单例模式（Singleton）

1、模式定义

2、UML类图

3、示例代码

Singleton.php

<?php

namespace DesignPatterns\Creational\Singleton;

/**
 * Singleton类
 */
class Singleton
{
    /**
     * @var Singleton reference to singleton instance
     */
    private static $instance;
    
    /**
     * 通过延迟加载（用到时才加载）获取实例
     *
     * @return self
     */
    public static function getInstance()
    {
        if (null === static::$instance) {
            static::$instance = new static;
        }

        return static::$instance;
    }

    /**
     * 构造函数私有，不允许在外部实例化
     *
     */
    private function __construct()
    {
    }

    /**
     * 防止对象实例被克隆
     *
     * @return void
     */
    private function __clone()
    {
    }

    /**
     * 防止被反序列化
     *
     * @return void
     */
    private function __wakeup()
    {
    }
}

4、测试代码

<?php

namespace DesignPatterns\Creational\Singleton\Tests;

use DesignPatterns\Creational\Singleton\Singleton;

/**
 * SingletonTest用于测试单例模式
 */
class SingletonTest extends \PHPUnit_Framework_TestCase
{

    public function testUniqueness()
    {
        $firstCall = Singleton::getInstance();
        $this->assertInstanceOf('DesignPatterns\Creational\Singleton\Singleton', $firstCall);
        $secondCall = Singleton::getInstance();
        $this->assertSame($firstCall, $secondCall);
    }

    public function testNoConstructor()
    {
        $obj = Singleton::getInstance();

        $refl = new \ReflectionObject($obj);
        $meth = $refl->getMethod('__construct');
        $this->assertTrue($meth->isPrivate());
    }
}