Posts Tagged ‘Design Patterns’

Waterproof AS 3.0 Singleton

November 24th, 2006 6 comments

While reading Advanced ActionScript 3 with Design Patterns I’ve once again came across the AS 3.0 Singleton solution that has to deal with the absence of private constructors (I’ll save my rage and cursings about who had the idea to remove them for now ;)) and while the authors are using the already well known method of using a SingletonEnforcer class outside the Singleton’s package to verify that the class cannot be instanciated via the constructor they also note that this can be doublecrossed by giving null or undefined as a constructor parameter. As this is rather sub optimal I was wondering why they don’t just check inside the Singleton constructor for a null/undefined argument and throw an exception accordingly?!
I’m sure somebody else had the same idea already but here’s my idea of a waterproof Singleton class that cannot be misused from the outside (As always, if I missed any detail that accidentally breaks hell loose feel free to correct me) …

	public class WaterproofSingleton {
		static private var _instance:Singleton;

		public function Singleton(singletonEnforcer:SingletonEnforcer) {
			if ((singletonEnforcer as SingletonEnforcer) == null) {
				throw (new Error("Direct instantiation of a Singleton is not allowed!"));

		public static function getInstance():Singleton {
			if (Singleton._instance == null) {
				Singleton._instance = new Singleton(new SingletonEnforcer());
			return Singleton._instance;

class SingletonEnforcer {}

Learn some patterns – Advanced ActionScript 3.0 with Design Patterns

November 23rd, 2006 1 comment

I’ve been reading this book by Joey Lott and Danny Patterson since yesterday and thought I’d loose a couple of words about it …
The book starts with a chapter about application design where it explains how to go through the analysis-, design- , implementation and testing phase. It introduces how to utilize Use Case- and Class Diagrams and then gives a lesson on how to use FlexUnit for unit testing. In the second chapter is a good explanation about why and how to use Interfaces (instead of Inheritance) and after that it goes into detail with teaching nine of the more common Design Patterns, namely MVC, Singleton, Factory/Template Method, Proxy, Iterator, Composite, Decorator, Command, Memento and State. In part 3 of the book you’ll find in-depth information about AS 3.0 features like Events, data IO, E4X and Regular Expressions.

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Understanding Interfaces and Polymorphism

November 18th, 2006 2 comments

I’ve been using Interfaces a couples of time now when needed but I never fully understood those down-sized Class construct wannabes. I knew they are useful when it is required to have an universal type for different objects but I haven’t fully grasped to scope why they are so useful otherwise. Also the fact that they can’t contain properties and only public method declarations confused me.

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