Confuse when choose which class to extends

Question

Dive in thousand classes , interfaces maybe more.
How to decide which one to extend while create model or constructors.. to build module from scratch
Sometimes it makes me so confuse. It too many
Anyone can give me advice

Answer 1

Here are the classes you need to extend:

• Magento\Framework\Model\AbstractModel for flat table model
• Magento\Framework\Model\AbstractExtensibleModel for eav models
• Magento\Framework\Model\ResourceModel\Db\AbstractDb for flat table resource models
• Magento\Eav\Model\Entity\AbstractEntity for eav resource models
• Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection for flat entity collection resource model
• Magento\Eav\Model\Entity\Collection\AbstractCollection for eav entities collection resource model
• Magento\Backend\App\Action for the admin actions.

As for the constructor, if your class does not have any other dependencies that the parent class, don't bother writing it.
If you have other dependencies, then just copy the original class constructor and inside it add just parent::__construct(dependencies here) then start adding your additional dependencies.
Let's take an example: You are extending Magento\Framework\Model\AbstractModel and you want to add your dependencies.
The constructor for Magento\Framework\Model\AbstractModel looks like this:

public function __construct(
    \Magento\Framework\Model\Context $context,
    \Magento\Framework\Registry $registry,
    \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
    \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
    array $data = []
) {
    $this->_registry = $registry;
    $this->_appState = $context->getAppState();
    $this->_eventManager = $context->getEventDispatcher();
    $this->_cacheManager = $context->getCacheManager();
    $this->_resource = $resource;
    $this->_resourceCollection = $resourceCollection;
    $this->_logger = $context->getLogger();
    $this->_actionValidator = $context->getActionValidator();

    if (method_exists($this->_resource, 'getIdFieldName')
        || $this->_resource instanceof \Magento\Framework\DataObject
    ) {
        $this->_idFieldName = $this->_getResource()->getIdFieldName();
    }

    parent::__construct($data);
    $this->_construct();
}

you copy it in your class and transform it to this:

public function __construct(
    \Magento\Framework\Model\Context $context,
    \Magento\Framework\Registry $registry,
    \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
    \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
    array $data = []
) {
    parent::__construct($context, $registry, $resource, $resouceCollection, $data)
}

Now, let's say your class requires a new dependency. SomeClass $someClass You can add it anywhere in the constructor parameters, but make sure you add it above the parameters with default values.
I recommend adding it on top of all the parameters, to make it clearer.
So your class constructor becomes:

public function __construct(
    SomeClass $someClass, //added this dependency
    \Magento\Framework\Model\Context $context,
    \Magento\Framework\Registry $registry,
    \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
    \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
    array $data = []
) {
    $this->someClass = $someClass;//added this dependency
    parent::__construct($context, $registry, $resource, $resouceCollection, $data)
}

Now make sure you create a protected class member called someClass.