CategoryWidget.php

<?php
namespace NicolasBejean\CategoryWidget\Block\Widget;

class CategoryWidget extends \Magento\Framework\View\Element\Template implements \Magento\Widget\Block\BlockInterface
{
    /**
     * Category helper
     *
     * @var \Magento\Catalog\Helper\Category
     */
    protected $_categoryHelper;

    /**
     * Category Flat Config
     *
     * @var \Magento\Catalog\Model\Indexer\Category\Flat\State
     */
    protected $categoryFlatConfig;

    /**
     * Top Menu
     *
     * @var \Magento\Theme\Block\Html\Topmenu
     */
    protected $topMenu;

    /**
     * Category factory
     *
     * @var \Magento\Catalog\Model\CategoryFactory
     */
    protected $_categoryFactory;

    protected $widgetTitle;
    protected $widgetContent;

    /**
     * @param \Magento\Framework\View\Element\Template\Context $context
     * @param \Magento\Catalog\Helper\Category $categoryHelper
     * @param \Magento\Catalog\Model\Indexer\Category\Flat\State $categoryFlatState
     * @param \Magento\Catalog\Model\CategoryFactory $categoryFactory
     * @param \Magento\Theme\Block\Html\Topmenu $topMenu
     * @param \Magento\Framework\Image\AdapterFactory $imageFactory
     */
    public function __construct(
        \Magento\Framework\View\Element\Template\Context $context,
        \Magento\Catalog\Helper\Category $categoryHelper,
        \Magento\Catalog\Model\Indexer\Category\Flat\State $categoryFlatState,
        \Magento\Catalog\Model\CategoryFactory $categoryFactory,
        \Magento\Theme\Block\Html\Topmenu $topMenu,
        \Magento\Framework\Image\AdapterFactory $imageFactory
    ) {
        $this->_categoryHelper = $categoryHelper;
        $this->categoryFlatConfig = $categoryFlatState;
        $this->_categoryFactory = $categoryFactory;
        $this->topMenu = $topMenu;
        $this->_imageFactory = $imageFactory;

        parent::__construct($context);
    }

    /**
     * Return categories helper
     */
    public function getCategoryHelper()
    {
        return $this->_categoryHelper;
    }

    public function getCategoryModel($id)
    {
         $_category = $this->_categoryFactory->create();
         $_category->load($id);
         return $_category;
    }

    /**
     * Retrieve current store categories
     *
     * @param bool|string $sorted
     * @param bool $asCollection
     * @param bool $toLoad
     * @return \Magento\Framework\Data\Tree\Node\Collection|\Magento\Catalog\Model\Resource\Category\Collection|array
     */

    /**
     * Récupère le titre du widget
    */
    public function getWidgetTitle()
    {
        return $this->getData('widget_title');
    }

    /**
     * Récupère le contenu du widget
     */
    public function getWidgetContent()
    {
        return $this->getData('widget_content');
    }

    /**
     * Récupère la sélection de catégories
     */
    public function getCategorySelection()
    {
        $rootCat = $this->getData('category_selection');


        $category = $this->_categoryFactory->create();
        $collection = $category
            ->getCollection()
            ->addAttributeToSelect('image')
            ->addIdFilter($rootCat);
        return $collection;
    }

    /**
     * Récupère le template du widget
     */
    public function getTemplate()
    {
        return $this->getData('template');
    }

    /**
     * Récupère la largeur de l'image du widget
     */
    public function getCategoryImageWidth()
    {
        return $this->getData('category_image_width');
    }

    /**
     * Récupère la hauteur de l'image du widget
     */
    public function getCategoryImageHeight()
    {
        return $this->getData('category_image_height');
    }

    /**
     * Permet d'optimiser les images de la catégorie
     */
    public function getResizeImage($image, $width = null, $height = null)
    {
        if ($image) :
            if (is_string($image)) :
                $isRelativeUrl = substr($image, 0, 1) === '/';
                if ($isRelativeUrl) :
                    $image = ltrim($image, '/media/');
                else :
                    $image = 'catalog/category/'.$image;
                endif;
            endif;
        endif;

        if(is_null($width)) :
            $width = $this->getCategoryImageWidth();
        endif;

        if(is_null($height)):
            $height = $this->getCategoryImageHeight();
        endif;

        $absolutePath = $this->_filesystem->getDirectoryRead(\Magento\Framework\App\Filesystem\DirectoryList::MEDIA)->getAbsolutePath('').$image;

        $imageResized = $this->_filesystem->getDirectoryRead(\Magento\Framework\App\Filesystem\DirectoryList::MEDIA)->getAbsolutePath('resized/'.$width.'/').$image;

        /**
         * Créé imageFactory
         */
        $imageResize = $this->_imageFactory->create();
        $imageResize->open($absolutePath);
        $imageResize->constrainOnly(TRUE);
        $imageResize->keepTransparency(TRUE);
        $imageResize->keepFrame(FALSE);
        $imageResize->keepAspectRatio(TRUE);

        $imageResize->resize($width,$height);

        /**
         * Enregistre l'image dans le dossier de destination
         */
        $destination = $imageResized ;
        $imageResize->save($destination);

        $resizedURL = $this->_storeManager->getStore()->getBaseUrl(\Magento\Framework\UrlInterface::URL_TYPE_MEDIA).'resized/'.$width.'/'.$image;

        return $resizedURL;
    }
}