What's the advantage of factory classes?

Question

I figured out that magento 2 uses a proxy class which then creates an instance of a class. I don't understand why this proxy class is used, whats the advantage of it? Why not just creating the instance normally like always?

Example:

vendor\magento\module-catalog\Model\Product\Link.php:

public function __construct(
    ...
    \Magento\Catalog\Model\ResourceModel\Product\Link\Product\CollectionFactory $productCollectionFactory,
    ...
    array $data = []
) {
    ...
    $this->_productCollectionFactory = $productCollectionFactory;
    ...
}

public function getProductCollection()
{
    $collection = $this->_productCollectionFactory->create()->setLinkModel($this);
    return $collection;
}

The method create() from $this->_productCollectionFactory->create() is defined in generated\code\Magento\Catalog\Model\ResourceModel\Product\Link\Product\CollectionFactory.php:

public function create(array $data = [])
{
    return $this->_objectManager->create($this->_instanceName, $data);
}

It uses objectManager to create the object from the class defined in $this->_instanceName which is \Magento\Catalog\Model\ResourceModel\Product\Link\Product\Collection

This looks totaly complicated and unecessary, why all this trouble? Why not just creating the object directly from start in the constructor:

public function __construct(
    ...
    \Magento\Catalog\Model\ResourceModel\Product\Link\Product\Collection $productCollection,
    ...
    array $data = []
) {
    ...
    $this->_productCollection = $productCollection;
    ...
}

public function getProductCollection()
{
    $collection = $this->_productCollection->setLinkModel($this);
    return $collection;
}

Please help me to understand the advantage of the factory way.

Answer 1

We use proxy in Magento 2 when we are trying to delay the instance to be loaded. some class do not incur any data to load and some do

If you have a class that has some dependency to it and some of these dependencies can load data when their constructor is called, that would incur an overhead in your class if you need the class but yet you don't want the data to be loaded.

When a class has data (eg: customer, product, order..), I was recommended to use factory.

I did check the class you mention to convince myself of the load impact

you may follow the code trail as below:

1. \Magento\Catalog\Model\ResourceModel\Product\Link\Product\Collection
2. \Magento\Catalog\Model\ResourceModel\Product\Collection::__construct
3. \Magento\Catalog\Model\ResourceModel\Collection\AbstractCollection::__construct
4. \Magento\Eav\Model\Entity\Collection\AbstractCollection::__construct
in this constructor, we have the line $this->_prepareStaticFields(); loading  data. 

This function is likely one amongst more if you were to contemplate this constructor more thoroughly. However, as an architecture point of view, even if your class may seem pretty much light to load, it may not be the case on further Magento releases and it is for this reason important to use the factory to prevent unnecessary code to be triggered..