Design Patterns TODO: add more from the kongress2002-design_patterns presentation What are design patterns? "Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice." ? Christopher Alexander, talking about buildings and towns - Many common programming problems - They've been solved over and over again - Recognize which problems are common - Name them to create a common vocabulary - Patterns are blueprints - They are meant to be modified - Not all suspension bridges are the same What do patterns not do? - Patterns are not rules - Patterns are not implementations! When to use them - When the Problem they solve is a problem you have ... - ... and when the Consequences are acceptable - Solve problems you have, not problems you think you might have Common Patterns - Abstract Factory - Singleton - Observer - Model/View/Controller Abstract Factory - Provides an interface for creating one of a family of objects without specifying that object's concrete class name. - Example: DB::connect()


require_once 'DB.php';

$type = 'mysql';
$options = array(...);
$dbh = &DB::connect($type, $options);

Implementing Abstract Factory


    function &factory($type)
    {
        @include_once("DB/${type}.php");

        $classname = "DB_${type}";

        if (!class_exists($classname)) {
            return PEAR::raiseError(null, DB_ERROR_NOT_FOUND,
                                    null, null, null, 'DB_Error', true);
        }

        return $obj =& new $classname;
    }

Singleton - Ensures that there is only ever one instance of a class. - Can only be implemented as a suggestion in PHP4 Implementing Singleton


    function &singleton()
    {
        static $registry;

        if (!isset($registry)) {
            $registry = new Registry();
        }

        return $registry;
    }

Implementing Singleton in PHP5 PHP5's improved object model allows a better singleton implementation:


    /**
     * The one instance.
     */
    static private $instance = false;

    /**
     * Make the constructor private.
     */
    private function __construct() {}

    /**
     * Use this static method to get at the one instance.
     */
    static public function singleton()
    {
        if (!self::$instance) {
            self::$instance = new Singleton();
        }
        return self::$instance;
    }

Observer - Also called Subject-Observer, Listener, or Publish-Subscribe - Let an arbitrary number of objects react to state changes Implementing Observer: Subject


    function attach(&$observer)
    {
        $this->_observers[$observer->getId()] =& $observer;
    }

    function detach(&$observer)
    {
        unset($this->_observers[$observer->getId()]);
    }

    function notify()
    {
        foreach ($this->_observers as &$observer) {
            $observer->update($this);
        }
    }

Implementing Observer: Observer


    function update(&$subject)
    {
        // Do what needs to be done.
    }

Model/View/Controller - MVC is a specialized Observer pattern, with Composite and Strategy completing the picture - Observer describes how Views and Models interact - Composite lets Views be made up of other Views - Strategy lets Views use different Controllers MVC: Model/View - Think of structured HTML as a Model, and CSS as a View - Goal is to completely separate data and logic from presentation - Allows redesigns or new views (different stylesheet) without changing the Model MVC: View Compositing - When Smalltalk-80 introduced MVC it had a CompositeView - The Composite pattern solves the problem of wanting to treat groups of objects like individual objects MVC: View/Controller - The Controller decides how a View handles user input - Think of a command line interface, web interface, and web services as potential Controllers - Controllers take user input and give them to both Views and Models as appropriate PHP Implementations - http://www.google.com/search?q=php+mvc ... numerous - Phrame - MVCnPHP Some Resources - The Portland Pattern Repository (the original wiki) - php|architect - phpPatterns.com (not updated recently)