This is a code sample demonstrating the basics of Structs in Swift

StructsSample.swift

struct Car {
    let make: String
    let model: String
    var color: String
    private(set) var mileage: Int? = 0

    mutating func update(mileage: Int) {
        self.mileage = mileage
    }

    func tryUpdate(mileage: Int) {
        self.mileage = mileage // not allowed, because this func lacks the mutating keyword
    }

    func nextYearsMileage(adding: Int) -> Int {
        return self.mileage! + adding // allowed because this func does not mutate the value of mileage
    }

    enum Drive {
        case forward
        case backward

        var go: String {
            switch self {
            case .forward:
                return "Vroom Vroom"
            case .backward:
                return "Beep Beep Beep"
            }
        }
    }

    class Seller {
        var name: String
    }

    struct Tires {
        var size: Int
    }
}

// Stucts provide a default initializer, and can be intialized at mutable or imutable instances
var myTruck = Car(make: "GMC", model: "Sierra", color: "Black")

// if the struc is a variable, public properties can be mutated via direct assignment
print(myTruck.color) // -> Black
myTruck.color = "Blue"
print(myTruck.color) // -> Blue

// private setters can be mutated via mutating functions
print(myTruck.mileage!) // -> 0
myTruck.update(mileage: 100)
print(myTruck.mileage!) // -> 100

// non-mutating functions cannot chance values of struct properties
myTruck.tryUpdate(mileage: 200) // -> Error
//error: Structs.playground:15:14: error: cannot assign to property: 'self' is immutable
//self.mileage = mileage
//~~~~ ^
//
//Structs.playground:14:5: note: mark method 'mutating' to make 'self' mutable
//func tryUpdate(mileage: Int) {
//    ^
//    mutating

// initializing a constant make the object imutable
let workTruck = Car(make: "Ford", model: "f150", color: "White")
workTruck.update(mileage: 100)
print(workTruck.mileage!) // -> Error
//error: Structs.playground:46:11: error: cannot use mutating member on immutable value: 'workTruck' is a 'let' constant
//workTruck.update(mileage: 100)
//~~~~~~~~~ ^
//
//Structs.playground:45:1: note: change 'let' to 'var' to make it mutable
//let workTruck = Car(make: "Ford", model: "f150", color: "White")
//^~~
//var

// creating a new instance of the workTruck creates a separate instance, so mutations do not impact the original instance
var newTruck = workTruck
newTruck.update(mileage: 123)
print(newTruck.mileage!) // -> 123
print(workTruck.mileage!) // -> 0

// non-mutating funcs are allowed if they don't change internal values
print(newTruck.nextYearsMileage(adding: 100)) // -> 223

// Enums are allowed, however they are maintained at the Static level unless stored in an internal variable
print(Car.Drive.forward.go) // -> Vroom Vroom
print(Car.Drive.backward.go) // -> Beep Beep Beep