            ),
            array(
                "name" => "Menu1",
                "id" => "menu1",
                "url" => $url = $block->getUrl('url/menu1'),
                "has_active" => false,
                "is_active" => $currentUrl == $url ? true : false,
                "extra_class" => "menu1",
                "order" => 2,
            ),
            [...]
        );

        foreach ($nodesData as $nodeData) {
            $categoryNode = new \Magento\Framework\Data\Tree\Node($nodeData, 'id', $tree, $parentCategoryNode);
            $parentCategoryNode->addChild($categoryNode);
        }
    }
}

[apiuser]

Comment from BarryCarlyon, posted on GitHub Jun 17, 2016

As far as I can tell, this will only add items onto the end of the menu.

I am as yet to find a way to add items to the start of the menu.

(But using the same "name":

<observer name="catalog_add_topmenu_items" instance="Magento\Catalog\Observer\AddCatalogToTopmenuItemsObserver" />

so

<observer name="catalog_add_topmenu_items" instance="Vendor\ModuleName\Observer\AddItemsToMenu"/

Instead

Means you can override it it seems)

[apiuser]

Comment from BarryCarlyon, posted on GitHub Jun 17, 2016

But really an

$parentCategoryNode->prependChild($categoryNode);

Would be more useful

[apiuser]

Comment from BarryCarlyon, posted on GitHub Jun 17, 2016

I've ended up with:

etc/frontend/events.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="page_block_html_topmenu_gethtml_before">
        <observer name="catalog_add_topmenu_items" instance="Vendor\Module\Observer\TopMenu" />
    </event>
</config>

Observer/TopMenu.php

<?php

namespace Vendor\Module\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\Data\Tree\Node;

class Topmenu implements ObserverInterface {
    /**
     * @var \Psr\Log\LoggerInterface
     */
    protected $logger;

    /**
     * @param \Magento\Catalog\Observer\AddCatalogToTopmenuItemsObserver $orig
     * @param \Psr\Log\LoggerInterface $logger
     */
    public function __construct(
        \Magento\Catalog\Observer\AddCatalogToTopmenuItemsObserver $orig,
        \Psr\Log\LoggerInterface $logger
    ) {
        $this->logger = $logger;

        $this->orig = $orig;
    }

    /**
     * Apply customized static files to frontend
     *
     * @param \Magento\Framework\Event\Observer $observer
     * @return void
     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $menu = $observer->getMenu();
        $tree = $menu->getTree();

        $data = [
            'name'      => 'TestING',
            'id'        => 'test',
            'url'       => '/test/',
            'is_active' => false
        ];

        $node = new Node($data, 'id', $tree, $menu);
        $menu->addChild($node);

        // add cats by calling the original observer
        $this->orig->execute($observer);
    }
}

[apiuser]

Comment from Yonn-Trimoreau, posted on GitHub Jun 21, 2016

My solution rebuilds the tree entirely, it does not only add nodes to the end

[bobspongieux]

Forgive me, I was wrong, my first solution wasn't rebuilding the entire tree.

Here is the solution to add ordering functionality to your menu.

In app/code/Company/ModuleName/etc/frontend/di.xml :

<preference for="Magento\Theme\Block\Html\Topmenu" type="Company\ModuleName\Block\Html\OrderedTopmenu"/>

In app/code/Company/ModuleName/Block/Html/OrderedTopMenu.php :

<?php

namespace Company\ModuleName\Block\Html;

use Magento\Framework\Data\Tree\Node;
use Magento\Framework\Data\Tree\NodeFactory;
use Magento\Framework\Data\TreeFactory;
use Magento\Framework\View\Element\Template;

//
// FIXME: Menu ordering and CSS classes customization is home-made
//
//      See: https://github.com/magento/magento2/issues/2354
//      https://github.com/magento/magento2/issues/3446
//
class OrderedTopmenu extends \Magento\Theme\Block\Html\Topmenu
{
    /**
     * Custom version that handles an 'order' attribute. Defaults position is 100 if not specified.
     *
     * @param \Magento\Framework\Data\Tree\Node $menuTree
     * @param string $childrenWrapClass
     * @param int $limit
     * @param array $colBrakes
     * @return string
     */
    protected function _getHtml(
        \Magento\Framework\Data\Tree\Node $menuTree,
        $childrenWrapClass,
        $limit,
        $colBrakes = []
    ) {
        $children = [];

        foreach ($menuTree->getChildren() as $node) {
            $children[] = $node;
            $menuTree->getChildren()->delete($node);
        }

        usort($children, function($a, $b) {
            $i = $a->getOrder() ?: 100;
            $j = $b->getOrder() ?: 100;

            return $i - $j;
        });

        for ($i = 0; $i < count($children); $i++) {
            $menuTree->getChildren()->add($children[$i]);
        }

        return parent::_getHtml($menuTree, $childrenWrapClass, $limit, $colBrakes);
    }

    /**
     * Custom version to handle an 'extra_class' attribute.
     *
     * @see \Magento\Theme\Block\Html\Topmenu::_getMenuItemClasses()
     */
    protected function _getMenuItemClasses(\Magento\Framework\Data\Tree\Node $item)
    {
        $classes = parent::_getMenuItemClasses($item);

        if ($item->getExtraClass()) {
            $classes[] = $item->getExtraClass();
        }

        return $classes;
    }
}

In Company/ModuleName/etc/frontend/events.xml :

<event name="page_block_html_topmenu_gethtml_before">
    <observer name="company_modulename_add_items_to_menu" instance="Company\ModuleName\Observer\AddItemsToMenu"/>
</event>

In Company/ModuleName/Observer/AddItemsToMenu.php :

<?php

namespace Company\ModuleName\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;

class AddItemsToMenu implements ObserverInterface
{
    /**
     * @var \Magento\Store\Model\StoreManagerInterface $_storeManager
     */
    protected $_storeManager;

    /**
     * @var \Magento\Framework\UrlInterface $_urlInterface
     */
    protected $_urlInterface;

    /**
     * ForceLogin Observer constructor.
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param \Magento\Framework\UrlInterface $urlInterface
     */
    public function __construct(
            \Magento\Store\Model\StoreManagerInterface $storeManager,
            \Magento\Framework\UrlInterface $urlInterface
            ) {
                $this->_storeManager = $storeManager;
                $this->_urlInterface = $urlInterface;
    }

    /**
     * @param Observer $observer
     * @return void
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        /** @var \Magento\Framework\Data\Tree\Node $parentCategoryNode */
        $parentCategoryNode = $observer->getMenu();
        $block = $observer->getEvent()->getBlock();
        $tree = $parentCategoryNode->getTree();
        $currentUrl = $this->_urlInterface->getCurrentUrl();

        // Create menu array
        $nodesData = array(
                array(
                        "name" => __('Item name'),
                        "id" => "item-id",
                        "url" => $url = rtrim($block->getUrl('company_modulename/url'), '/'),
                        "has_active" => false,   // If one of his children is activated
                        "is_active" => $currentUrl == $url ? true : false,
                        "extra_class" => "extra-class",
                        "order" => 110
                ),
                ...
                ...
                ...
        );

        foreach ($nodesData as $nodeData) {
            $categoryNode = new \Magento\Framework\Data\Tree\Node($nodeData, 'id', $tree, $parentCategoryNode);
            $parentCategoryNode->addChild($categoryNode);
        }
    }
}

In Company/ModuleName/view/frontend/layout/default.xml :

<referenceBlock name="catalog.topnav">
    <action method="setTemplate">
        <argument name="template" xsi:type="string">Magento_Theme::html/topmenu.phtml</argument>
    </action>
</referenceBlock>