Sfrgttk

Not sure if it is Magento related question. But need to understand this.

I have seen some of the parameters in construct are assigned to null value.

For example:

/**

 * Initialize dependencies.

 *

 * @param MagentoFrameworkModelContext $context

 * @param MagentoFrameworkRegistry $registry

 * @param MagentoFrameworkApiExtensionAttributesFactory $extensionFactory

 * @param AttributeValueFactory $customAttributeFactory

 * @param MagentoSalesModelOrderFactory $orderFactory

 * @param MagentoStoreModelStoreManagerInterface $storeManager

 * @param MagentoCatalogApiProductRepositoryInterface $productRepository

 * @param MagentoFrameworkModelResourceModelAbstractResource $resource

 * @param MagentoFrameworkDataCollectionAbstractDb $resourceCollection

 * @param array $data

 * @param MagentoFrameworkSerializeSerializerJson|null $serializer

 * @SuppressWarnings(PHPMD.ExcessiveParameterList)

 */

public function __construct(

    MagentoFrameworkModelContext $context,

    MagentoFrameworkRegistry $registry,

    MagentoFrameworkApiExtensionAttributesFactory $extensionFactory,

    AttributeValueFactory $customAttributeFactory,

    MagentoSalesModelOrderFactory $orderFactory,

    MagentoStoreModelStoreManagerInterface $storeManager,

    MagentoCatalogApiProductRepositoryInterface $productRepository,

    MagentoFrameworkModelResourceModelAbstractResource $resource = null,

    MagentoFrameworkDataCollectionAbstractDb $resourceCollection = null,

    array $data = [],

    MagentoFrameworkSerializeSerializerJson $serializer = null

) {

    parent::__construct(

        $context,

        $registry,

        $extensionFactory,

        $customAttributeFactory,

        $resource,

        $resourceCollection,

        $data

    );

    $this->serializer = $serializer ?: MagentoFrameworkAppObjectManager::getInstance()

        ->get(MagentoFrameworkSerializeSerializerJson::class);

    $this->_orderFactory = $orderFactory;

    $this->_storeManager = $storeManager;

    $this->productRepository = $productRepository;

}

MagentoFrameworkSerializeSerializerJson $serializer = null is assigned to null. And in the function it is assigned to object.

$this->serializer = $serializer ?: MagentoFrameworkAppObjectManager::getInstance()

            ->get(MagentoFrameworkSerializeSerializerJson::class);

This is done for backward compatibility. For instance, if a class was inherited and the child class doesn't specify the needed argument the class initialization doesn't fail, the missed dependency will be initialized by ObjectManager.

Such workaround was added in Magento 2.2.0 release when all public code was marked as @api to don't break extensions which might use the private code. Starting from 2.2.0, the code which not marked as @api might be changed in a patch release and it won't be a breaking change because other classes should not depend on the private implementation.

For more details, please, see Adding a constructor parameter section in the official documentation.