Lazy Initialization Pattern

Lazy initialization is the tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed.

In a software design pattern view, lazy initialization is often used together with a factory method pattern. This combines three ideas:

• using a factory method to get instances of a class (factory method pattern)
• storing the instances in a map, so you get the same instance the next time you ask for an instance with same parameter (compare with a singleton pattern)
• using lazy initialization to instantiate the object the first time it is requested (lazy initialization pattern).

A Simple PHP Example

php

class User
{
protected $_id;

public function getId()
{
   return $this->_id;
}
}

class Post
{
protected $_userId;

protected $_text;

/**
*
* @var User
*/
protected $_user;


public function setUser(User $user)
{
   $this->_userId = $user->getId();
   $this->_user = $user;
}

public function getUser()
{
   if (!$this->_user) {
       $this->_user = new User($this->_userId);
   }
   return $this->_user;
}

public function setText($text)
{
   $this->_text = $text;
}

public function getText()
{
   return $this->_text;
}
}

A PHP 5 Example

php

class View
{
protected $_values = array();

public function set($name, $value)
{
   $this->_values[$name] = $value;
}

public function render($file)
{
   extract($this->_values);
   ob_start();
   include($file);
   return ob_get_clean();
}
}

class Page
{
/**
* View object.
*
* @var View
*/
protected $_view;

public function __construct()
{
   // remove attribute to use the __get magic method in first access
   unset($this->_view);
}

public function __get($name)
{
   if ($name == '_view') {
       // load view object
       $this->_view = new View();
       return $this->_view;
   }
}

public function actionIndex()
{
   $this->_view->set('title', 'Lazy Initialization');
   print $this->_view->render('lazy.tpl');
}
}