AS3: Static class versus Singleton

Question

I know SO pals are not fans of "versus" questions but... even rephrasing the tittle a bit, the versus part is still there, so, why hide it.

Basically, I'd like to know when and why should I use a singleton or a static class, what can offer a singleton that a static class cant, and vice versa.

For a long time I used both, and I cant see a reason why I shouldn't use one over the other.

Thanks.

Answer 1

Both are basically global variables with caveats. Static classes prevent inheritance, and Singletons are ugly and add overhead with a property lookup. Both make automated testing difficult.

AS3 supports global variables, so why not use those?

Static:

package com.example
{
    public class Config
    {
        public static var VERSION:uint = 1;
    }
}

Singletons:

package com.example
{
    public class Config
    {
        public static var instance:Config = new Config();

        public var version:uint = 1;

        public function Config()
        {
            //Boiler plate preventing multiple instances
        }
    }
}

Global variable:

package com.example
{
    public var config:Config = new Config();
}
class Config
{
    public var version:uint = 1;
}

Now, let's say even though you only want a single instance of the class in your production app, you need multiple instances to write tests. You can make a public class Config, and use com.example.config = new Config() to reset. All the places that reference your global variable are now using the new instance, and you can even do fancy things like inheritence.

For example:

if(inDebugMode)
{
    com.example.config = new DebugConfig();
}
{
    com.example.config = new Config();
}