Objective-C: Use singleton vs. use class as an object?

Question

I've been wondering in what cases it is really necessary to adopt the singleton pattern in objective-C (e.g., define a dedicated class and create a single instance), that using the class as an object won't do.

Particularly, I'm thinking of the following solution:

1. Define and use appropriate class methods, instead of instance methods on the singleton instance;
2. Use static variables (file-scope globals), instead of instance variables of the singleton instance;
3. Use the class object when registering as an observer for notifications, instead of the singleton instance. Although the class object is an objective-C object in its own right (right?), this would require that the notification handler registered be a class method; (is this possible?)

For example, instead of having a Texture class (model object) and a TextureManager singleton (resource manager), you could have all texture creation/cleanup implemented as class methods and static variables of the same Texture class (factory pattern plus some resource management).

Any thoughts on this design?

EDIT: Now that I think of it, and still in the Texture example above, even if I keep the two classes separate (Texture and TextureManager) I must choose between A. Having the manager be a singleton, and operate it with instance methods, or B. Having the manager be an instanceless, auxiliary class. To clarify:

Texture* myTexture = [[TextureManager defaultManager] textureWithName:@"TextureName"]; 
// (singleton, client uses instance methods)

versus

Texture* myTexture = [TextureManager textureWithName:@"TextureName"]; 
// (Class standing in for singleton, client uses class methods)

The latter looks more straightforward and less cumbersome/verbose, but I wonder which design is "more correct". Of course, the former allows for more than one TextureManager instance shall the need arise (not in my case).

Answer 1

I have been thinking about the same thing and I think I have an answer for you.

It depends on what you need to do with it. Neither is necessarily more "correct".

Read on if you want the details of how I came to my conclusion or scroll down to the tl;dr section.

As you said, it would appear (externally) less cumbersome to access the singleton to have the class manage the singleton for you. Essentially you would do this by replacing the singleton's factory method with an initializer method. Looking at Apple's documentation on this you can see where they show a "shared" method that acts as the factory to produce the singleton upon demand.

static MyGizmoClass *sharedGizmoManager = nil;

+ (MyGizmoClass*)sharedManager
{
    if (sharedGizmoManager == nil) {
        sharedGizmoManager = [[super allocWithZone:NULL] init];
    }
    return sharedGizmoManager;
}

Instead of doing this you could replace the method with a void initializer like so:

+ (void)initializeMyGizmo
{
    if (sharedGizmoManager == nil) {
        sharedGizmoManager = [[super allocWithZone:NULL] init];
    }
    // whatever else needs to be done to the singleton to initialize it
}

and then ONLY ever use class methods and allow the MyGizmoClass to manage updates to the singleton like [MyGizmoClass setGizmoName:@"Gadget"].

NOTE: In this scenario it would be confusing to someone looking at the .h file to see properties, in which case they may come to the conclusion that they should create an instance of the object themselves, or be able to have access to the singleton in some form or fashion. So if you were to go the route of encapsulating access to the singleton it would not be wise to use public variables.

To that point:

If you do limit access to solely through the class itself you lose any getters and setters or other free things that come along with properties. This means that if MyGizmoClass were to have as part of it's model an NSString *gizmoName you would be forced to create custom getters and setters for this "property" and keep it either as an ivar or property in an interface extension in the .m file (i.e. private) of the singleton class, or as an adjacent static variable.

So this begs the question (and is what got me pondering in the first place), should we even include the line static MyGizmoClass *sharedGizmoManager = nil; at all or can we nix the internal interface extension altogether and replace any possible ivars or properties that we want to limit access to with static implementations in the implementation?

I answered that already...

It depends on what you need to do with it.

tl;dr

First Scenario

If you ever (even the slightest chance) need to subclass your TextureManager or could create multiple instances of it (making it no longer a singleton) it would be better to stick to the regular Apple convention for a singleton.

This includes multiple "singletons" wherein you might have several TextureManagers preconfigured with different settings.

In this case you would use properties as you need them (publicly or privately) as well as ivars. You could also use a mix of ivars and statics but you would still always need to have a static instance of your TextureManager inside of the TextureManager implementation.

Second Scenario

If you ONLY will ever need ONE instance of the TextureManager and it will run completely standalone with no intermixing further down the line then you could completely remove the static instance of your class within the implementation in the .m file and replace ivars and properties with static variables within that implementation.

This can be useful if you are storing off properties or settings in CoreData and only need them for configuration.

Just remember in this case you will have to create all getters and setters for the static variables and will only be able to access them using class methods (but that's sorta the point).

Other Interesting Stuff

This answer offers an interesting solution to the question of when and how to call the "initializer" method or create the singleton. This can be used with each scenario to either initialize the singleton in the first scenario, or preload defaults into the class-level statics in the second scenario.

If you want to stick with a static singleton in the implementation you might look at this article to give you a better idea at the true "global scope" of your singleton.

Answer 2

Yes you can definitely make a Texture class without needing a singleton.

Singletons probably should not be created and used as an object.

Singletons can be used for many important things.
I certainly don't know all of the things they can be used for, but i will tell you what i have used them for in the past.

I usually use singletons for level navigation in a game with many levels (like Angry Birds).
By level navigation, i mean... when a player completes a certain level in a game i simply call a class method on the singleton and pass in the level number, then the singleton's class method figures out which level is next (if user presses 'next level' button).

Answer 3

I can help you understand the Singleton class better and when it applies.

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Pattern : Singleton

Intent : Enforce that a class can only have a single instance, as well as making that instance accessible to any other object.

Motivation : Sometimes we need to make sure that there exists only a single object of a certain type in our problem domain. Example: A student carries around only a single backpack, which he can fill with books. We would not want to relate him to secondary backpack, with even more books.

Use when :

• There is need for only a single instance of a class, and that instance must be accessible from different objects within your code.
• When you (possibly) need to be able to add more functionality to that class by subclassing it.

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