Swift Programming: The Big Nerd Ranch Guide

Extending Your Own Type

You will need to create a new type before you can add protocol conformance to it through an extension. Make a new struct to represent a Car type. You will later use an extension on Car to add conformance to the Vehicle protocol.

Listing 21.4 A Car struct

...
typealias Velocity = Double
extension Velocity {
    var kph: Velocity { return self * 1.60934 }
    var mph: Velocity { return self }
}
protocol Vehicle {
    var topSpeed: Velocity { get }
    var numberOfDoors: Int { get }
    var hasFlatbed: Bool { get }
}
struct Car {
    let make: String
    let model: String
    let year: Int
    let color: String
    let nickname: String
    var gasLevel: Double {
        willSet {
            precondition(newValue <= 1.0 && newValue >= 0.0,
                         "New value must be between 0 and ...