Doc/Dev/Debug
- 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
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Many common programming problems
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They've been solved over and over again
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Recognize which problems are common
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Name them to create a common vocabulary
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Patterns are blueprints
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They are meant to be modified
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Not all suspension bridges are the same
++ What do patterns not do?
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Patterns are not rules
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Patterns are not implementations!
++ When to use them
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When the Problem they solve is a problem you have ...
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... and when the Consequences are acceptable
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Solve problems you have, not problems you think you might have
++ Common Patterns
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Abstract Factory
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Singleton
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Observer
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Model/View/Controller
+++ Abstract Factory
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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
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Ensures that there is only ever one instance of a class.
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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
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Also called Subject-Observer, Listener, or Publish-Subscribe
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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
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MVC is a specialized Observer pattern, with Composite and Strategy completing the picture
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Observer describes how Views and Models interact
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Composite lets Views be made up of other Views
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Strategy lets Views use different Controllers
++++ MVC: Model/View
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Think of structured HTML as a Model, and CSS as a View
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Goal is to completely seperate data and logic from presentation
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Allows redesigns or new views (different stylesheet) without changing the Model
++++ MVC: View Compositing
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When Smalltalk-80 introduced MVC it had a !CompositeView
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The Composite pattern solves the problem of wanting to treat groups of objects like individual objects
++++ MVC: View/Controller
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The Controller decides how a View handles user input
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Think of a command line interface, web interface, and web services as potential Controllers
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Controllers take user input and give them to both Views and Models as appropriate
++ PHP Implementations
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http://www.google.com/search?q=php+mvc ... numerous
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[http://phrame.sourceforge.net/ Phrame]
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[http://www.tonybibbs.com/index.php?topic=MVCnPHP MVCnPHP]
++ Some Resources
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The [http://c2.com/ppr/ Portland Pattern Repository] (the original wiki)
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[http://www.phparchitect.com/ php|architect]
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[http://www.phppatterns.com/ phpPatterns.com] (not updated recently)