How to add associated values to enums

Swift version: 5.1

Paul Hudson    @twostraws    May 28th 2019

Enums with associated values let you associate extra data with an enum case. This helps make them significantly more useful, because we can create gradations of cases rather than have them be absolute.

For example, we could create a Weather enum that can store various weather types: sunny, cloudy, windy, and rainy. However, that doesn’t really describe those conditions very well – how cloudy is it? Is it gale force winds or just a breeze? Is the rain definitely going to happen, or is it a fairly remote chance?

With enum associated values we can describe these situations more accurately. For example:

enum Weather {
    case sunny
    case cloudy(coverage: Int)
    case windy(speed: Int)
    case rainy(chance: Int)
}

That leaves “sunny” as a simple value, but the other three all have associated values – how cloudy it is, what the speed of the wind is, and how likely the rain is.

Using those values we can now create instances of those enums:

let london = Weather.cloudy(coverage: 90)
let gusty = Weather.windy(speed: 10)
let guaranteedRain = Weather.rainy(chance: 100)

Available from iOS 8.0 – learn more in my book Pro Swift

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