Let's talk about PHP singleton mode again Singleton_PHP tutorial

The singleton pattern is a creational pattern. It is the simplest pattern among design patterns. Of course, its use is ubiquitous.

The singleton pattern ensures that a class has only one instance and provides a global access point to access it.

When we need to control the number of instances of a class and the caller can access it from a public, well-known access point, we can consider using the singleton pattern.

We use UML to design the singleton pattern. Of course, in the design part of the future design pattern, we will use UML to describe our design, which will be more visual.

We can see from the UML design diagram that in order for a class to have only one instance, it must create a static variable, and then we use a public static Instance() method to create it, but in order to avoid the class itself The constructor can create objects. We set the constructor to protected or private, so that the outside can only create a static Singleton class through the Instance() method. It seems that we have achieved our goal. Next, let’s look at the code:

public class Singleton  {
    private static Singleton instance;
    protected Singleton()
    public static Singleton Instance() {
        if(instance != null) instance = new Singleton();
        return instance;
    }
}

From this point of view, we can do the following steps to implement the singleton pattern:

1. Create a static variable in the class, the variable type is the current class;
2. Create a public static method in the class so that users can create static objects of this class through this method;
3. Finally, set the constructor to protected or private.

Program List: the simplest singleton class

<?php
class Fruit
{
    static private $_color;
    private function __construct()
    {
    }
	
    static public function singleton() {
        return isset(self::$_color) ? self::$_color : self::$_color = new self();
    }
}
?>

Program List: Extensible singleton class

An extensible singleton class may seem impossible, but the following program comes close.

    
<?php
class Test extends Fruit {
    public static function getInstance()
	{
        return Fruit::getSingleton(get_class());
    }
}
?>
<?php
class Fruit {
    /***********************
     * HOW TO USE
     * 
     * Inherit(extend) from Singleton and add getter:
     * 
     *  //public getter for singleton instance
     *     public static function getInstance(){
     *        return Singleton::getSingleton(get_class());
     *    }
     * 
     */
    
    private static $instanceMap = array();
    //protected getter for singleton instances
    protected static function getSingleton($className)
	{
        if(!isset(self::$instanceMap[$className]))
		{
            
       		$object = new $className;
            //Make sure this object inherit from Singleton
            if($object instanceof Fruit)
			{    
                self::$instanceMap[$className] = $object;
            }
            else
			{
                throw SingletonException("Class '$className' do not inherit from Singleton!");
            }
        }
        return self::$instanceMap[$className];
    }    
    
    //protected constructor to prevent outside instantiation
    protected function __construct(){
    }
    
    //denie cloning of singleton objects
    public final function __clone(){
        trigger_error('It is impossible to clone singleton', E_USER_ERROR);
    }    
}
?>
<?php
class Apple extends Fruit {
    
    protected $rndId;
    
    protected function __construct(){
        $this->rndId = rand();
    }    
    
    public function whatAmI(){
        echo 'I am a Apple('.$this->rndId.')<br />';
    }
    public static function getInstance(){
        return Fruit::getSingleton(get_class());
    }
}
class GreenApple extends Apple {
    public function whatAmI(){
        echo 'I am a GreenApple('.$this->rndId.')<br />';
    }
    
    public static function getInstance(){
        return Fruit::getSingleton(get_class());
    }
}
$apple1 = Apple::getInstance();
$apple2 = GreenApple::getInstance();
$apple1->whatAmI();// should echo 'I am a A(some number)
$apple2->whatAmI();// should echo 'I am a B(some number)
$apple1 = Apple::getInstance();
$apple2 = GreenApple::getInstance();
$apple1->whatAmI();// should echo 'I am a A(same number as above)
$apple2->whatAmI();// should echo 'I am a B(same number as above)
// $a = new A();// this should fail
// $b = new B();// this should fail
?>

Program execution result:

I am a Apple(4462)
I am a GreenApple(8207)
I am a Apple(4462)
I am a GreenApple(8207)

Program List: singleton class and its derived classes

  
<?php
class Fruit
{
  // Hold an instance of the class
  private static $instance;
 
  // A private constructor; prevents direct creation of object
  protected function __construct()
  {
      echo 'I am constructed';
  }
  // The singleton method
  public static function singleton($classname = __CLASS__)
  {
      if (!isset(self::$instance)) {
          self::$instance = new $classname;
      }
      return self::$instance;
  }
}
class Apple extends Fruit {
   public static function singleton()
   {
       return parent::singleton(__CLASS__); // NOTE The singleton method MUST return an instance.
   }
   public function showColor()
   {
       echo 'My Color is Red.';
   }
}
$subclassInstance = Apple::singleton();
$subclassInstance->showColor();
?>

Program execution result:

I am constructed
My Color is Red.

http://www.bkjia.com/PHPjc/752380.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/752380.htmlTechArticleThe singleton pattern is a creational pattern. It is the simplest pattern among design patterns. Of course, its use It's also everywhere. The singleton pattern ensures that a class has only one instance and provides...