What's the correct method to subclass a singleton class in Objective -C?

Question

I have created a singleton class and I want to create a class which is subclass of this singleton class, what is the correct method to do it

Answer 1

Jon Skeet makes a good point about whether you’d really have a singleton if you’re allowed to instantiate both the class and its subclass. Putting that aside, here’s a pattern you can use so that so you only have to define the shared-instance getter once, in the parent class:

// this code goes in the implementation of the superclass

static Sprocket *defaultSprocket;

+ (instancetype) defaultSprocket
{
    if (defaultSprocket == nil)
        defaultSprocket = [[[self class] alloc] init];
    return defaultSprocket;
}

This approach has the following advantages:

• Using [self class] allows e.g. [SprocketSubclass defaultSprocket] to return an instance of SprocketSubclass instead of Sprocket
• Using instancetype allows the compiler to type-check the result of this method: it’ll be Sprocket when you invoke it as +[Sprocket defaultSprocket] but SprocketSubclass when you invoke it as +[SprocketSubclass defaultSprocket].

Notably, you can define this accessor method in the base class and then you don’t have to do anything in the subclasses!

(Hat tips to NSHipster for explaining why instancetype is so cool and bbum for reminding me of it recently.)