Not quite a Singleton

Question

I'm using ActionScript3 to develop a Data Access Layer for an application another developer is working on in my team. One of the objects, lets call it User is expensive to construct. Each user has a unique ID number so I can tell if they've been created before or not, and all User objects are stored in an array somewhere else.

I want to be able to restrict it so that there can be only once instance of a User per ID. I.E. each user has to be unique, and requests for an already constructed User should receive the pre-constructed User.

If I was using a sane, well-designed language I would simple make it a private constructor and force all requests for the object to go through a function that cached things. But flash doesn't allow private constructors (only public and internal, neither of which will work). How can I achieve this?

Answer 1

If you want to do the private construtor then you can achieve this like this:

package somepackage {
  public class User {
    public function User(SingletonEnforcer s):void {
      if (s == null)
        throw new Exception("Sorry mate!!!");
    }
    public static GetUserWithId(id:String):User {
      //user with this id exists then return it.
      //create a new user, initialize it, cache it and return
    }
  }
}
class SingletonEnforcer {
  //this is only visible to this class (User).
}

Answer 2

You could use a dictionary to store user ids mapped to User instances. This way you could check to see if there was already an existing user before creating a new one. Here is some pseudo code:

public class UserLookup()
{
    var _lookup:Dictionary = new Dictionary();

    function getUser( id:int ):User 
    {
        if( _lookup[id] ) {
            return _lookup[id]
        } else {
            var user = new User( id );
            _lookup[ id ] = user;
            return user;
        }
    }
}