Basic Class Syntax in Swift

27th February 2017

Like most programming languages, Swift comes with the ability to create classes. Here are some examples of basic class syntax in Swift 3.

Definition

To define the class, use the class keyword followed by the name of your class. Classes their entire definition within a pair of curly braces.

class Colour {

}

You can then create an instance of the class.

var colour = Colour()

Properties

Put simply, a property is a constant or variable that is stored as part of an instance of a particular class.

Variables are defined using the var keyword, and constants are defined using let.

class Person {
    var name: String = "John Doe"
    let isHuman: Bool = true
}

We can access these properties by using the dot notation after creating an instance of the class.

var person = Person()
print(person.name) // Prints: John Doe

You also have the ability to change the property value.

var person = Person()
person.name = "Jane Doe"
print(person.name) // Prints: Jane Doe

Note that this does not mean the property value in the class will be permanently changed. This is just for the particular class instance.

var person = Person()
person.name = "Jane Doe"
print(person.name) // Prints: Jane Doe

var person2 = new Person()
print(person2.name) // Prints: John Doe

Methods

Methods are functions that belong to instances, such as a particular class. They are defined using the func keyword. The code within the method should be within a pair of curly braces and the method should be contained within the class declaration.

class Person {
    func sayHello() {
        print("Hello!")
    }
}

Similar to properties, methods can be accessed using the dot notation.

class Person {
    func sayHello() {
        print("Hello!")
    }
}

var person = Person()
person.sayHello() // Prints: Hello!

Initialisation

Classes can also implement an initialisation process. This usually involves setting an initial value for each stored property on that instance. It is written using the init keyword.

class Square {
    var height: Int
    var width: Int

    init() {
        // perform some initialisation here
        height = 20
        width = 20
    }
}

var square = Square()
print("We have a square that has a height of \(square.height) and a width of \(square.width)")
// Prints: We have a square that has a height of 20 and a width of 20

Note: This article is based on Swift version 3.