You say Constructor Chaining, Swift says Initializer Delegation

One of the things that one must get used to with a new language is dealing with new conventions. Now coming from mostly a Java and Ruby background, there is a notion of constructors and constructor chaining, basically when one constructor calls another one. So when it comes to Swift, there are a few rules around the methods used to create objects.

Firstly, the language guide refers to constructors as initializers. So basically anytime you want to use the term constructor, use the term initializer when you are in Swift land. More specifically, constructor chaining is known as initializer delegation.

Initializers fall into two categories, Designated and Convenience.Designated initializers are the primary initializers and are responsible for initializing all properties of a class. So in this case, the only initializer here is known as the Designated one.

class Person {  
    let name: String
    let age: Int

    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}

Now if you want to add other initializers to delegate to the Designated one, they will be known as Convenience initializers. So let's add one that defaults the age. Convenience initializers have the keyword conveniencebefore them.

class Person {  
    let name: String
    let age: Int

    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }

    convenience init(name: String) {
        self.init(name: name, age: 100)
    }
}

Convenience initializers need to delegate to another Convenienceinitializer or a Designated initializer.

Here's an example of a Convenience initializer calling another Convenienceone.

class Person {  
    let name: String
    let age: Int

    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }

    convenience init(name: String) {
        self.init(name: name, age: 30)
    }

    convenience init() {
        self.init(name: "Homer")
    }
}

If you wanted to, you could create another Designated initializer. Now we have two Designated initializers and two Convenience initializers.

class Person {  
    let name: String
    let age: Int

    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }

    init(age: Int, name: String) {
        self.age = age
        self.name = name
    }

    convenience init(name: String) {
        self.init(name: name, age: 30)
    }

    convenience init() {
        self.init(name: "Homer")
    }
}

What about subclasses?

Once again, with initializers, there are certain rules that need to be adhered to when subclassing. So Designated initializers must call otherDesignated initializers in their immediate parent class.

So let's take a look at an example. We are adding a Student class as a subclass of the Person class. It will add a property student number.

class Student : Person {

    let number: String

    init(name: String, age: Int, number: String) {
        self.number = number
        super.init(name: name, age: age)
    }
}

Here we are calling the Designated initializer in the Person class. If we tried to call one of the Convenience initializers in the Person class, a compile error would occur. Also note, we need to assign the number property before calling the initializer in the parent class as it is a requirement to ensure all properties are initialized in child classes before their respective parent initializer is called.

Summary

Basically in summary, as explained in the language guide, Designatedinitializers delegate up and Convenience initializers delegate across. I hope this article helped in some way to improve your understanding of initializers in Swift.

References

• Swift Language Guide