learn-co-curriculum / swift-calculated-property

Swift — Calculated Property

Objectives

1. Learn the calculated property syntax.
2. Identify when to utilize a calculated property.
3. Create a non-numerical calculated property.

Before getting started, create a new Swift-based Xcode project.

Calculated Properties

Calculated properties can be used for properties that rely on the state of other properties. Instead of constantly updating the value of the property that depends on the state of other properties whenever those states change, we can write a calculated property which only determines its present value at the moment that it is accessed. The syntax for a calculated property generally follows:

    var propertyName: Type { return <#calculation#> }

Objective-C: This is generally equivalent to writing a read-only property with a custom getter that returns a value based on the values of other properties.

Consider a Circle class that has a diameter property:

class Circle {
    var diameter: Double = 1
    
    init(diameter: Double) {
        self.diameter = diameter
    }
}

The diameter property holds state, the width of the circle. If we wish to add another property that returns the circumference, it would be appropriate to create this as a calculated property that determines its value based on the current state of the circle's diameter. Since the mathematical definition of a circle's circumference is its diameter multiplied by pi (≈ 3.141592654), we can program this calculation into the circumference property:

class Circle {
    var diameter: Double = 1
    
    var circumference: Double { return diameter * M_PI }
    
    init(diameter: Double) {
        self.diameter = diameter
    }
}

From the AppDelegate we can create an instance of our class and print the circumference:

let circle = Circle(diameter: 1.0)
let bigCircle = Circle(diameter: 2.0)
        
print(circle.circumference)
print(bigCircle.circumference)

This will print:

3.14159265358979
6.28318530717959

Non-numerical Calculated Properties

Calculated properties don't have to be mathematical calculations. They can return strings, booleans, or any other type as well.

Consider our Student class from previous examples:

//  Student.swift

import Foundation

class Student {
    let username: String
    var firstName: String
    var lastName: String
    var email: String = ""
    var phone: String = ""
    
    init(username: String, firstName: String, lastName: String) {
        self.username = username
        self.firstName = firstName
        self.lastName = lastName
    }
}

If we want to add a fullName property, it would be appropriate to include it as a calculated property since the firstName and lastName properties are already holding the values we need. To avoid any potential that these properties could get desynchronized, we should set up fullName as a calculated property that interpolates firstName and lastName into a single string:

    var fullName: String { return "\(firstName) \(lastName)" }

Including it in the class's definition might look like this:

//  Student.swift

import Foundation

class Student {
    let username: String
    var firstName: String
    var lastName: String
    var email: String = ""
    var phone: String = ""
    
    var fullName: String { return "\(firstName) \(lastName)" }
    
    init(username: String, firstName: String, lastName: String) {
        self.username = username
        self.firstName = firstName
        self.lastName = lastName
    }
}

Now from the AppDelegate we can initialize an instance of our Student class with the necessary values and access (print, in this case) the fullName property just like the class's other properties:

let susan = Student(username: "susanmarielovaglio", firstName: "Susan", lastName: "Lovaglio")
        
print(susan.fullName)

This will print: Susan Lovaglio

If we change the state of the underlying properties that are used for the calculated property, the changes will be automatically reflected in the result of accessing the calculated property:

let susan = Student(username: "susanmarielovaglio", firstName: "Susan", lastName: "Lovaglio")
        
print(susan.fullName)

susan.firstName = "Siouxsie"
susan.lastName = "and the Banshees"

print(susan.fullName)

This will print:

Susan Lovaglio
Siouxsie and the Banshees

Siouxsie and the Banshees

View this lesson on Learn.co