[php-di v7] Service Locator

[partikus]

Follow up - Service Locator introduced before by @falkenhawk in #7

copied from #7

ServiceLocator

• Serving "lazy" dependencies for classes using ServiceSubscriberInterface.
• A ServiceSubscriber exposes its dependencies via static getSubscribedServices() method.
• A ServiceLocator instance could then be injected into a class via constructor or a property - the instance would be already configured with dependences read from getSubscribedServices(), but the dependences won't be instantiated until first get - that's how "laziness" is introduced
• DI\Definition\Reference checks if it's a ServiceLocator entry by comparing its name with DI\Definition\Reference::$serviceLocatorClass
• Reference definitions are passed with additional parameter - $requestingName which generally points to name of the class which implements ServiceSubscriberInterface - to resolve ServiceLocator for that class
• Suggested as a lightweight alternative for heavyweight proxies from ocramius/proxy-manager

note:

• it's based on https://github.com/ovos/PHP-DI/tree/feature/annotations-autowiring-options branch to include a change for AnnotationBasedAutowiring::readConstructor() method, which was introduced there (second parameter passed to Reference instance)

Usage:

class MyClass implements ServiceSubscriberInterface
{
    /**
     * @var ServiceLocator
     */
    protected $serviceLocator;

    /**
     * @param ServiceLocator $serviceLocator
     */
    public function __construct(ServiceLocator $serviceLocator)
    {
        $this->serviceLocator = $serviceLocator;
    }

    /**
     * Lazy instantiate heavy dependencies on-demand
     */
    public static function getSubscribedServices(): array
    {
        return [
            SomeClass::class,
            'heavyDependency' => HeavyService::class,
        ];
    }

    public function doOperation()
    {
        $someClass = $this->serviceLocator->get(SomeClass::class);
        return $someClass->doSomething();
    }

    public function getSomethingFromThatHeavyDependency()
    {
        // this method may be rarely used, and it might be good idea to skip resolving the dependency every time during instantiation for performance reasons
        return $this->serviceLocator->get('heavyDependency')->getSomething();
    }
}

[falkenhawk]

please remove this from constructor parameters, this should not be altered from outside. Ref: https://github.com/ovos/PHP-DI/pull/7/files#diff-addce6eaa59f011f6a80f000d9b021f007d8410be8044e3a747a6aa93ac54053R55