null-object.md

title
Null Object Proxy

Null Object Proxy

A Null Object proxy is a null object pattern implementation. The proxy factory creates a new object with defined neutral behaviour based on another object, class name or interface.

What is null object proxy?

In your application, when you cannot return the object related to the request, the consumer of the model must check for the return value and handle the failing condition gracefully, thus generating an explosion of conditionals throughout your code.

Fortunately, this seemingly-tangled situation can be simplified by creating a polymorphic implementation of the domain object, which would implement the same interface as one of the objects in question, only that its methods would not do anything, therefore offloading client code from doing repetitive checks for ugly null values when the operation is executed.

Usage examples

class UserMapper
{   
    private $adapter;
    
    public function __construct(DatabaseAdapterInterface $adapter) {
        $this->adapter = $adapter;
    }

    public function fetchById($id) {
        $this->adapter->select('users', ['id' => $id]);
        
        if (!$row = $this->adapter->fetch()) {
            return null;
        }
        
        return $this->createUser($row);
    }
     
    private function createUser(array $row) {
        $user = new Entity\User($row['name'], $row['email']);
        
        $user->setId($row['id']);
        
        return $user;
    }
}

If you want to remove conditionals from client code, you need to have a version of the entity conforming to the corresponding interface. With the Null Object Proxy, you can build this object :

$factory = new \ProxyManager\Factory\NullObjectFactory();

$nullUser = $factory->createProxy('Entity\User');

var_dump($nullUser->getName()); // empty return

You can now return a valid entity :

class UserMapper
{   
    private $adapter;
    
    public function __construct(DatabaseAdapterInterface $adapter) {
        $this->adapter = $adapter;
    }

    public function fetchById($id) {
        $this->adapter->select('users', ['id' => $id]);
        
        return $this->createUser($this->adapter->fetch());
    }
     
    private function createUser($row) {
        if (!$row) {
            $factory = new \ProxyManager\Factory\NullObjectFactory();

            return $factory->createProxy('Entity\User');
        }
        
        $user = new Entity\User($row['name'], $row['email']);
        
        $user->setId($row['id']);
        
        return $user; 
    }
}

Proxying interfaces

You can also generate proxies from an interface FQCN. By proxying an interface, you will only be able to access the methods defined by the interface itself, and like with the object, the methods are empty.

Tuning performance for production

See Tuning ProxyManager for Production.