SwiftUI無法將變數傳遞給高階函式映射

我有一個包含數字欄位的模型陣列的應用程式。我希望能夠總結任何給定的資料欄位。我可以通過使用資料欄位名稱（例如 $0.cups）對 map 執行此操作。但是，我想以編程方式為“杯子”發送一個變數，但我沒有嘗試過任何作業。

這是一些示例代碼。我想將要總計的欄位名稱傳遞給函式 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

看起來這應該很簡單，但我在很多搜索中都沒有找到任何答案。任何指導將不勝感激。Xcode 13.3，iOS 15.4

uj5u.com熱心網友回復：

一個選項是KeyPath- 它不是String您正在使用的，而是您正在尋找的屬性的編譯時安全路徑。一個實作可能如下所示：

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

如果您真的想使用 a String，可以考慮使用@dynamicMemberLookup：https ://www.hackingwithswift.com/articles/55/how-to-use-dynamic-member-lookup-in-swift

uj5u.com熱心網友回復：

不清楚你想要什么，也許你可以嘗試這樣的事情，“......將我想要總計的欄位名稱傳遞給函式 sumOnAttribute......”：

struct ContentView: View {
    @State private var namefield: String = ""  // <-- here
    @State private var sumfield: String = ""   // <-- here
    @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)
    ]
    
    var body: some View {
        
        VStack(spacing: 10) {
            TextField("name field", text: $namefield).border(.red) // <-- here
            TextField("sum field", text: $sumfield).border(.red)   // <-- here
            Text("Do Some Math")
                .font(.title)
            Text(answer)
                .font(.headline)
            Button {
                answer = String(sumOnAttribute(filterField: namefield, sumField: sumfield)) // <-- here
            } label: {
                Text("Make it So")
            }
            .font(.system(size: 20))
        }
    }
    
    func sumOnAttribute(filterField: String, sumField: String) -> Double {
        let filteredAttribute = startingModels.filter{ $0.name == filterField }
        // -- here
        var nums: [Double] = []
        switch sumField {
          case "cups": nums = filteredAttribute.map{ $0.cups }
          case "bags": nums = filteredAttribute.map{ $0.bags }
          case "age": nums = filteredAttribute.map{ $0.age }
          default: break
        }
        return nums.reduce(0,  )
    }
}

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