Dependency Injection, Lazy loading, Proxy and circular dependency

Question

This is a language independent question but I write the sample codes in PHP.

We have a two classes :

1. User class
2. UserRepository class

UserRepository class deals with the interaction with DB and loading the required User in to a User object and return it.

Within the User class let's say there is an email attribute which is not loaded in the first place, and it would be lazy-loaded from Database whenever it's needed. But User class should not be aware of the UserRepository class, so we create a email proxy here. Which is aware of the UserRepository and wheveer the email is needed it would ask it from the UserRepository.

So here is the Code so far :

User class

class User
{
    private $username;
    private $email_proxy;
    public function __construct($username,EmailProxy $email_proxy)
    {
        $this->username = $username;
        $this->email_proxy = $email_proxy;
    }

    public function get_username()
    {
        return $this->username;
    }

    public function get_email()
    {
        return $this->email_proxy->get_email($this->get_username());
    }
}

UserRepository class

class UserRepository
{
    private $db;
    public function __construct($db)
    {
        $this->db = $db;
    }

    /**
     * @param string username
     * @return string email , email address of the user
     */
    public function get_email_by_username($username)
    {
        // interaction with DB, ...
        return $user_email;
    }
} 

EmailProxy class

class EmailProxy
{
    private $user_repository;
    public function __construct(UserRepository $repository)
    {
        $this->user_repository = $repository;
    }
    public function get_email($username)
    {
        return $this->user_repository->get_email_by_username($username);
    }
}

And here is the usage sample :

$user_repository = new UserRepository($db_instance);
$email_proxy = new EmailProxy();
$user = new User('my_username', $email_proxy);

So far so good. But here is the tricky part which I need your help about. By it's nature, a UserRepository should be responsible for fetching a User object from DataBase, constructing a User object and returning it. Like below :

class UserRepository
{
    private $db;
    public function __construct($db)
    {
        $this->db = $db;
    }

    public function get_email_by_username($username)
    {
        // interaction with DB, ...
        return $user_email;
    }

    public function get_user_by_username($username)
    {
        // fetch the User from DB and ...
        return new User($fetched_username) // missing the EmailProxy
    }
}

So my question is how do pass the EmailProxy to the User object which is created by UserRepository ?

• Do you inject the UserProxy instance to UserRepository so you could inject it to newly created User objects ?

• Would you use a Dependency Injection Container for this?

• Would you use a factory?

EDIT

EmailProxy is already aware of UserRepository, if we pass EmailProxy to UserRepository as well, it would be a circular dependency.

Any code/comments would be appreciated.

Thanks,

Answer 1

It's a tricky question and I'd love to ear how people handle it. In the mean time, here's what I've come up to myself.

I would put the creation of the User and UserRepository classes in a Factory

class ServiceFactory
{
    private $instances=[];
    public function createUser($data)
    {
        return new User($data)
            ->setEmailProxy($this->createEmailProxy());

    }

    public function createEmailProxy()
    {
        if (!isset($this->instances['EmailProxy'])) {
            $this->instances['EmailProxy'] = new EmailProxy(
                $this->createUserRepository()
            );
        }
        return $this->instances['EmailProxy'];
    }

    public function createUserRepository()
    {
        if (!isset($this->instances['UserRepository'])) {
            $this->instances['UserRepository'] = new UserRepository(
                $db, 
                $this->createDtoFactory()
               );
        }
        return $this->instances['UserRepository'];
    }

    public function createDtoProvider()
    {
        return new DtoProvider($this);
    }

}

Here's the code of the DtoProvider class

class DtoProvider
{
    private $serviceFactory;

    public function __construct($factory) 
    {
        $this->serviceFactory = $factory;
    }

    public function createUser($data)
    {
        return $this->serviceFactory->createUser($data);
    }

    public function createOtherStuff($data)
    {
        return $this->serviceFactory->createOtherStuff($data);
    }
}

The purpose of this class is hide the factory(ies) from the services. You only provide them what they need.

Now your UserRepository should look something like that

class UserRepository
{
    private $db;
    private $services;
    public function __construct($db, $serviceProvider)
    {
        $this->db = $db;
        $this->services = $serviceProvider;
    }

    public function get_email_by_username($username)
    {
        // interaction with DB, ...
        return $user_email;
    }

    public function get_user_by_username($username)
    {
        // fetch the User from DB and ...
        return $this->services->createUser($data)
    }
}

It seems a bit overkill (and it propably is) but it allows you to refactor your code quite easyly which is good for long term application.

I guess it all depends whether you prefer code that is easy to read or code that is easy (quick) to write.

Answer 2

Perhaps a reference to the User object inside your UserRepository::get_email_by_username() method.

Something like:

public function get_email_by_username(&$username)
{
    // interaction with DB, ...
    $username->setEmail($email_proxy);
}

http://php.net/manual/en/language.references.pass.php

---

In reply to your comment, you can simply call the UserRepository method when you create the user:

public function get_user_by_username($username)
{
    $email_proxy = this->get_email_by_username($username);
    return new User($fetched_username, $email_proxy);
}

However, that seems too obvious an answer and I suspect you're asking something entirely different than is posted. You may want to rephrase your question:

So my question is how do pass the EmailProxy to the User object which is created by UserRepository ?