Day9-Acceess Control.md

Access Control

Initializers

Initializers are special methods that provide different ways to create your struct. All structs come with one by default, called their memberwise initializer – this asks you to provide a value for each property when you create the struct.

struct User {
    var username: String
}

When we create one of those structs, we must provide a username:

var user = User(username: "twostraws")

We can provide our own initializer to replace the default one. For example, we might want to create all new users as “Anonymous” and print a message, like this:

struct User {
    var username: String

    init() {
        username = "Anonymous"
        print("Creating a new user!")
    }
}

You don’t write func before initializers, but you do need to make sure all properties have a value before the initializer ends.

Now our initializer accepts no parameters, we need to create the struct like this:

var user = User()
user.username = "twostraws"

Referring to the current instance

Inside methods you get a special constant called self, which points to whatever instance of the struct is currently being used. This self value is particularly useful when you create initializers that have the same parameter names as your property.

For example, if you create a Person struct with a name property, then tried to write an initializer that accepted a name parameter, self helps you distinguish between the property and the parameter – self.name refers to the property, whereas name refers to the parameter.

struct Person {
    var name: String

    init(name: String) {
        print("\(name) was born!")
        self.name = name
    }
}

Lazy properties

As a performance optimization, Swift lets you create some properties only when they are needed. As an example, consider this

FamilyTree struct – it doesn’t do much, but in theory creating a family tree for someone takes a long time:

struct FamilyTree {
    init() {
        print("Creating family tree!")
    }
}

We might use that FamilyTree struct as a property inside a Person struct, like this:

struct Person {
    var name: String
    var familyTree = FamilyTree()

    init(name: String) {
        self.name = name
    }
}

var ed = Person(name: "Ed")

But what if we didn’t always need the family tree for a particular person? If we add the lazy keyword to the familyTree property, then Swift will only create the FamilyTree struct when it’s first accessed:

lazy var familyTree = FamilyTree()

So, if you want to see the “Creating family tree!” message, you need to access the property at least once:

ed.familyTree

Static properties and methods

All the properties and methods we’ve created so far have belonged to individual instances of structs, which means that if we had a Student struct we could create several student instances each with their own properties and methods:

struct Student {
    var name: String

    init(name: String) {
        self.name = name
    }
}

let ed = Student(name: "Ed")
let taylor = Student(name: "Taylor")

You can also ask Swift to share specific properties and methods across all instances of the struct by declaring them as static.

To try this out, we’re going to add a static property to the Student struct to store how many students are in the class. Each time we create a new student, we’ll add one to it:

struct Student {
    static var classSize = 0
    var name: String

    init(name: String) {
        self.name = name
        Student.classSize += 1
    }
}

Because the classSize property belongs to the struct itself rather than instances of the struct, we need to read it using Student.classSize:

print(Student.classSize)

Access control

Access control lets you restrict which code can use properties and methods. This is important because you might want to stop people reading a property directly, for example.

We could create a Person struct that has an id property to store their social security number:

struct Person {
    var id: String

    init(id: String) {
        self.id = id
    }
}

let ed = Person(id: "12345")

Once that person has been created, we can make their id be private so you can’t read it from outside the struct – trying to write ed.id simply won’t work.

Just use the private keyword, like this:

struct Person {
    private var id: String

    init(id: String) {
        self.id = id
    }
}

Now only methods inside Person can read the id property. For example:

struct Person {
    private var id: String

    init(id: String) {
        self.id = id
    }

    func identify() -> String {
        return "My social security number is \(id)"
    }
}

Another common option is public, which lets all other code use the property or method.