PHP singleton pattern

【Foreword】

This article summarizes the PHP singleton pattern related

[Main body] Singleton mode

Simple understanding: a singleton is a class that can only be instantiated once and can only get one object

Shopping malls use singleton pattern

Idea: divided into steps

1. Create a common class; 2. Protect the constructor and encapsulate it (can not be called after protection); 3. Therefore, it needs to be called internally after encapsulation, so as to open an interface to the outside world;

4. Set as static, remove control, so that the call is not instantiated; 5. Add judgment; 6. Final prohibits inheritance

A single instance object, i.e. only one object can be instantiated

① Instances of common classes

<?php
    class Single{
        public $rand;
        public function __construct(){
            $this->rand = mt_rand(10,300);//mt_rand() random number
        }
    }
    var_dump(new Single());//115
    var_dump(new Single());//148
?>

Here I instantiate the class twice and get two objects with different values. This concludes that a class can instantiate multiple objects.

②Singleton pattern case

Instantiate into the class, plus conditional judgment

<?php
    class Single{
        public $rand;
        static public $ob;
        protected  function __construct(){
            $this->rand = mt_rand(10,300);//random number
        }
        static public function out(){
            if (Single::$ob === null) {//Determine whether to instantiate
                Single::$ob = new Single();
            }
            return Single::$ob;
        }
    }
    var_dump(Single::out());//Output 222
    var_dump(Single::out());//Output 222
?>

same output twice

The above example has not been completed, because when the inherited subclass is instantiated again, it will still produce multiple different results

<?php
    class Single{
        public $rand;
        static public $ob;
        //final is not allowed to be overridden by subclasses
        protected  function __construct(){
            $this->rand = mt_rand(10,300);//random number
        }
        static public function out(){
            if (Single::$ob === null) {//Determine whether to instantiate
                Single::$ob = new Single();
            }
            return Single::$ob;
        }
    }
    class Test extends Single{
        public function __construct(){
            echo rand(20,300);//This is overridden by subclasses
        }
    }
    new Test();
    new Test();
?>

For this we use final to prohibit overriding, because final classes cannot be inherited, and final methods cannot be overridden by subclasses

PHP singleton pattern

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