使用状态变量作为 SwiftUI 中函数的输入

我有两个文本字段，用于更改两个 @State 变量（即startingMileage 和endingMileage）的值，以及一个步进器，用于更改名为fuelAdded 的第三个@State 变量的值。我正在尝试使用用户的输入并进行计算来计算每加仑的英里数。在操场上，我的代码按预期工作。但是，它在 SwiftUI 项目中不起作用。

尝试在操场上运行代码如下：

func CalcMPG(start: String, end: String, fuel: Double) -> Int {
    let start = Int(start) ?? 1
    let end = Int(end) ?? 1
    let fuel = Int(fuel)
    let mpg = (end-start) / fuel
    return mpg
}


var endingMileage:String = "9250"
var startingMileage:String = "9000"
var fuelAdded:Double = 20

let milesPerGallon = CalcMPG(start: startingMileage, end: endingMileage, fuel: fuelAdded)
print("Fuel Efficiency: \(milesPerGallon) mpg")

这按预期工作。

struct ContentView : View {

    @State var startingMileage: String = ""
    @State var endingMileage: String = ""
    @State var fuelAdded: Double = 10
    @State var carModel: String = ""
    @State var showMPGInfo = false
    @State var milesPerGallon: Int = 10

    func CalcMPG(start: String, end: String, fuel: Double) -> Int {
        let start = Int(start) ?? 1
        let end = Int(end) ?? 1
        let fuel = Int(fuel)
        let mpg = (end-start) / fuel
        return mpg
    }


    var body: some View {
        NavigationView {
            VStack{
                HStack {
                    Text("Car Model:")
                    Spacer()
                    TextField($carModel, placeholder: Text("Toyota Corolla"))
                        .textFieldStyle(.roundedBorder)
                }
                HStack {
                    Text("Starting ODO:")
                    Spacer()
                    TextField($startingMileage, placeholder: Text("8000"))
                    .textFieldStyle(.roundedBorder)
                    Text("miles")
                }

                HStack {
                    Text("Ending ODO:")
                    Spacer()
                    TextField($endingMileage, placeholder: Text("9000"))
                        .textFieldStyle(.roundedBorder)
                    Text("miles")
                }
                HStack {
                    Stepper(value: $fuelAdded, in: 0...20, step: 0.5) {
                        Text("Fuel Added: \(fuelAdded, specifier: "%0.1f") gallons")
                    }
                }
                Button(action: {
                    self.showMPGInfo.toggle() }){
                        Text("Show/Hide MPG")
                }
                    if showMPGInfo {
                        Spacer()
                        milesPerGallon = CalcMPG(start: startingMileage, end: endingMileage, fuel: fuelAdded)
                        Text("Fuel effiency: \(milesPerGallon) MPG")
                            .font(.largeTitle)
                    }
                }.padding()
                .navigationBarTitle(Text("Gas Mileage Calculator"))
        }
    }
}

当用户单击“显示/隐藏 MPG”时。我预计最终结果文本为“燃油效率：xx MPG”

但是，我收到以下错误：

ContentView.swift:34:19：无法推断复杂的闭包返回类型；添加显式类型以消除歧义

它对我来说没有任何意义...它突出显示了导航视图中的起始 VStack。

有什么想法吗？

问题是分配给milesPerGallon与用于构建视图层次结构参数的“函数构建器语法”不能很好地配合。如果我们替换，它会变得更加明显@State var milesPerGallon通过局部变量（即is,它不携带视图所依赖的状态，仅携带中间值）：

if showMPGInfo {
    Spacer()
    let milesPerGallon = CalcMPG(start: startingMileage,
                                 end: endingMileage, fuel: fuelAdded)
    Text("Fuel effiency: \(milesPerGallon) MPG")
        .font(.largeTitle)
}

现在编译器错误是

Closure containing a declaration cannot be used with function builder 'ViewBuilder'
  

有关函数生成器语法的更多信息，请参阅是什么启用了 SwiftUI 的 DSL？ https://stackoverflow.com/questions/56434549/what-enables-swiftuis-dsl（其中还有文档的链接）。

最简单的解决方案是避免使用局部变量并直接插入文本：

if showMPGInfo {
    Spacer()
    Text("Fuel effiency: \(CalcMPG(start: startingMileage, end: endingMileage, fuel: fuelAdded)) MPG")
        .font(.largeTitle)
}

其他解决方案是在“立即评估的闭包”中计算文本字段：

if showMPGInfo {
    Spacer();
    { () -> Text in
        let milesPerGallon = CalcMPG(start: startingMileage,
                                      end: endingMileage,
                                      fuel: fuelAdded)
        return Text("Fuel effiency: \(milesPerGallon) MPG")
    }()
    .font(.largeTitle)
}

或定义辅助功能

func resultField(start: String, end: String, fuel: Double) -> Text {
    let  milesPerGallon = CalcMPG(start: startingMileage, end: endingMileage, fuel: fuelAdded)
    return Text("Fuel effiency: \(milesPerGallon) MPG")
}

并将其用作

if showMPGInfo {
    Spacer()
    resultField(start: startingMileage, end: endingMileage,
                fuel: fuelAdded)
        .font(.largeTitle)
}

可能还有其他解决方法，但这就是我到目前为止想到的。