1.7. Singleton

BU DESEN ANTI-PATTERN OLARAK NİTELENDİRİLMİŞTİR, DAHA İYİ BİR TEST EDİLEBİLİRLİK (TESTABILITY) VE BAKIM YAPILABİLİRLİK (MAINTAINABILITY) İÇİN BAĞIMLILIK ENJEKSİYONU (DEPENDENCY INJECTION) KULLANINIZ!

1.7.1. Amaç

Bir uygulamada bir nesnenin yalnızca bir örneğine (instance) ilgili tüm istemleri işletmek.

1.7.2. Örnekler

  • Veritabanı Bağlantıcısı (Database Connector)
  • Kayıtçı (Logger)
  • Uygulama için kilitleme (lock) dosyası (dosya sisteminde sadece bir tane var …).

1.7.3. UML Diyagramı

Alt Singleton UML Diagram

1.7.4. Kod

Bu kodu Github üzerinde de bulabilirsiniz.

Singleton.php

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<?php

declare(strict_types=1);

namespace DesignPatterns\Creational\Singleton;

use Exception;

final class Singleton
{
    private static ?Singleton $instance = null;

    /**
     * gets the instance via lazy initialization (created on first usage)
     */
    public static function getInstance(): Singleton
    {
        if (static::$instance === null) {
            static::$instance = new static();
        }

        return static::$instance;
    }

    /**
     * is not allowed to call from outside to prevent from creating multiple instances,
     * to use the singleton, you have to obtain the instance from Singleton::getInstance() instead
     */
    private function __construct()
    {
    }

    /**
     * prevent the instance from being cloned (which would create a second instance of it)
     */
    private function __clone()
    {
    }

    /**
     * prevent from being unserialized (which would create a second instance of it)
     */
    public function __wakeup()
    {
        throw new Exception("Cannot unserialize singleton");
    }
}

1.7.5. Test

Tests/SingletonTest.php

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<?php

declare(strict_types=1);

namespace DesignPatterns\Creational\Singleton\Tests;

use DesignPatterns\Creational\Singleton\Singleton;
use PHPUnit\Framework\TestCase;

class SingletonTest extends TestCase
{
    public function testUniqueness()
    {
        $firstCall = Singleton::getInstance();
        $secondCall = Singleton::getInstance();

        $this->assertInstanceOf(Singleton::class, $firstCall);
        $this->assertSame($firstCall, $secondCall);
    }
}