Overriding _getHtml function in topmenu.php makes navbar disappear

Question

I'm trying to override the __getHtml function in topmenu.php

class Topmenu extends \Magento\Theme\Block\Html\Topmenu
{
    protected function _getHtml(
        \Magento\Framework\Data\Tree\Node $menuTree,
        $childrenWrapClass,
        $limit,
        array $colBrakes = []
    ) {
        $html = '';

        $children = $menuTree->getChildren();
        $childLevel = $this->getChildLevel($menuTree->getLevel());
        $this->removeChildrenWithoutActiveParent($children, $childLevel);

        $counter = 1;
        $childrenCount = $children->count();

        $parentPositionClass = $menuTree->getPositionClass();
        $itemPositionClassPrefix = $parentPositionClass ? $parentPositionClass . '-' : 'nav-';

        /** @var Node $child */
        foreach ($children as $child) {
            $child->setLevel($childLevel);
            $child->setIsFirst($counter === 1);
            $child->setIsLast($counter === $childrenCount);
            $child->setPositionClass($itemPositionClassPrefix . $counter);

            $outermostClassCode = '';
            $outermostClass = $menuTree->getOutermostClass();

            if ($childLevel === 0 && $outermostClass) {
                $outermostClassCode = ' class="' . $outermostClass . '" ';
                $this->setCurrentClass($child, $outermostClass);
            }

            if ($this->shouldAddNewColumn($colBrakes, $counter)) {
                $html .= '</ul></li><li class="column"><ul>';
            }

            $html .= '<li ' . $this->_getRenderedMenuItemAttributes($child) . '>';
            $html .= '<a href="' . $child->getUrl() . '" ' . $outermostClassCode . '><span>' . $this->escapeHtml(
                $child->getName()
            ) . '</span></a>' . $this->_addSubMenu(
                $child,
                $childLevel,
                $childrenWrapClass,
                $limit
            ) . '</li>';
            $counter++;
        }

        if (is_array($colBrakes) && !empty($colBrakes) && $limit) {
            $html = '<li class="column"><ul>' . $html . '</ul></li>';
        }

        return $html;
    }
}

It's the original function because I first wanted to test the override. But this makes the navbar completely disappear.

Overriding another function like _addSubMenu works, though.

Answer 1

I just had the very same problem and I was able to "xdebug" it: when you override the Topmenu _getHtml like this, copying the original code from the parent object, you end up trying to use/call a series of private methods from the parent! And because of the php visibility your descendant object does not have access to those private methods.

This also explains why you were able to override a different method without problems: the _addSubMenu method is not calling any private methods, only protected which is ok.

The list of private methods _getHtml calls is: removeChildrenWithoutActiveParent, getChildLevel, shouldAddNewColumn and setCurrentClass. I just copied them to my own descendant class.