4.1. Service Locator

THIS IS CONSIDERED TO BE AN ANTI-PATTERN!

Service Locator is considered for some people an anti-pattern. It violates the Dependency Inversion principle. Service Locator hides class” dependencies instead of exposing them as you would do using the Dependency Injection. In case of changes of those dependencies you risk to break the functionality of classes which are using them, making your system difficult to maintain.

4.1.1. Propósito

Implementar una arquitectura débilmente acoplada con el fin de obtener un código más testeable, mantenible y ampliable. El patrón DI y el patrón Service Locator son una implementación del patrón Inverse of Control.

4.1.2. Uso

Con ``ServiceLocator”” puede registrar un servicio para una interfaz determinada. Al usar la interfaz, puede recuperar el servicio y usarlo en las clases de la aplicación sin conocer su implementación. Puede configurar e inyectar el objeto Service Locator en el arranque.

4.1.3. Diagrama UML

Alt ServiceLocator UML Diagram

4.1.4. Código

Puedes encontrar el código en GitHub

Service.php

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<?php

namespace DesignPatterns\More\ServiceLocator;

interface Service
{

}

ServiceLocator.php

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<?php

declare(strict_types=1);

namespace DesignPatterns\More\ServiceLocator;

use OutOfRangeException;
use InvalidArgumentException;

class ServiceLocator
{
    /**
     * @var string[][]
     */
    private array $services = [];

    /**
     * @var Service[]
     */
    private array $instantiated = [];

    public function addInstance(string $class, Service $service)
    {
        $this->instantiated[$class] = $service;
    }

    public function addClass(string $class, array $params)
    {
        $this->services[$class] = $params;
    }

    public function has(string $interface): bool
    {
        return isset($this->services[$interface]) || isset($this->instantiated[$interface]);
    }

    public function get(string $class): Service
    {
        if (isset($this->instantiated[$class])) {
            return $this->instantiated[$class];
        }

        $args = $this->services[$class];

        switch (count($args)) {
            case 0:
                $object = new $class();
                break;
            case 1:
                $object = new $class($args[0]);
                break;
            case 2:
                $object = new $class($args[0], $args[1]);
                break;
            case 3:
                $object = new $class($args[0], $args[1], $args[2]);
                break;
            default:
                throw new OutOfRangeException('Too many arguments given');
        }

        if (!$object instanceof Service) {
            throw new InvalidArgumentException('Could not register service: is no instance of Service');
        }

        $this->instantiated[$class] = $object;

        return $object;
    }
}

LogService.php

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<?php

declare(strict_types=1);

namespace DesignPatterns\More\ServiceLocator;

class LogService implements Service
{

}

4.1.5. Test

Tests/ServiceLocatorTest.php

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<?php

declare(strict_types=1);

namespace DesignPatterns\More\ServiceLocator\Tests;

use DesignPatterns\More\ServiceLocator\LogService;
use DesignPatterns\More\ServiceLocator\ServiceLocator;
use PHPUnit\Framework\TestCase;

class ServiceLocatorTest extends TestCase
{
    private ServiceLocator $serviceLocator;

    public function setUp(): void
    {
        $this->serviceLocator = new ServiceLocator();
    }

    public function testHasServices()
    {
        $this->serviceLocator->addInstance(LogService::class, new LogService());

        $this->assertTrue($this->serviceLocator->has(LogService::class));
        $this->assertFalse($this->serviceLocator->has(self::class));
    }

    public function testGetWillInstantiateLogServiceIfNoInstanceHasBeenCreatedYet()
    {
        $this->serviceLocator->addClass(LogService::class, []);
        $logger = $this->serviceLocator->get(LogService::class);

        $this->assertInstanceOf(LogService::class, $logger);
    }
}