PHP design pattern singleton pattern

Introduction

The singleton pattern refers to a design pattern in which a class has only one object instance in the entire application. It is usually used to create objects, ensure that there is only one instance of a class, and instantiate itself and provide this instance to the entire system.

It is a common design pattern. In computer systems, thread pools, caches, log objects, dialog boxes, printers, database operations, and graphics card drivers are often designed as singletons.

Features

A class has one and only one instance in the entire application

The class must create this instance by itself

It must provide this instance to the entire system by itself

Function

The biggest effect of using the singleton pattern is to reduce the system cost overhead, saving resources. For example, when we use PHP for daily development, we often need to connect to various relational and non-relational databases such as MySQL or redis multiple times. If we do not use the singleton mode, we need to perform new multiple times to create new instances, which will consume a lot of resources. memory resources and system resources.

Points

1. A static member variable is needed to save the only instance of the class

private static $_instance;

2. The constructor and clone function must be declared private to prevent external programs from new classes and thus lose the meaning of the singleton mode

// 私有化构造方法，防止new实例
private function __construct()
{
    $this->_db = pg_connect('demo');
}
// 覆盖__clone()方法，禁止克隆
private function __clone()
{
}

3. A public static method (usually the getInstance method) that accesses this instance must be provided to return a reference to the unique instance

// 用于返回对象实例的公共静态方法
public static function getInstance()
{
    // 判断当前实例是否已经创建
    if (!(self::$_instance instanceof self) ) {
        // 如果没有，则创建
        self::$_instance = new self();
    }
    // 最终返回实例
    return self::$_instance;
}

Explanation

Now I will take a MySQL operation class as an example to demonstrate how to write it.

Code:

<?php

/**
 * Class DB 数据库操作类
 */
class Db
{
    private $_db;
    private static $_instance;

    // 私有化构造方法，防止外部new实例
    private function __construct(...)
    {
        // 这里主要用于连接数据库，细节此处省略
        $this->_db = pg_connect(...); 
    }

    // 覆盖__clone()方法，禁止克隆
    private function __clone() 
    {
        
    }
    
    // 返回对象实例
    public static function getInstance()
    {
        // 判断是否存在对象实例
        if(! (self::$_instance instanceof self) ) {
            // 不存在则创建保存
            self::$_instance = new self();
        }
        // 返回对象实例
        return self::$_instance;
    }

    // 其他方法
    public function addUserInfo(...)
    {
        
    }

    // 其他方法
    public function getUserInfo(...)
    {

    }
}


// 演示
$db = Db::getInstance();
$db->addUserInfo(...);
$db->getUserInfo(...);

Summary

Having talked so much about the singleton mode, it does not mean that it has no shortcomings. Below I have listed the advantages and disadvantages of using the singleton mode. Take a look and you can choose according to your actual needs. use.

Advantages and disadvantages of singleton mode:

Advantages:

Improve the design of the system

is an improvement on global variables

Save system resource overhead

Disadvantages:

Difficult to debug

Hidden dependencies

Cannot overwrite a singleton with wrong type of data