Singleton(单例模式)

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一个可访问的实例。

<?php
/**
 * Singleton class
 */
final class Product
{
 
    /**
     * @var self
     */
    private static $instance;
 
    /**
     * @var mixed
     */
    public $mix;
 
 
    /**
     * Return self instance
     *
     * @return self
     */
    public static function getInstance() {
        if (!(self::$instance instanceof self)) {
            self::$instance = new self();
        }
        return self::$instance;
    }
 
    private function __construct() {
    }
 
    private function __clone() {
    }
}
 
$firstProduct = Product::getInstance();
$secondProduct = Product::getInstance();
 
$firstProduct->mix = 'test';
$secondProduct->mix = 'example';
 
print_r($firstProduct->mix);
// example
print_r($secondProduct->mix);

　　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
 
abstract class FactoryAbstract {
 
    protected static $instances = array();
 
    public static function getInstance() {
        $className = static::getClassName();
        if (!(self::$instances[$className] instanceof $className)) {
            self::$instances[$className] = new $className();
        }
        return self::$instances[$className];
    }
 
    public static function removeInstance() {
        $className = static::getClassName();
        if (array_key_exists($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();
    }
}
// using:
 
class FirstProduct extends Factory {
    public $a = [];
}
class SecondProduct extends FirstProduct {
}
 
FirstProduct::getInstance()->a[] = 1;
SecondProduct::getInstance()->a[] = 2;
FirstProduct::getInstance()->a[] = 3;
SecondProduct::getInstance()->a[] = 4;
 
print_r(FirstProduct::getInstance()->a);
// array(1, 3)
print_r(SecondProduct::getInstance()->a);
// array(2, 4)