Can you pass the name of a property to a function in Swift?

July 19, 2023

I have a struct containing a bunch of properties referencing different types of statistics. A sample object is:

class Stats {
    stats: [Stat]
}
struct Stat {
    id: UUID
    countA: Int
    countB: Int
    countC: Int
    totalAAttempts: Int
    totalBAttempts: Int
    totalCAttempts: Int
    ...
}

I am displaying the percentages with a bunch of functions in the class…

class Stats {
    stats: [Stat]
    var countAPerc: Double {
        let totalA = stats.reduce(0) { total, stat in
            total + stat.countA
        }
        let totalAttemps = s.reduce(0) { total, stat in
            total + stat.totalAAttempts
        }
        return Double(totalA) / Double(totalAttempts)
    }
    var: countAPercDisplay: String {
        String(format: "%.1f", countAPerc * 100) + "%"
    }
    var countBPerc: Double {
        let totalB = stats.reduce(0) { total, stat in
            total + stat.countB
        }
        let totalAttemps = s.reduce(0) { total, stat in
            total + stat.totalBAttempts
        }
        return Double(totalB) / Double(totalAttempts)
    }
    var: countBPercDisplay: String {
        String(format: "%.1f", countBPerc * 100) + "%"
    }
    ...
}

Is it possible to specify the name of the property as a parameter in a function to prevent the same code from being written over and over? Something like:

func summarizeProperty(_ countProperty: PROPERTYNAME1, _ totalProperty: PROPERTYNAME2) -> Double {
    let totalCount = stats.reduce(0) { total, stat in
        total + stat.PROPERTYNAME1
    }
    let totalAttemps = s.reduce(0) { total, stat in
        total + stat.PROPERTYNAME2
    }
    return Double(totalCount) / Double(totalAttempts)
}

func summarizedProperty(_ countProperty: PROPERTYNAME1, _ totalProperty: PROPERTYNAME2) -> String {
    let summarized = summarizeProperty(PROPERTYNAME1, PROPERTYNAME2)
    return String(format: "%.1f", summarized * 100) + "%"
}

And then call it…

Text(summarizedProperty(countA, totalAAttempts))

Thanks in advance

>Solution :

The tool you want here is a KeyPath:

func summarizeProperty(_ countProperty: KeyPath<Stat, Int>, _ totalProperty: KeyPath<Stat, Int>) -> Double {
    let totalCount = stats.reduce(0) { total, stat in
        total + stat[keyPath: countProperty]
    }
    let totalAttempts = stats.reduce(0) { total, stat in
        total + stat[keyPath: totalProperty]
    }
    return Double(totalCount) / Double(totalAttempts)
}

A KeyPath<Stat, Int> is property on Stat that returns an Int. It is accessed using the keyPath: subscript and generally created using the \ syntax :

Stats().summarizeProperty(\.countA, \.totalAAttempts)

KeyPaths are normal values and can be passed around like this:

func summarizedProperty(_ countProperty: KeyPath<Stat, Int>, _ totalProperty: KeyPath<Stat, Int>) -> String {
    let summarized = summarizeProperty(countProperty, totalProperty)
    return String(format: "%.1f", summarized * 100.0) + "%"
}