Proper application of encapsulation - swift

Question

I've been researching the concept of encapsulation and found some decent threads about the topic like this one and this one. But I haven't been able to find an answer to a particular question I have. I'll use an example in Swift.
Say you have an object that is of type RoadTrip:
class RoadTrip() {
private var duration: Double
private var totalMiles: Double
private var gallonsOfFuel: Double
var averageMilesPerGallon: Double
}
Now let's say the app is going to calculate the averageMilesPerGallon which is the only public property:
func calculateAverageMilePerGallon() -> Double {
let mpg = totalMiles / gallonsOfFuel
return mpg
}
Should the calculation of average miles per gallon be a private method of the RoadTrip object that executes and updates its averageMilesPerGallon or would it be acceptable to have the calculation performed by another method in a separate utility class that then updates the averageMilesPerGallon property of the RoadTrip object using a mutator method that will set the value?
EDIT: Here's my single class that contains my app's basic calculations. I approached it this way based on what I learned in the Stanford course on iTunes, but I'm beginning to think in my case I should move much of this to my LiftEvent class:
infix operator ^^ { }
func ^^ (radix: Double, power: Double) -> Double {
return Double(pow(Double(radix), Double(power)))
}
class CalculatorBrain: NSObject {
var weightLifted: Double?
var repetitions: Double?
var oneRepMax: Double?
let dataManager = LiftEventDataManager()
func calculateOneRepMax(weightLifted: Double, repetitions: Int ) -> Double {
let preferredFormulaID = UserDefaultsManager.sharedInstance.preferredFormula!
let formulas = dataManager.fetchSelectableFormulas()
let formulaName = formulas[preferredFormulaID].formulaName
switch formulaName {
case "Epley":
oneRepMax = weightLifted * (1 + Double(repetitions)/30.0)
return oneRepMax!
case "Baechle":
oneRepMax = weightLifted * (36/(37 - Double(repetitions)))
return oneRepMax!
case "Brzychi":
oneRepMax = weightLifted * ( 1 + ( 0.033 * Double(repetitions)))
return oneRepMax!
case "Lander":
oneRepMax = 100 * weightLifted / (101.3 - (2.67123 * Double(repetitions)))
return oneRepMax!
case "Lombardi":
oneRepMax = weightLifted * (Double(repetitions) ^^ 0.10)
return oneRepMax!
case "Mayhew":
oneRepMax = 100 * weightLifted / (52.2 + (41.9 * (2.71828 ^^ (-0.055 * Double(repetitions)))))
return oneRepMax!
case "O'Conner":
oneRepMax = weightLifted * (1 + 0.025 * Double(repetitions))
return oneRepMax!
default:
return 0.0
}
}
private func calculatePercentOfWeight(maxWeight: Double, percent: Double) -> Double {
return maxWeight * percent
}
func calculateWeightPercentages(maxWeight: String) -> [Int: Double] {
let weightPercentages = [1.0, 0.95, 0.90, 0.85, 0.80, 0.75, 0.70, 0.65, 0.60, 0.55, 0.50, 0.45, 0.40, 0.35, 0.30, 0.25]
var percentages = [Int: Double]()
for percent in weightPercentages {
let integerPercent = Int(percent * 100)
percentages[integerPercent] = calculatePercentOfWeight(Double(maxWeight)!, percent: percent)
}
return percentages
}
func convertBetweenUnits(fromUnit: Int, toUnit: Int, value: Double) -> Double {
let units = dataManager.fetchUnits()
let from = units[fromUnit].conversionRatio as Double
let to = units[toUnit].conversionRatio as Double
let result = Double(value * to / from)
return result
}
}

Answer 1

I think this is the ideal use-case for a computed property:
class RoadTrip {
private let duration: Double
private let totalMiles: Double
private let gallonsOfFuel: Double
private var averageMilesPerGallon: Double {
return totalMiles / gallonsOfFuel
}
}