Patterns are ways to describe best practices and good designs.
They show a flexible solution to common programming problems.
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 19-25. 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');
}
}
}
?>
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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');
?>
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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 19-26. 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);
}
}
?>
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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;
?>
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