Magento – Magento2 – Layered Navigation how to create multiselect functionality using code

Well, turns out this behavior is a feature, not a bug.

In the multiselect attribute's Storefront Properties is a setting "Use in Layered Navigation". Two two choices cause the attribute to be used in layered navigation. These are:

• Filterable (with results)
• Filterable (no results)

Evidently, selecting "with results", means that in Navigation this attribute's filter only shows attribute values which, if selected as a filter, would change the list of products shown. So Magento hides that value if there are zero items having that value. But Magento also hides that value if all the currently shown products have that value.

So that may make it easier for the user to choose filter values which will definitely whittle down the list. However it hides from the user the attribute values that the shown items have, suggesting that the filtered items lack those attribute values.

In the example I gave in the question statement, the user would think that the two products have only Blue or Green color schemes, and be unaware that these are Red-Green and Red-Blue schemes.

At any rate, the "Filterable (no results)" option will result in the corresponding Layered Nav filter showing all allowed values and their product counts, whether the value corresponds to zero, some, or all currently shown items.

I have not discovered a happy medium setting where Magento would hide filter values with zero hits, but show all other values.

You should extend \Magento\Catalog\Model\Layer\Category or \Magento\Catalog\Model\Layer\Search class, like this:

In your etc/di.xml add:

...
<preference for="Magento\Catalog\Model\Layer\Category" type="[Vendor]\[Your_Extension]\Model\Override\Layer\Category" />
<type name="[Vendor]\[Your_Extension]\Model\Override\Layer\Category">
    <arguments>
        <argument name="context" xsi:type="object">Magento\Catalog\Model\Layer\Category\Context</argument>
    </arguments>
</type>
...

Create your model with your new or overrided functions in [Vendor][Your_Extension]\Model\Override\Layer\Category:

<?php

namespace [Vendor]\[Your_Extension]\Model\Override\Layer;

use Magento\Catalog\Api\CategoryRepositoryInterface;
use Magento\Catalog\Model\ResourceModel;

class Category extends \Magento\Catalog\Model\Layer
{
    /**
     * @param $context
     * @param StateFactory $layerStateFactory
     * @param \Magento\Catalog\Model\ResourceModel\Product\Attribute\CollectionFactory $attributeCollectionFactory
     * @param \Magento\Catalog\Model\ResourceModel\Product $catalogProduct
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param \Magento\Framework\Registry $registry
     * @param CategoryRepositoryInterface $categoryRepository
     * @param array $data
     */
    public function __construct(
        \Magento\Catalog\Model\Layer\ContextInterface $context,
        \Magento\Catalog\Model\Layer\StateFactory $layerStateFactory,
        \Magento\Catalog\Model\ResourceModel\Product\Attribute\CollectionFactory $attributeCollectionFactory,
        \Magento\Catalog\Model\ResourceModel\Product $catalogProduct,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Framework\Registry $registry,
        CategoryRepositoryInterface $categoryRepository,
        array $data = []
    ) {
        parent::__construct(
            $context,
            $layerStateFactory,
            $attributeCollectionFactory,
            $catalogProduct,
            $storeManager,
            $registry,
            $categoryRepository,
            $data
        );
    }

    public function newFunction($param1){
        //Your code here
        return $this;
    }
}