Merge pull request #5989 from WoltLab/wsc-rpc-api-const

[GitHub/WoltLab/WCF.git] / wcfsetup / install / files / lib / system / SingletonFactory.class.php

<?php

namespace wcf\system;

use wcf\system\exception\SystemException;

/**
 * Base class for singleton factories.
 *
 * @author  Alexander Ebert
 * @copyright   2001-2019 WoltLab GmbH
 * @license GNU Lesser General Public License <http://opensource.org/licenses/lgpl-license.php>
 */
abstract class SingletonFactory
{
    /**
     * list of singletons
     * @var SingletonFactory[]
     */
    protected static $__singletonObjects = [];

    /**
     * Singletons do not support a public constructor. Override init() if
     * your class needs to initialize components on creation.
     */
    final protected function __construct()
    {
        $this->init();
    }

    /**
     * Called within __construct(), override if necessary.
     */
    protected function init()
    {
    }

    /**
     * Object cloning is disallowed.
     */
    final protected function __clone()
    {
    }

    /**
     * Object serializing is disallowed.
     */
    final public function __sleep()
    {
        throw new SystemException('Serializing of Singletons is not allowed');
    }

    /**
     * Returns an unique instance of current child class.
     *
     * @return  static
     * @throws  SystemException
     */
    final public static function getInstance()
    {
        $className = static::class;
        if (!isset(self::$__singletonObjects[$className])) {
            // The previously used `null` value forced us into using `array_key_exists()` which has a bad performance,
            // especially with the growing list of derived classes that are used today. Saves a few ms on every request.
            self::$__singletonObjects[$className] = false;
            self::$__singletonObjects[$className] = new $className();
        } elseif (self::$__singletonObjects[$className] === false) {
            throw new SystemException(
                "Infinite loop detected while trying to retrieve object for '" . $className . "'"
            );
        }

        return self::$__singletonObjects[$className];
    }

    /**
     * Returns whether this singleton is already initialized.
     *
     * @return  bool
     */
    final public static function isInitialized()
    {
        $className = static::class;

        return isset(self::$__singletonObjects[$className]);
    }
}