Talking protocols

Playground.swift

// Playground - noun: a place where people can play
// Please see http://ashfurrow.com/blog/protocols-and-swift/ to know what this is all about

protocol FoodConsumer {
    var calorieCount: Double { get set }
    var hydrationLevel: Double { get set }
}

protocol Food {
    //By having a simple generic function we don't have to cast the returned value when we call the method on the food
    func beConsumedBy<T where T:FoodConsumer>(consumer: T, grams: Double) -> T
}

struct Cat: FoodConsumer {
    var calorieCount: Double = 0
    var hydrationLevel: Double = 0
}

struct Kibble: Food {
    let caloriesPerGram: Double = 40
    
    func beConsumedBy<T where T : FoodConsumer>(consumer: T, grams: Double) -> T {
        var newConsumer = consumer
        newConsumer.calorieCount += grams * caloriesPerGram
        return newConsumer
    }
}

struct FancyFeast: Food {
    let caloriesPerGram: Double = 80
    let milliLitresWaterPerGram: Double = 0.2
    
    //Boring to type, but autocompletion does all the work anyway
    func beConsumedBy<T where T : FoodConsumer>(consumer: T, grams: Double) -> T {
        var newConsuner = consumer
        newConsuner.calorieCount += grams * caloriesPerGram
        newConsuner.hydrationLevel += grams * milliLitresWaterPerGram
        return newConsuner
    }
}

extension Cat {
    //We can directly return Cat here
    func eat(food: Food, grams: Double) -> Cat {
        return food.beConsumedBy(self, grams: grams)
    }
}

let catFood = Kibble()
let wetFood = FancyFeast()
var dave = Cat()

//And no need to cast anymore
dave = dave.eat(catFood, grams: 30)
dave = dave.eat(wetFood, grams: 20)