SwiftUI 无法将变量传递给高阶函数映射

Question

我有一个应用程序，其中包含一系列包含数字字段的模型。我想成为能够对任何给定的数据字段求和。我可以通过使用数据字段来使用地图名称（例如 $0.cups）。但是我想以编程方式发送一个 “杯子”的变量，但我没有尝试过。

这是一些示例代码。我想将要合计的字段名传入函数 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({ [=10=].name == filterField })
    
        //[=10=].sumField does not work, [=10=].value(sumField) does not work
        let nums = filteredAttribute.map({ [=10=].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

看起来应该很简单，但是我搜索了很多都没有找到任何答案。任何指导将不胜感激。 Xcode13.3，iOS15.4

Answer 1

一个选项是 KeyPath -- 它不是您正在使用的 String，而是您正在查找的属性的 compile-time-safe 路径为了。实现可能如下所示：

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({ [=10=].name == filterField })
        return filteredAttribute.map({ [=10=][keyPath:sumField] }).reduce(0, +)
    }
}//struct

如果你真的想使用String，你可以考虑使用@dynamicMemberLookup：https://www.hackingwithswift.com/articles/55/how-to-use-dynamic-member-lookup-in-swift

Answer 2

不清楚你想要什么，也许你可以尝试这样的事情，“...将我想要汇总的字段名称传递给函数 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{ [=10=].name == filterField }
        // -- here
        var nums: [Double] = []
        switch sumField {
          case "cups": nums = filteredAttribute.map{ [=10=].cups }
          case "bags": nums = filteredAttribute.map{ [=10=].bags }
          case "age": nums = filteredAttribute.map{ [=10=].age }
          default: break
        }
        return nums.reduce(0, +)
    }
}

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