Sometimes this is what you also want to have in your software. To achieve different behaviour you normally just implement two different classes or aggregates and since they have the same “meaning” they normally share a interface (a.k.a Separated Interface). But who decides which implementation fits into the current environment/context?

The easiest way is to have a Factory Method with a small condition to decide this, but this method might grow very fast if you have various options. In this situation its also a matter of form to move this decision into some kind of configuration(-file) so that there’s only on file which differs in various environments.

Martin Fowler suggests to place some kind of mapping into the configuration-file, so that your Factory knows which implementation to instantiate in the current context. The key for the mapping in this case is the name of the Separated Interface.

As you see in the example code below, there are Cookie Tins which create contain Cookies and depending on the current month you want to use a more or less restrictive Cookie Tin. In May, June and July you restrict your application to create max. 5 cookies, the rest of the year you don’t care

	// The Separated Interface
interface CookieTin {
	public function getCookieInstance();
}

class NormalTin implements CookieTin {
	public function getCookieInstance() {
		return new Cookie();
	}
}

class DietTin implements CookieTin {
	protected $i=0;
	public function getCookieInstance() {
		return ($this->i++ < 5)?new Cookie():new NullCookie();
	}
}

// The Factory which supports Plugin-Creation
class CookieTinFactory {
	public static function getPlugin( $class ) {
		try {
			return new $GLOBALS['Plugins'][$class]();
		} catch( Exception $e) {
			// maybe call some default implementation if the mapping is wrong
		}
	}
}

class Cookie {
	public function printCalories() {
		echo ’200 ‘;
	}
}

class NullCookie extends Cookie{
	public function printCalories() { }
}

// This is usually in a configuration-file, normally this should
// also sit in a XML structure….

// That’s what it would be in 300 days/year
$GLOBALS['Plugins']['CookieTin'] = “NormalTin”;
// That’s what we have to turn it to before summer
if(in_array(date(‘n’),array(5,6,7))) {
	$GLOBALS['Plugins']['CookieTin'] = “DietTin”;
}

$tin = CookieTinFactory::getPlugin(‘CookieTin’);
$i=10;
while($i–) {
	$cookie = $tin->getCookieInstance();
	$cookie->printCalories();
}

A very common use-case for this is the changed behaviour in a testing-environment compared to the production-environment.
There's something else in the example beside the Plugin Pattern - I also made use of the Special Case (the NullCookie) pattern which I'm going to show in a future post....

So the example just shows a cookie-machine which makes use of the prototype-pattern to create new cookies (depending on the cookie you throw in before)… yummy

abstract class Cookie {
function __clone() { }
abstract public function printFlavor();
}

class CoconutCookie extends Cookie {
public function printFlavor() {
echo ‘Coconut Flavor<br/>’;
}
}
class ChocolateCookie extends Cookie {
public function printFlavor() {
echo ‘Chocolate Flavor<br/>’;
}
}

class CookieMachine {
protected $cookie;
public function __construct(Cookie $cookie) {
$this->cookie = $cookie;
}
public function makeCookie() {
return clone $this->cookie;
}
}

The client-code can look like this:

$coconutCookie = new CoconutCookie();
$coconutCookieMachine = new CookieMachine($coconutCookie);

$chocolateCookie = new ChocolateCookie();
$chocolateCookieMachine = new CookieMachine($chocolateCookie);

//while(true) {
for($i=0;$i<5;$i++) {
$coconutCookieMachine->makeCookie()->printFlavor();
$chocolateCookieMachine->makeCookie()->printFlavor();
}

Explaining it the tasty way: let’s say you have exactly one well know place where you store all your cookies and whenever you need one you can easy point to that place. Normally that’s really comfortable because whatever you do with it, you don’t have to search some place where you can find a cookie, you just go to that single “cookie tin” (or wherever you store cookies) and put a new cookie into it

In modern OOP the singleton is not very popular, Eric Evans simple calls it anti-pattern. It makes it much harder to test your software and normally the user of the class, not the class itself, should know how many instances are needed.

But nevertheless for small applications it’s very comfortable to use it in some situations and as long as you’re also familiar with the drawbacks I think it’s ok to consider using it….

So the example shows a cookie tin and how to use it and by the way it also implements some kind of lowlevel registry by using the PHP5 magic methods, but that’s another story

/**
* CookieTin a.k.a Singleton object
* we want to force our application
* to hold exactly one CookieTin object
*/
class CookieTin {
	private $_cookies=array();

	/* generic singleton part start*/

	private static $_instance=NULL;
	// prevent instatiation from the outside
	private function __construct() { }

	// prevent cloning
	private function __clone() {}

	public static function getInstance() {
		if(!self::$_instance) {
			self::$_instance = new self();
		}
		return self::$_instance;
	}
	/* generic singleton part end */

	/* registry part start */
	public static function __set($key,Cookie $value) {
		echo “call setter\n”;
		self::$_instance->_cookies[$key] = $value;
	}

	public static function __get($key) {
		echo “call getter\n”;
		return self::$_instance->_cookies[$key];
	}

	public static function __isset($key) {
		echo “call isset\n”;
		return isset(self::$_instance->_cookies[$key]);
	}

	public static function __unset($key) {
		echo “call unset\n”;
		unset(self::$_instance->_cookies[$key]);
	}
/* registry part end */
}
/**
* Dummy Cookie
*/
class Cookie {
	public $name,$flavor;
}

$tin = CookieTin::getInstance();
if(!isset($tin->granniescookie)) {
	$tin->granniescookie = new Cookie();
	$tin->granniescookie->name=“Grannies chocolate wonder”;
	$tin->granniescookie->flavor=“chocolate”;
}

CookieTin::getInstance()->granniescookie->flavor = “chocolte chips”;

var_dump($tin);
unset(CookieTin::getInstance()->granniescookie);
var_dump($tin);

The really important things happen in the last lines where the CookieTin is accessed statically and this change also (as you will see yourself) is reflected in the local instance if the $tin.