Patterns are ways to describe best practices and good designs.
They show a flexible solution to common programming problems.
The Factory pattern allows for the instantiation of objects
at runtime. It is called a Factory Pattern since it is
responsible for "manufacturing" an object.
例子 19-23. Factory Method
<?php class Example { // The factory method public static function factory($type) { if (include_once 'Drivers/' . $type . '.php') { $classname = 'Driver_' . $type; return new $classname; } else { throw new Exception ('Driver not found'); } } } ?>
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Defining this method in a class allows drivers to be loaded on the
fly. If the Example class was a database
abstraction class, loading a MySQL and
SQLite driver could be done as follows:
<?php // Load a MySQL Driver $mysql = Example::factory('MySQL');
// Load a SQLite Driver $sqlite = Example::factory('SQLite'); ?>
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The Singleton pattern applies to situations in which
there needs to be a single instance of a class.
The most common example of this is a database connection.
Implementing this pattern allows a programmer to make this
single instance easily accessible by many other objects.
例子 19-24. Singleton Function
<?php class Example { // Hold an instance of the class private static $instance; // A private constructor; prevents direct creation of object private function __construct() { echo 'I am constructed'; }
// The singleton method public static function singleton() { if (!isset(self::$instance)) { $c = __CLASS__; self::$instance = new $c; }
return self::$instance; } // Example method public function bark() { echo 'Woof!'; }
// Prevent users to clone the instance public function __clone() { trigger_error('Clone is not allowed.', E_USER_ERROR); }
}
?>
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This allows a single instance of the Example
class to be retrieved.
<?php // This would fail because the constructor is private $test = new Example;
// This will always retrieve a single instance of the class $test = Example::singleton(); $test->bark();
// This will issue an E_USER_ERROR. $test_clone = clone($test);
?>
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