单例模式

单例模式是一种创建型设计模式， 让你能够保证一个类只有一个实例， 并提供一个访问该实例的全局节点。

  • 将默认构造函数设为私有，防止其他对象使用单例类的 new运算符。
  • 新建一个静态构建方法作为构造函数。该函数会 “偷偷” 调用私有构造函数来创建对象，并将其保存在一个静态成员变量中。 此后所有对于该函数的调用都将返回这一缓存对象。
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class Singleton
{
    private static $instances = [];

    protected function __construct(){}
    
    protected function __clone(){}

    public function __wakeup()
    {
        throw new \Exception("Cannot unserialize singleton");
    }
    
    public static function getInstance()
    {
        $subclass = static::class;
        if (!isset(self::$instances[$subclass])) {
            self::$instances[$subclass] = new static();
        }
        return self::$instances[$subclass];
    }
}

class Logger extends Singleton
{
   
    private $fileHandle;
    
    protected function __construct()
    {
        $this->fileHandle = fopen('php://stdout', 'w');
    }
    
    public function writeLog(string $message): void
    {
        $date = date('Y-m-d');
        fwrite($this->fileHandle, "$date: $message\n");
    }

    public static function log(string $message): void
    {
        $logger = static::getInstance();
        $logger->writeLog($message);
    }
}

class Config extends Singleton
{
    private $hashmap = [];

    public function getValue(string $key): string
    {
        return $this->hashmap[$key];
    }

    public function setValue(string $key, string $value): void
    {
        $this->hashmap[$key] = $value;
    }
}

/**
 * The client code.
 */
Logger::log("Started!");

// Compare values of Logger singleton.
$l1 = Logger::getInstance();
$l2 = Logger::getInstance();
if ($l1 === $l2) {
    Logger::log("Logger has a single instance.");
} else {
    Logger::log("Loggers are different.");
}

// Check how Config singleton saves data...
$config1 = Config::getInstance();
$login = "test_login";
$password = "test_password";
$config1->setValue("login", $login);
$config1->setValue("password", $password);
// ...and restores it.
$config2 = Config::getInstance();
if (
    $login == $config2->getValue("login") &&
    $password == $config2->getValue("password")
) {
    Logger::log("Config singleton also works fine.");
}

Logger::log("Finished!");