Eamonn Faherty

Background Reading
None

The Problem?
You want to ensure you have only a single instance of an object and want global access to it. This may be because the object uses a resource, such as sounds, or because the object needs a lot of configuration, such as a currency formatter, or needs global access, an event dispatcher.

Examples?
Currency formatter, sound controller, global event dispatcher

How?
The Singleton:

package singletonpattern {

	/**
	 * @author eamonn faherty
	 */
	public class SocketManagerSingleton {
		private static var _instace : SocketManagerSingleton;

		public static function getInstace() : SocketManagerSingleton {
			ensureInstanceExists();
			return _instace;
		}

		private static function ensureInstanceExists() : void {
			if (_instace == null) {
				_instace = new SocketManagerSingleton();
			}
		}

		public function send(message : String) : void {
		}
	}
}

Example Usage:

package singletonpattern {
	import common.Example;		

	/**
	 * @author eamonn faherty
	 */
	public class SingletonPatternExample extends Example {

		public function SingletonPatternExample() {
			var socket : SocketManagerSingleton = SocketManagerSingleton.getInstace();

			socket.send("Hello world");
		}
	}
}

Why is it good?
• It works well; if you use an instance protector then you can be sure that only a single instance will exist.
• It is well understood; it is widely used in the actionscript world.
• It is easy to understand; it is a simple pattern that needs little extra code.
• It is easy to implement; actionscript allows us to check the caller of a method and allows us to use private classes.

Why is it bad?
• It tightly couples the classes that use the singleton to the fact that they are using a singleton.
• It is not possible to extend the singleton class if you wanted to add functionality.
• You cannot use your singleton class again; you cannot extend it or reuse it with composition.
• If you are unit testing a class that uses a singleton you need to add some code to your singleton to make testing possible.

Further Reading
Read about the factory and inversion of control patterns.