Singleton Design Pattern
Issue no. 3 on design patterns: The singleton pattern.
Whenever we need a place to save and retrieve data throughout a script without using globals the singleton pattern comes in handy. It allows only one instance of itself. What seems like a drawback is quite useful: Wherever in your source you instantiate such an object: You can be sure that you’ll always get the instance of the same object. Remember the parameterized factory from the last chapter? Every time we wanted to create a new database-access-object we had to pass “mysql” or “pgsql” to the create-method. Wouldn’t it be easier if we just saved the database type to a singleton and retrieved it everytime we need it?
//------------------------------------------------
// Singleton
//------------------------------------------------
// Definition
class Singleton
{
// "This is a handle to myself"
private static $instance;
// database type
public $dbtype = "";
// Private clone-function and private constructor
// make sure, that nobody can clone the object or
// instantiate it using the standard constructor.
private function __construct() {}
private function __clone() {}
// This one returns the instance handle to itself.
// It's static, because it has to be accessible
// before any object is available
public static function GetInstance()
{
// "Am I already existing?"
if (!isset(self::$instance))
{
// "No? Create myself and save the handle
// to myself."
$class = __CLASS__;
self::$instance = new $class;
}
// "Return handle to mysqlf"
return self::$instance;
}
}
// ...
// somewhere in our code
function SetupScript()
{
// Get handle to our singleton object
$GlobalInfos = Singleton::GetInstance();
// Set the $content variable to 42
$GlobalInfos->dbtype = "mysql";
}
// ...
// somewhere else in our code
SetupScript();
// ... deep down in the code
// Get handle to our singleton object
$Registry = Singleton::GetInstance();
// Remeber that from the last chapter?
$myDbObj= MyClassFactory::Create($Registry->dbtype);