Magento 2 dynamically change dependency injected based on configuration?

Question

I have a simple interface, PurgerInterface that has the methods needed to purge items from a cache backend (Varnish or Fastly). I have some custom configuration (standard system.xml stuff) that allows an admin to decide which backend they'd like to use (Varnish or Fastly).

I also have an Observer that listens to various events and will purge the appropriate caches. Here is the issue: I would like to inject, in my observer constructor, this purger interface like so:

public function __construct( CacheConfig $config, \Psr\Log\LoggerInterface $logger, PurgerInterface $purger) { ....

Now, the issue is that the concrete implementation of the PurgerInterface that I'd like injected is dependent on a configuration. If the Varnish backend is selected, I'd like VarnishPurger injected. If Fastly is selected, I'd like FastlyPurger.

Now, I'm familiar with other frameworks where you are able to dynamically bind items in the "service container" (I'm thinking Laravel here). Basically the logic / code controlling dependency injection can be interacted with, so when the container asks for a PurgerInterface I can run some code to check which concrete class I should return, and that is the class that is injected.

So, is this possible?

Answer 1

First, you need to create the factory that will be responsible for creating needed class base on configuration values. like

class Factory 
{
  ....
    public function create($data)
    {
        $className = $this->getClassFromConfiguration();
        $mapperInstance = $this->_objectManager->create($className, $data);

        if (false == $mapperInstance instanceof YourAdapterInterface) {
            throw new \Exception(
                'Object is not instance on YourAdapterInterface'
            );
        }
        return $mapperInstance;
    }
}

If you need to implements this for shared instance you should create Proxy.

class Proxy implements \Magento\Framework\ObjectManager\NoninterceptableInterface
{

    /**
     * @var \Magento\Config\Model\Config\Structure\Element\Group
     */
    protected $_subject;

    /**
     * @param Factory $factory
     */
    public function __construct($factory)
    {
        $this->factory = $objectManger;
    }

    /**
     * Retrieve subject
     *
     * @return YourAdapterInterface
     */
    protected function _getSubject()
    {
        if (!$this->_subject) {
            $this->_subject = $this->factory->create()
        }
        return $this->_subject;
    }

    /**
     * Method from interface
     *
     */
    public function methodName()
    {
        return $this->_getSubject()->methodName();
    }

    ... implementation of all methods from the interface by pattern

}

and use this class as the preference for interface