PHP设计模式

Singleton(单例模式):单例模式是最常见的模式之一，在Web应用的开发中，常常用于允许在运行时为某个特定的类创建仅有一个可访问的实例。

<?php

/**
 * Singleton class[单例模式]
 * @author ITYangs<ityangs@163.com>
 */
final class Mysql
{

    /**
     *
     * @var self[该属性用来保存实例]
     */
    private static $instance;

    /**
     *
     * @var mixed
     */
    public $mix;

    /**
     * Return self instance[创建一个用来实例化对象的方法]
     *
     * @return self
     */
    public static function getInstance()
    {
        if (! (self::$instance instanceof self)) {
            self::$instance = new self();
        }
        return self::$instance;
    }

    /**
     * 构造函数为private,防止创建对象
     */
    private function __construct()
    {}

    /**
     * 防止对象被复制
     */
    private function __clone()
    {
        trigger_error('Clone is not allowed !');
    }
}

// @test
$firstMysql = Mysql::getInstance();
$secondMysql = Mysql::getInstance();

$firstMysql->mix = 'ityangs_one';
$secondMysql->mix = 'ityangs_two';

print_r($firstMysql->mix);
// 输出： ityangs_two
print_r($secondMysql->mix);
// 输出： ityangs_two

在很多情况下，需要为系统中的多个类创建单例的构造方式，这样，可以建立一个通用的抽象父工厂方法：

<?php
/**
 * Singleton class[单例模式:多个类创建单例的构造方式]
 * @author ITYangs<ityangs@163.com>
 */
abstract class FactoryAbstract {

    protected static $instances = array();

    public static function getInstance() {
        $className = self::getClassName();
        if (!(self::$instances[$className] instanceof $className)) {
            self::$instances[$className] = new $className();
        }
        return self::$instances[$className];
    }

    public static function removeInstance() {
        $className = self::getClassName();
        if (array_key_exists($className, self::$instances)) {
            unset(self::$instances[$className]);
        }
    }

    final protected static function getClassName() {
        return get_called_class();
    }

    protected function __construct() { }

    final protected function __clone() { }
}

abstract class Factory extends FactoryAbstract {

    final public static function getInstance() {
        return parent::getInstance();
    }

    final public static function removeInstance() {
        parent::removeInstance();
    }
}
// @test

class FirstProduct extends Factory {
    public $a = [];
}
class SecondProduct extends FirstProduct {
}

FirstProduct::getInstance()->a[] = 1;
SecondProduct::getInstance()->a[] = 2;
FirstProduct::getInstance()->a[] = 11;
SecondProduct::getInstance()->a[] = 22;

print_r(FirstProduct::getInstance()->a);
// Array ( [0] => 1 [1] => 11 )
print_r(SecondProduct::getInstance()->a);
// Array ( [0] => 2 [1] => 22 )