Swift @Binding 在@StateObject 和 View 上被发布了两次

概述

• 我有一辆@StateObject，它被用来开一辆NavigationSplitView.
• 我使用@StateObject上的@Published个属性来驱动视图的 Select .一百零二
• 当我进入NavigationSplitView人的最终 Select 时，我想创造一个Selection分.
• 这应该会发布一个属性only once.

import SwiftUI
import Combine

struct Consumption: Identifiable, Hashable {
    var id: UUID = UUID()
    var name: String
}

struct Frequency: Identifiable, Hashable {
    var id: UUID = UUID()
    var name: String
}

struct Selection: Identifiable {
    var id: UUID = UUID()
    var consumption: Consumption
    var frequency: Frequency
}


class SomeModel: ObservableObject {
    
    let consump: [Consumption] = ["Coffee","Tea","Beer"].map { str in Consumption(name: str)}
    let freq: [Frequency] = ["daily","weekly","monthly"].map { str in Frequency(name: str)}

    @Published var consumption: Consumption?
    @Published var frequency: Frequency?
    @Published var selection: Selection?
    
    private var cancellable = Set<AnyCancellable>()

    init(consumption: Consumption? = nil, frequency: Frequency? = nil, selection: Selection? = nil) {
        self.consumption = consumption
        self.frequency = frequency
        self.selection = selection
        
        $frequency
            .print()
            .sink { newValue in
                if let newValue = newValue {
                    print("THIS SHOULD APPEAR ONCE")
                    print("---------------")
                    self.selection = .init(consumption: self.consumption!, frequency: newValue)
                } else {
                    print("NOTHING")
                    print("---------------")
                }
            }.store(in: &cancellable)
    }
    
}

问题

视图上的@StateObject是发布property twice的.(请参阅下面的代码)

struct ContentView: View {
    
    @StateObject var model: SomeModel = .init()
    
    var body: some View {
        NavigationSplitView {
            VStack {
                List(model.consump, selection: $model.consumption) { item in
                    NavigationLink(value: item) {
                        Label(item.name, systemImage: "circle.fill")
                    }
                }
            }
        } content: {
            switch model.consumption {
            case .none:
                Text("nothing")
            case .some(let consumption):
                List(model.freq, id:\.id, selection: $model.frequency) { item in
                    NavigationLink(item.name, value: item)
                }
                .navigationTitle(consumption.name)
            }
        } detail: {
            switch model.selection {
            case .none:
                Text("nothing selected")
            case .some(let selection):
                VStack {
                    Text(selection.consumption.name)
                    Text(selection.frequency.name)
                }
            }
        }

    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

我的解决方案

如果我通过在视图上创建@State个属性来更改视图，然后使用OnChange修改器更新模型，我会将属性设置为update once.这就是我想要的.(请参阅下面的代码)

 struct ContentView: View {
     
     @StateObject var model: SomeModel = .init()
     
     @State private var consumption: Consumption?
     @State private var frequency: Frequency?

     var body: some View {
         NavigationSplitView {
             VStack {
                 List(model.consump, selection: $consumption) { item in
                     NavigationLink(value: item) {
                         Label(item.name, systemImage: "circle.fill")
                     }
                 }
             }.onChange(of: consumption) { newValue in
                 self.model.consumption = newValue
             }
         } content: {
             switch model.consumption {
             case .none:
                 Text("nothing")
             case .some(let consumption):
                 List(model.freq, id:\.id, selection: $frequency) { item in
                     NavigationLink(item.name, value: item)
                 }
                 .navigationTitle(consumption.name)
                 .onChange(of: frequency) { newValue in
                     self.model.frequency = newValue
                 }
             }
         } detail: {
             switch model.selection {
             case .none:
                 Text("nothing selected")
             case .some(let selection):
                 VStack {
                     Text(selection.consumption.name)
                     Text(selection.frequency.name)
                 }
             }
         }

     }
 }

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

问题/帮助

在@StateObject + @Published和View + @State之间，@Binding的行为有区别吗？

我不明白为什么在我之前的视图中，该属性被发布了两次.

我的解决方案需要做更多的工作，但它通过使用@State和onChange个视图修饰符来工作.