SwiftUI Can't Pass String to KeyPath

Question

I have an app that references a large dataset that I load from an external public site as a comma separated value file. I parse the data to a an array of a model called WaterPointModel. An abbreviated version is:

struct WaterPointModel: Identifiable {

    let id = UUID()

    let STATE: String
    let COUNTY: String
    let AQWFrTo: Double
    let AQWGWSa: Double
    let AQWGWTo: Double
    //many more
}

I then want to summarize(reduce) the data. My function for this is:

func sumOnAttributeUSList(sumField: KeyPath<WaterPointModel,Double>) -> Double {
    return dataStore.waterPointModels.map({ $0[keyPath:sumField] }).reduce(0, +)
}

Next I want to call this to build a report:

let aqWFrTo = sumOnAttributeUSList(sumField: \.AQWFrTo)

This all works. However there are other reports where I need to pass a string to create that keypath. Say I have a lookup table where I lookup "abc" and get "AQWFrTo". I would like to do something like this in a loop:

let abc = "AQWFrTo"
let aqWFrTo = sumOnAttributeUSList(sumField: \WaterPointModel.abc)

I have not been able to make any version of this work. Any guidance would be appreciated.

Xcode 13.3.1, iOS 15.4

This is just Swift and is not related to SwiftUI, which is a UI framework built on the Swift language. — jnpdx, Apr 21 '22 at 06:21
It is possible to get key path by string using Mirror ([see this discussion](https://forums.swift.org/t/getting-keypaths-to-members-automatically-using-mirror/21207/4)) and then you can just overload your sum function with String argument. — Asperi, Apr 21 '22 at 08:03
Here is a result of experiments with different dynamic accesses to property [TestWays4DynamicMemberLookup.swift](https://github.com/Asperi-Demo/4SwiftUI/blob/master/PlayOn_iOS/PlayOn_iOS/Findings/TestWays4DynamicMemberLookup.swift) — Asperi, Apr 21 '22 at 10:01

score 1 · Accepted Answer · answered Apr 21 '22 at 06:31

1

A simple approach is this:

func toKeypath(_ str: String) -> KeyPath<WaterPointModel,Double>? {  // <-- with or without optional
    switch str {
        case "AQWFrTo": return \.AQWFrTo
        case "AQWGWSa": return \.AQWGWSa
        case "AQWGWTo": return \.AQWGWTo
        // ...
        default: return nil  // <-- or a default Keypath
    }
}

let aqWFrTo = sumOnAttributeUSList(sumField: toKeypath("AQWFrTo"))

answered Apr 21 '22 at 06:31

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Super. This will work for me. – JohnSF Apr 21 '22 at 16:56

SwiftUI Can't Pass String to KeyPath

1 Answers1