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()
<?php
require_once 'DB.php';
$type = 'mysql';
$options = array(...);
$dbh = &DB::connect($type, $options);
?>
Implementing Abstract Factory
<?php
function &factory($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
<?php
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:
<?php
/**
* 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
<?php
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
<?php
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
Some Resources