Magento 2: what is the $data array constructor parameter?

Question

So I noticed that in most models and blocks, there's this array $data = [] given as the last parameter of the constructor.

For example \Magento\Catalog\Block\Product\ListProduct

public function __construct(
    \Magento\Catalog\Block\Product\Context $context,
    \Magento\Framework\Data\Helper\PostHelper $postDataHelper,
    \Magento\Catalog\Model\Layer\Resolver $layerResolver,
    CategoryRepositoryInterface $categoryRepository,
    \Magento\Framework\Url\Helper\Data $urlHelper,
    array $data = []
) {
    $this->_catalogLayer = $layerResolver->get();
    $this->_postDataHelper = $postDataHelper;
    $this->categoryRepository = $categoryRepository;
    $this->urlHelper = $urlHelper;
    parent::__construct(
        $context,
        $data
    );
}

I also know that, when dealing with preferences, you still have to keep that parameter in the end of your constructor parameters list when you add more parameters than the original constructor.

So I've got several questions regarding this array:

• what is it ?
• how to use it ?
• why do we need to keep it at the end of the constructor parameters list when declaring preferences for a block that adds more parameters ?

Answer 1

The $data can be used to populate data on your object since the constructor of \Magento\Framework\DataObject is this

public function __construct(array $data = [])
{
    $this->_data = $data;
}

or similar for Magento\Framework\Api\AbstractSimpleObject

/**
 * Initialize internal storage
 *
 * @param array $data
 */
public function __construct(array $data = [])
{
    $this->_data = $data;
}

which a lot of classes in Magento extend from.

A common use is in conjunction with a factory. For example in Magento\Sales\Model\Order\CustomerManagement we have:

$this->addressFactory->create(['data' => $addressData]);

which essentially populates the $data array at creation.

Having to keep the $data = [] at the end of the list of parameters is normal php behaviour since you are assigning a default value - the empty array.

Answer 2

The $data[] can be used to hold values passed from di.xml's arguments replacement mechanism, when \Magento\Catalog\Block\Product\ListProduct intantiated.

In your case this could be populated by

<type name="\Magento\Catalog\Block\Product\ListProduct">
         <arguments>
             <argument name="data" xsi:type="array">
                 <item name="0" xsi:type="string">anystring</item>
                 <item name="1" xsi:type="string">xyz</item>
                <item name="2" xsi:type="string">abs</item>
            </argument>
         </arguments>
     </type>

You can even pass object and other formats to an array using above di mechanism. Further you can use those arguments while extending functionality. We can say, it is a kind of va_list in c.

This can be used when you don't want to modify the number of arguments in a constructor and want to inject other classes in your extended functionalities.