2.6. Dependency Injection

2.6.1. Purpose

To implement a loosely coupled architecture in order to get better testable, maintainable and extendable code.

2.6.2. Usage

DatabaseConfiguration gets injected and DatabaseConnection will get all that it needs from $config. Without DI, the configuration would be created directly in DatabaseConnection, which is not very good for testing and extending it.

2.6.3. Examples

  • The Doctrine2 ORM uses dependency injection e.g. for configuration that is injected into a Connection object. For testing purposes, one can easily create a mock object of the configuration and inject that into the Connection object
  • many frameworks already have containers for DI that create objects via a configuration array and inject them where needed (i.e. in Controllers)

2.6.4. UML Diagram

Alt DependencyInjection UML Diagram

2.6.5. Code

You can also find this code on GitHub

DatabaseConfiguration.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\DependencyInjection;

class DatabaseConfiguration
{
    public function __construct(
        private string $host,
        private int $port,
        private string $username,
        private string $password
    ) {
    }

    public function getHost(): string
    {
        return $this->host;
    }

    public function getPort(): int
    {
        return $this->port;
    }

    public function getUsername(): string
    {
        return $this->username;
    }

    public function getPassword(): string
    {
        return $this->password;
    }
}

DatabaseConnection.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\DependencyInjection;

class DatabaseConnection
{
    public function __construct(private DatabaseConfiguration $configuration)
    {
    }

    public function getDsn(): string
    {
        // this is just for the sake of demonstration, not a real DSN
        // notice that only the injected config is used here, so there is
        // a real separation of concerns here

        return sprintf(
            '%s:%s@%s:%d',
            $this->configuration->getUsername(),
            $this->configuration->getPassword(),
            $this->configuration->getHost(),
            $this->configuration->getPort()
        );
    }
}

2.6.6. Test

Tests/DependencyInjectionTest.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\DependencyInjection\Tests;

use DesignPatterns\Structural\DependencyInjection\DatabaseConfiguration;
use DesignPatterns\Structural\DependencyInjection\DatabaseConnection;
use PHPUnit\Framework\TestCase;

class DependencyInjectionTest extends TestCase
{
    public function testDependencyInjection()
    {
        $config = new DatabaseConfiguration('localhost', 3306, 'domnikl', '1234');
        $connection = new DatabaseConnection($config);

        $this->assertSame('domnikl:1234@localhost:3306', $connection->getDsn());
    }
}