NS_DESIGNATED_INITIALIZER

// an header

- (instancetype)initWithSomeObject:(SomeObject *)someObject NS_DESIGNATED_INITIALIZER;

This will make initWithSomeObject: the designated initializer. In a nutshell the designated initializer is the only one allowed to call super, and the one that sets all the variables and constants that need to be set on initialisation, the other initializers are only wrappers around the designated one.

Using NS_DESIGNATED_INITIALIZER we can make the compiler enforce the designated initializer behaviour for us.

There are several advantages in having a designated initializer, in my opinion the most important ones are clarity and single responsibility. Clarity, because by looking at it a reader can immediately see everything needed by the object on initialisation, all it’s dependencies; and single responsibility, because we centralise the actual initialisation in one point.

__attribute((unavailable("...")))

Before starting I would like to point out that sublcassing is in most cases a bad idea, and that composition should be used instead, as much as possible. That said sometimes we have to subclass, for example when using UIViewControllers. There are also cases in which we might need to add an initialization parameter to the subclass.

We just saw how we can use NS_DESIGNATED_INITIALIZER to promote one init... method as the designated initializer, but this won’t prevent consumers to call other init... methods. What would happen then if the superclass init was called? The instance wouldn’t be created with the required objects 😱.

We can use __attribute((unavailable)) to mark a superclass initializer as off limits:

// super class header

- (instancetype)initWithSomeObject:(SomeObject *)object NS_DESIGNATED_INITIALIZER;

// subclass header

- (instancetype)initWithSomeSuperDuperObject:(SuperDuperObject *)object NS_DESIGNATED_INITIALIZER;

- (instancetype)initWithSomeObject:(SomeObject *)object __attribute((unavailable));

This way the compiler will ensure that consumers of the subclass won’t call the super class initializer.

You can add a message for the other developers using __attribute((unavailable("message"))), or be more concise and only use __unavailable.