用PHP扩展单例 [英] Extending singletons in PHP

问题描述

我正在使用Web应用程序框架，其中一部分由许多服务组成，所有服务都以单例形式实现。它们都扩展了一个Service类，其中实现了单例行为，如下所示：

I'm working in a web app framework, and part of it consists of a number of services, all implemented as singletons. They all extend a Service class, where the singleton behaviour is implemented, looking something like this:

class Service {
    protected static $instance;

    public function Service() {
        if (isset(self::$instance)) {
            throw new Exception('Please use Service::getInstance.');
        }
    }

    public static function &getInstance() {
        if (empty(self::$instance)) {
            self::$instance = new self();
        }
        return self::$instance;
    }
}

现在，如果我有一个名为FileService的类实现像这个：

Now, if I have a class called FileService implemented like this:

class FileService extends Service {
    // Lots of neat stuff in here
}

...调用FileService :: getInstance（）不会产生FileService实例，就像我想要的那样，但是一个服务实例。我假设这里的问题是服务构造函数中使用的self关键字。

... calling FileService::getInstance() will not yield a FileService instance, like I want it to, but a Service instance. I assume the problem here is the "self" keyword used in the Service constructor.

还有其他方法可以实现我想要的吗？单例代码只有几行，但我仍然希望尽可能避免任何代码冗余。

Is there some other way to achieve what I want here? The singleton code is only a few lines, but I'd still like to avoid any code redundance whenever I can.

推荐答案

代码：

abstract class Singleton
{
    protected function __construct()
    {
    }

    final public static function getInstance()
    {
        static $instances = array();

        $calledClass = get_called_class();

        if (!isset($instances[$calledClass]))
        {
            $instances[$calledClass] = new $calledClass();
        }

        return $instances[$calledClass];
    }

    final private function __clone()
    {
    }
}

class FileService extends Singleton
{
    // Lots of neat stuff in here
}

$fs = FileService::getInstance();

如果您使用PHP< 5.3，添加它：

If you use PHP < 5.3, add this too:

// get_called_class() is only in PHP >= 5.3.
if (!function_exists('get_called_class'))
{
    function get_called_class()
    {
        $bt = debug_backtrace();
        $l = 0;
        do
        {
            $l++;
            $lines = file($bt[$l]['file']);
            $callerLine = $lines[$bt[$l]['line']-1];
            preg_match('/([a-zA-Z0-9\_]+)::'.$bt[$l]['function'].'/', $callerLine, $matches);
        } while ($matches[1] === 'parent' && $matches[1]);

        return $matches[1];
    }
}

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