使用 ForEach + Stepper SwiftUI 增加多个结构变量

Question

我有一份披萨配料清单。列表中的每一项都是一个结构：

struct Topping: Identifiable {
    let name: String
    let price: Double
    @State var amount: Int
    let id = UUID()
}

列表在文件 MenuDataService.swift 中的 Class 中创建，使用静态 let 定义，然后在 ViewModel 文件中初始化：

//FILE MenuDataService.swift
Class MenuDataService {
    static let toppingsList: [Topping] = [Topping(name: "Cheese", price: 3.00, amount: 0), Topping(name: "Sauce", price: 3.00, amount: 0)]
}

//FILE MenuViewModel.swift
Class MenuViewModel: ObservableObject {
    @Published var toppingsList: [Topping]
    init() {
        toppingsList = MenuDataService.toppingsList
        self.toppingsList = toppingsList
    }
}

在视图中，我正在视图中使用 .environmentobject 访问 object。我正在使用 ForEach 循环遍历浇头列表，显示浇头的名称和数量，然后是应该递增或递减 'amount' 变量的步进器。

@EnvironmentObject private var vm = MenuViewModel
let toppingsList: [Topping]
var body: some View {
    VStack {
        ForEach(toppingsList, id: \.id) { topping in
            Stepper("\(topping.name): \(topping.amount)", value: topping.$amount)
        }
    }
}

struct ExtraToppingsView_Previews: PreviewProvider {
    static var previews: some View {
        ExtraToppingsView(toppingsList: MenuViewModel.init().toppingsList)
        .environmentObject(MenuViewModel())
    }
}

我在 Stepper() 代码行中遇到此错误：在安装到视图之外访问状态的值。这样会导致初始值的Binding常量，不会更新。

单击 'Cheese' 步进器中的“+”时，它应该递增变量，并将标题更改为 'Cheese: 1'，但标题保持不变。我假设这与“@State var amount”变量有关，或者与步进器中的“$amount”变量有些绑定。如果有人能指出我正确的方向，将不胜感激。

Answer 1

您的大部分代码都很接近——只是需要细微的差别才能使其正常工作。查看我的更改的内联评论：

struct Topping: Identifiable {
    let id = UUID()
    let name: String
    let price: Double
    var amount: Int //don't include `@State` in non-Views
}

//no changes
class MenuDataService {
    static let toppingsList: [Topping] = [Topping(name: "Cheese", price: 3.00, amount: 0), Topping(name: "Sauce", price: 3.00, amount: 0)]
}

class MenuViewModel: ObservableObject {
    @Published var toppingsList: [Topping]
    init() {
        toppingsList = MenuDataService.toppingsList
        //remove unnecessary duplicate assignment
    }
}

//This is the parent view -- may be called something different in your code
struct ContentView : View {
    @StateObject private var vm = MenuViewModel() //declare as @StateObject
    
    var body: some View {
        ExtraToppingsView()
            .environmentObject(vm) //pass the environment object
    }
}

struct ExtraToppingsView : View {
    @EnvironmentObject private var vm : MenuViewModel //no need to take the toppingsList parameter separately -- just use the environment object 

    var body: some View {
        VStack {
            ForEach($vm.toppingsList) { $topping in //use element binding to get a mutable binding to each item
                Stepper("\(topping.name): \(topping.amount)", value: $topping.amount) //see change of the position of the $ character
            }
        }
    }
}

struct ExtraToppingsView_Previews: PreviewProvider {
    static var previews: some View {
        ExtraToppingsView()
            .environmentObject(MenuViewModel())
    }
}

有关元素绑定的更多信息：https://www.swiftbysundell.com/articles/bindable-swiftui-list-elements/

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