1.7. Singleton

DIESES MUSTER IST EIN ANTI-PATTERN! FÜR BESSERE TESTBARKEIT UND WARTBARKEIT, VERWENDE DAS DEPENDENCY INJECTION MUSTER!

1.7.1. Zweck

Um nur eine einzelne Instanz eines Objekts in der Anwendung zu verwenden, die alle Aufrufe abarbeitet.

1.7.2. Beispiele

  • DB Konnektor
  • Logger
  • Lock file für die Anwendung (falls diese nur einmal auf dem System laufen darf …)

1.7.3. UML Diagramm

Alt Singleton UML Diagram

1.7.4. Code

Du findest den Code auch auf GitHub

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. Теst

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);
    }
}