如何在 SwiftUI 中使用枚举绑定数据？

Question

我正在尝试绑定到枚举实例的 rawValue

enum Brand: Int, CaseIterable, Codable {
    case toyota = 0
    case mazda
    case suzuki
}

struct Car {
    var brand: Brand
}

struct CarView: View { 

    @Binding car: Car  
 
    var body: some View {
        SomeView(selectionInt: $car.brand)
    }
}

但我收到此错误：

无法将“绑定”类型的值转换为预期参数 输入“绑定”。

如果尝试使用 $car.brand.rawValue 代替（在 SomeView 的参数中），但我收到此错误：

无法分配给属性：“rawValue”是不可变的。

如何将我的视图绑定到模型实例的 rawValue？

Answer 1

使用brandProxy 来维护其他Car 变量的完整性

import SwiftUI
enum Brand: Int, CaseIterable, Codable {
    case toyota = 0
    case mazda
    case suzuki
    case unknown
    
    func description() -> String {
        var result = "unknown"
        switch self {
        case .toyota:
            result = "toyota"
        case .mazda:
            result = "mazda"
        case .suzuki:
            result = "suzuki"
        case .unknown:
            result = "unknown"
        }
        return result
    }
}
struct CarView: View {
    //This makes it mutable
    @State var car: Car
    var brandProxy: Binding<Int> {
        Binding<Int>(
            get: {
                car.brand.rawValue
            },
            set: {
                car.brand = Brand(rawValue: $0) ?? Brand.unknown
            }
        )
    }
    var body: some View {
        VStack{
            Text(car.brand.description())
            SomeView(selectedIndex: brandProxy)
        }
    }
}

Answer 2

对于这种绑定，你需要一个自定义的 Binding 来帮助 SwiftUI 绑定数据！

信息：您不能更改枚举的 rawValue，因为它们是不可变的！ 并且尝试以绑定方式更改它们对您没有帮助，它们是不可变的！ 你可以让他们不设置他们！

因此，有了这些信息，您就可以知道为什么 SwiftUI 不希望将它绑定在第一个位置！因为如果 SwiftUI 使该 Binding 成为可能，那将是对枚举 rawValues 的重大违反！然后我们将尝试以这种方式使用绑定能力，即我们改变汽车的 Int 而不是 Enum。以代码为例。

---

---

import SwiftUI

struct ContentView: View {
    
    @State private var selectedCar: Car = Car(brand: Brand.suzuki)
    
    var body: some View {

        VStack {
            
            Picker("", selection: $selectedCar.brand) {
                
                ForEach(Brand.allCases, id: \.self) { car in
                    
                    Text(car.description)
                    
                }
                
            }
            .pickerStyle(SegmentedPickerStyle())
        }
        .padding()

        
        CarView(selectionInt: Binding.init(get: { () -> Int in return selectedCar.brand.rawValue },
                                           set: { newValue in
                                            
                                            if let unwrappedBrand = Brand(rawValue: newValue) { selectedCar = Car(brand: unwrappedBrand) } }) )
        
    }
}

---

struct CarView: View {
    
    @Binding var selectionInt: Int
    
    var body: some View {
        
        Text("selected Int: " + selectionInt.description)
            .padding()
        
        let selectedCar = (Brand(rawValue: selectionInt)?.description ?? "unknown")
        
        Text( "selected car: " + selectedCar)
            .padding()
        
        
        VStack(spacing: 20.0) {
            
            Button("select toyota") { selectionInt = 0 }
            Button("select mazda") { selectionInt = 1 }
            Button("select suzuki") { selectionInt = 2 }
            
        }
        .font(Font.body.weight(Font.Weight.bold))
        
    }
}

---

enum Brand: Int, CaseIterable, Codable, CustomStringConvertible {
    
    case toyota = 0
    case mazda = 1
    case suzuki = 2
    
    var description: String {
        
        switch self {
        case .toyota: return "toyota"
        case .mazda: return "mazda"
        case .suzuki: return "suzuki"
        }
    }
}

---

struct Car {
    var brand: Brand
}