ActionScript 3.0 Design Patterns

Key OOP Concepts Used with the Singleton Pattern | 103

That looks pretty simple. However, looking at it carefully, you’ll find a lot going on.

Let’s go over the key points:

Private static instantiation of the instance variable typed as Singleton

The private characteristic of the variable means you can’t access it outside the

class. Being static means you can access it by the method in the class set up to

instantiate a Singleton object.

Private class constructor

Example 3-1 is pretty straightforward in ActionScript 2.0. In ActionScript 3.0,

we are faced with the dilemma that private functions cannot be used as construc-

tors in packages. Classes to be instantiated need to be public and part of a pack-

age. This will be discussed further when creating the equivalent of a private class

constructor.

Public static function for creating a class instance

ThegetInstance( )method is a wonderfully simple function. It checks to see if

an instance has been created, and, if it hasn’t, then it creates one. Otherwise it

simply returns the existing Singleton instance.

The simplicity of the Singleton is clearer after seeing how it actually works in a con-

crete example. However, working with ActionScript 3.0, we’re stuck because we

can’t create a private class constructor. This next section provides a workaround to

this dilemma.

Creating and Using a Private Class Constructor

Rather than showing how your code will fail if you attempt to create a private con-

structor function inside a package, you’re going to see how to effectively make your

very own private class, and use it as a constructor. In fact, you will see how to imple-

ment it in a script.

Example 3-1. Classic Singleton

class Singleton
{
private static var instance:Singleton;
private function Singleton( )
{
}
public static function getInstance( ):Singleton
{
if (instance == null) {
Singleton.instance = new Singleton( );
}
return Singleton.instance;
}
}