• Design Patterns

[[toc]]

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()

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


++++ Implementing Abstract Factory

    $classname = "DB_${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

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

    return $registry;
}

++++ Implementing Singleton in PHP5

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

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

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

++++ Implementing Observer: Observer

+++ 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
• [http://phrame.sourceforge.net/ Phrame]
• [http://www.tonybibbs.com/index.php?topic=MVCnPHP MVCnPHP]

++ Some Resources

• The [http://c2.com/ppr/ Portland Pattern Repository] (the original wiki)
• [http://www.phparchitect.com/ php|architect]
• [http://www.phppatterns.com/ phpPatterns.com] (not updated recently)