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Patterns

Patterns are ways to describe best practices and good designs. They show a flexible solution to common programming problems.

Factory

The Factory pattern allows for the instantiation of objects at runtime. It is called a Factory Pattern since it is responsible for "manufacturing" an object. A Parameterized Factory receives the name of the class to instantiate as argument.

Example 10.25. Parameterized Factory Method

<?php
class Example
{
   
// The parameterized factory method
   
public static function factory($type)
   {
       if (include_once
'Drivers/' . $type . '.php') {
           
$classname = 'Driver_' . $type;
           return new
$classname;
       } else {
           throw new
Exception ('Driver not found');
       }
   }
}
?>

Defining this method in a class allows drivers to be loaded on the fly. If the Example class was a database abstraction class, loading a MySQL and SQLite driver could be done as follows:

<?php
// Load a MySQL Driver
$mysql = Example::factory('MySQL');

// Load a SQLite Driver
$sqlite = Example::factory('SQLite');
?>


Singleton

The Singleton pattern applies to situations in which there needs to be a single instance of a class. The most common example of this is a database connection. Implementing this pattern allows a programmer to make this single instance easily accessible by many other objects.

Example 10.26. Singleton Function

<?php
class Example
{
   
// Hold an instance of the class
   
private static $instance;
   
   
// A private constructor; prevents direct creation of object
   
private function __construct()
   {
       echo
'I am constructed';
   }

   
// The singleton method
   
public static function singleton()
   {
       if (!isset(
self::$instance)) {
           
$c = __CLASS__;
           
self::$instance = new $c;
       }

       return
self::$instance;
   }
   
   
// Example method
   
public function bark()
   {
       echo
'Woof!';
   }

   
// Prevent users to clone the instance
   
public function __clone()
   {
       
trigger_error('Clone is not allowed.', E_USER_ERROR);
   }

}

?>

This allows a single instance of the Example class to be retrieved.

<?php
// This would fail because the constructor is private
$test = new Example;

// This will always retrieve a single instance of the class
$test = Example::singleton();
$test->bark();

// This will issue an E_USER_ERROR.
$test_clone = clone $test;

?>