Understanding Initializers better in Swift

Hi Michael,

You asked to explain why we need to use init methods like for a kid, so I'll try to do my best. Consider we have a class Person. This class might have some stored properties and some methods like this:

class Person {
    let firstName: String
    let lastName: String

    func getFullName() -> String {
        return firstName + " " + lastName
    }
}

We have to remember that a class is just a blueprint of what you will really want to use in your program - an instance of a class. Instance of a class is an object that you can use and work with because somehow its firstName and lastName have real values and its methods can return something meaningful. But to make all of it happen you have to initialize your class. To initialize you need an init method inside your class and some arguments you could use to give your stored properties some values. So you have to understand that init method is just a method that gives values from outside of your class to properties you didn't give yet. I hope you get that let firstName: String is a property without value. The compiler will know that it has a String type but it is not enough. To instanciate a class you have to init all of its stored properties. You can add another property with a value and you'll see that you don't need to initialize it inside init method because it already has its value. Like this

class Person {
    let firstName: String
    let lastName: String
    let greeting = "Hello, "

    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }

    func getFullName() -> String {
        return firstName + " " + lastName
    }

    func greet() -> String {
        return greeting + getFullName()
    }
}

Then let's try to use our fully configured class outside in some other place:

class SomeViewController {

    @IBOutlet weak var titleLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()

        let michael = Person.init(firstName: "Michael", lastName: "Williams") 
        //you don't have to use init keyword here. 
        //compiler would get that you want to call init method only because you opened parentheses after class's name
        //you could easily write following
        //let michael = Person(firstName: "Michael", lastName: "Williams") withour init
        //and that would be much more appreciated in the community because it's 
        //cleaner and prettier. I used init. keyword only for you to easily get that
        //this is the same init() method from the Person class and we just call it from 
        //other class

        titleLabel.text = michael.greet()
    }
}

You can see that this class would not work if we didn't know firstName and lastName. In viewDidLoad() we init Person with some hardcoded values and they go to the init() method of Person class. Inside the init method we assign hadrcoded values we passed in viewDidLoad() to our stored properties.

Now let's talk about self. Names of stored properties and init method's arguments are the same. We have to use self keyword to differentiate class's properties from init method's arguments. If we use self. before some property, that means "this property (or method) belongs to the class (or struct) that you are working with right now".

Hope you get the part that self.firstName is the constant we declared in the top of our Person class - our stored property. And the firstName is the init method's argument. The last one has a value from outside and the first one with self. keyword has not. But we pass the value from argument to the property and that is a process of initialization. After we did that our class instance (constant michael in viewDidLoad()) now has all properties initialized and we can use all the methods it has.

I don't know if my explanations would be somewhat understandable for a kid but I hope I answered for some of your questions instead of confusing you more :)

Thank you for your clarification. That's what I needed to make it click into place.