Magento – Magento 2.3 How to get all the Multi Source Inventory (MSI) locations collection in custom module

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

---

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

---

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.

You can proceed on the wiki page of the project - https://github.com/magento-engcom/msi/wiki and from the right side of the page you will find "MSI User Guide" which describes how to set up the project and use its main features.

Also, there is a big Slack channel devoted to "#MSI" in Magento Community Slack, which you can apply for following this link https://tinyurl.com/engcom-signup There are about 300 active participants there.