Magento – Adding custom product attributes to layered navigation but not shown

I inadvertently created the attributes as system attributes, i added 'system' => 0 where i added the attribute and this seems to have resolved the issue.

One of the solution is to add a backend model to your attribute which is used to format / validate your attribute value before save and/or after load.

Add a backend class :

[
    'type' => 'int',
    'backend' => '\Foo\Bar\Model\Attribute\Backend\YourAttribute',
    'frontend' => '',
    'label' => 'XXXX',
    'input' => 'text',
    'frontend_class' => 'validate-greater-than-zero',
    'source' => '',
    'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_GLOBAL,
    'visible' => true,
    'required' => true,
    'user_defined' => false,
    'default' => 0,
    'searchable' => false,
    'filterable' => true,
    'comparable' => false,
    'visible_on_front' => false,
    'used_in_product_listing' => true,
    'unique' => false
]

Here is an example of your custom class \Foo\Bar\Model\Attribute\Backend\YourAttribute

<?php

namespace Foo\Bar\Model\Attribute\Backend;

/**
 * Class YourAttribute
 */
class YourAttribute extends \Magento\Eav\Model\Entity\Attribute\Backend\AbstractBackend
{

    /**
     * @var int $minimumValueLength
     */
    protected $minimumValueLength = 0;

    /**
     * @param \Magento\Framework\DataObject $object
     *
     * @return $this
     */
    public function afterLoad($object)
    {
        // your after load logic

        return parent::afterLoad($object);
    }

    /**
     * @param \Magento\Framework\DataObject $object
     *
     * @return $this
     */
    public function beforeSave($object)
    {
        $this->validateLength($object);

        return parent::beforeSave($object);
    }

    /**
     * Validate length
     *
     * @param \Magento\Framework\DataObject $object
     *
     * @return bool
     * @throws \Magento\Framework\Exception\LocalizedException
     */
    public function validateLength($object)
    {
        /** @var string $attributeCode */
        $attributeCode = $this->getAttribute()->getAttributeCode();
        /** @var int $value */
        $value = (int)$object->getData($attributeCode);
        /** @var int $minimumValueLength */
        $minimumValueLength = $this->getMinimumValueLength();

        if ($this->getAttribute()->getIsRequired() && $value <= $minimumValueLength) {
            throw new \Magento\Framework\Exception\LocalizedException(
                __('The value of attribute "%1" must be greater than %2', $attributeCode, $minimumValueLength)
            );
        }

        return true;
    }

    /**
     * Get minimum attribute value length
     * 
     * @return int
     */
    public function getMinimumValueLength()
    {
        return $this->minimumValueLength;
    }
}

If you want a simple example of that kind of class you can check

• \Magento\Customer\Model\Customer\Attribute\Backend\Website
• all the classes which extend \Magento\Eav\Model\Entity\Attribute\Backend\AbstractBackend
• the classes into backend_model column in eav_attribute table

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EDIT
If you want a class that do nearly the same thing as you want you can take a look at the SKU attribute validation \Magento\Catalog\Model\Product\Attribute\Backend\Sku
I also added the method in the example class

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EDIT
Another solution (maybe not the best one) is to create a plugin on the function \Magento\Eav\Helper\Data::getFrontendClasses and add your frontend class here that can be validated in front.