magento2 what exactly purpose of Factory class?

Question

From this documentation Factories are special objects that have only one purpose: to create an instance of one non-injectable class or interface non-injectable object are Objects that cannot be instantiated by the object manager. this \Magento\Catalog\Model\ResourceModel\Category\CollectionFactory is a factory class that really exist in vendor/magento/catalog directory

why we should use this function instead of inject directly \Magento\Catalog\Model\ResourceModel\Category\Collection in constructor , and why \Magento\Catalog\Model\ResourceModel\Category\Collection is a none-injectable class because from this link it is not instance of entity to provide or a model representing a database entity it is a collection of models

and another question is why \Magento\Catalog\Model\ResourceModel\Category\CollectionFactory already exist in vendor directory because from the documentation factories are an automatically generated class type. Factory classes do not need to be explicitly defined

Answer 1

In general you use factories to instantiate non-injectable objects, like the cms page object. Some objects require user input to instantiate so you cannot inject them via the constructor. \Magento\Catalog\Model\ResourceModel\Category\Collection is not one of those cases.

Factories are usually auto-generated, in the sense that, if you don't define one yourself, magento will auto-generate one for you:

if (!class_exists($className)) {
        return Generator::GENERATION_ERROR != $this->_generator->generateClass($className);
    }

And this answers your question about why you should use the Factory approach here. If you want to add a common behaviour for a certain collection, you have the chance to use your own Factory and easily modify the default behaviour.

But you can also inject \Magento\Catalog\Model\ResourceModel\Category\Collection in the constructor if you want to. This is not a case of a non-injectable object. It will still work, but it is not recommended.