SwiftUI Can't Pass Variable to Higher Order Function Map

Question

I have an app with an array of models that contain number fields. I'd like to be able to sum any given data field. I can do so with map by using the data field name ($0.cups for example). However I would like to programmatically send in a variable for "cups" and nothing I have tried works.

Here's some example code. I'd like to pass the field name that I want to total into the function sumOnAttribute

struct ContentView: View {

    @State private var answer: String = ""

    @State private var startingModels = [
        StartingModel(name: "abe", age: 1, bags: 2, cups: 3),
        StartingModel(name: "abe", age: 11, bags: 12, cups: 13),
        StartingModel(name: "abe", age: 21, bags: 22, cups: 23),
        StartingModel(name: "donna", age: 31, bags: 32, cups: 33),
        StartingModel(name: "elsie", age: 41, bags: 42, cups: 43),
        StartingModel(name: "farah", age: 51, bags: 52, cups: 53)
    ]//some data

    var body: some View {
    
        VStack(spacing: 10) {
            Text("Do Some Math")
                .font(.title)
            Text(answer)
                .font(.headline)
            Button {
                answer = String(sumOnAttribute(filterField: "abe", sumField: "cups"))
            } label: {
                Text("Make it So")
            }
            .font(.system(size: 20))
        }//v
    }//body

    func sumOnAttribute(filterField: String, sumField: String) -> Double {
    
        let filteredAttribute = startingModels.filter({ $0.name == filterField })
    
        //$0.sumField does not work, $0.value(sumField) does not work
        let nums = filteredAttribute.map({ $0.cups })

        let total = nums.reduce(0, +)
        return total
    }
}//struct

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

struct StartingModel: Identifiable {
    let id = UUID()
    let name: String
    let age: Double
    let bags: Double
    let cups: Double
}//model

Seems like this should be simple, but I have not found any answer in a lot of searches. Any guidance would be appreciated. Xcode 13.3, iOS 15.4

Answer 1

One option is a KeyPath -- it's not a String like you're using, but rather a compile-time-safe path to the property you're looking for. An implementation might look like this:

struct ContentView: View {

    @State private var answer: String = ""

    @State private var startingModels = [
        StartingModel(name: "abe", age: 1, bags: 2, cups: 3),
        StartingModel(name: "abe", age: 11, bags: 12, cups: 13),
        StartingModel(name: "abe", age: 21, bags: 22, cups: 23),
        StartingModel(name: "donna", age: 31, bags: 32, cups: 33),
        StartingModel(name: "elsie", age: 41, bags: 42, cups: 43),
        StartingModel(name: "farah", age: 51, bags: 52, cups: 53)
    ]//some data

    var body: some View {
    
        VStack(spacing: 10) {
            Text("Do Some Math")
                .font(.title)
            Text(answer)
                .font(.headline)
            Button {
                answer = String(sumOnAttribute(filterField: "abe", sumField: \.cups))
            } label: {
                Text("Make it So")
            }
            .font(.system(size: 20))
        }//v
    }//body

    func sumOnAttribute(filterField: String, sumField: KeyPath<StartingModel,Double>) -> Double {
        let filteredAttribute = startingModels.filter({ $0.name == filterField })
        return filteredAttribute.map({ $0[keyPath:sumField] }).reduce(0, +)
    }
}//struct

If you really wanted to use a String, you could consider using @dynamicMemberLookup: https://www.hackingwithswift.com/articles/55/how-to-use-dynamic-member-lookup-in-swift