The Factory Pattern 71

The method Assessor::getPropInfo() represents the logical introduction of a PropertyInfo factory as a protected method of the Assessor class. The Assessor::getProperty() method is the public factory that returns one of our three Property subclasses, depending on what property name is requested.

Factories for Lazy Loading

Another significant benefit to using a Factory is the ability to perform lazy loading. Where this sce-nario comes into play most often is when a factory can instantiate a number of subclasses that are defined in separate PHP source files.

Terminology — Lazy Loading

The term lazy loading refers to not performing expensive operations (generally IO operations like includ-ing PHP files or querying a database) before they are absolutely required by the script.

A common technique with web sites is to have multiple web pages dynamically controlled through a single script. Consider blog software that might have different pages for viewing the recent entries, a single entry with comments, a comment submitting page, an archive navigation page, a page for the administrator to edit page, and so forth. You might encapsulate the logic to generate each of these in a class, and use a Factory to load both the class definition and the object. Each of these class-es might be stored in a separate file in a ‘pages’ subdirectory of your application. The code to implement a lazy loading page factory might look like:

class PageFactory { function &getPage() {

$page = (array_key_exists(‘page’, $_REQUEST)) ? strtolower($_REQUEST[‘page’]) : ‘’; switch ($page) {

case ‘entry’: $pageclass = ‘Detail’; break; case ‘edit’: $pageclass = ‘Edit’; break; case ‘comment’: $pageclass = ‘Comment’; break; default:

$pageclass = ‘Index’; } if (!class_exists($pageclass)) {

require_once ‘pages/’.$pageclass.’.php’; }

return new $pageclass; } }



	®	The Factory Pattern 71 The method Assessor::getPropInfo() represents the logical introduction of a PropertyInfo factory as a protected method of the Assessor class. The Assessor::getProperty() method is the public factory that returns one of our three Property subclasses, depending on what property name is requested. Factories for Lazy Loading Another significant benefit to using a Factory is the ability to perform lazy loading. Where this sce-nario comes into play most often is when a factory can instantiate a number of subclasses that are defined in separate PHP source files. Terminology — Lazy Loading The term lazy loading refers to not performing expensive operations (generally IO operations like includ-ing PHP files or querying a database) before they are absolutely required by the script. A common technique with web sites is to have multiple web pages dynamically controlled through a single script. Consider blog software that might have different pages for viewing the recent entries, a single entry with comments, a comment submitting page, an archive navigation page, a page for the administrator to edit page, and so forth. You might encapsulate the logic to generate each of these in a class, and use a Factory to load both the class definition and the object. Each of these class-es might be stored in a separate file in a ‘pages’ subdirectory of your application. The code to implement a lazy loading page factory might look like:

	class PageFactory { function &getPage() { $page = (array_key_exists(‘page’, $_REQUEST)) ? strtolower($_REQUEST[‘page’]) : ‘’; switch ($page) { case ‘entry’: $pageclass = ‘Detail’; break; case ‘edit’: $pageclass = ‘Edit’; break; case ‘comment’: $pageclass = ‘Comment’; break; default: $pageclass = ‘Index’; } if (!class_exists($pageclass)) { require_once ‘pages/’.$pageclass.’.php’; } return new $pageclass; } }