SwiftUI TabView：从不同的视图设置选定的tabItem，但检测到重复选择

Question

我想显示一个TabView 来显示不同的屏幕。我的第一个屏幕（主页）显示三个按钮，可以选择三个屏幕中的一个来显示。 但是（！）我必须检查是否重复选择了一个选项卡以在这种情况下触发特殊操作。

检测 screen2 的重复选择工作正常，但我无法通过按钮设置选择。 我尝试使用@EnvironmentalObject，但在我的 TabView.selection 中未观察到对此对象的更改。

   import SwiftUI

   @main
   struct TestApp: App {
       static let kIndex0 = 0
       static let kIndex1 = 1
       static let kIndex2 = 2

       var appState = AppState()

       @State private var selection = TestApp.kIndex0

       var body: some Scene {
           // this code is required to detect a repeated selection of
           // the same tab to trigger a special action
           let index = Binding<Int>(
               get: { self.selection },
               set: {
                   if $0 == TestApp.kIndex1  &&  self.selection == $0 {
                       print("Trigger special action for index 1")
                   }
                   print("Pressed tab: \($0)  self.selction: \(self.selection)  app.selectedTab: \(appState.selectedTab)")

                   self.selection = $0
                   appState.selectedTab = $0
               })

           WindowGroup {
               TabView(selection: index) {
                   First()
                       .environmentObject(appState)
                      .tabItem {
                           Image(systemName: "1.circle")
                           Text("Home")
                       }.tag(TestApp.kIndex0)
                
                   Text("Second Content View")
                       .tabItem {
                           Image(systemName: "2.circle")
                           Text("Screen Two")
                       }.tag(TestApp.kIndex1)

                   Text("Third Content View")
                       .tabItem {
                           Image(systemName: "3.circle")
                           Text("Screen Three")
                       }.tag(TestApp.kIndex2)
               }
           }
       }
   }


   class AppState: ObservableObject {
       @Published var selectedTab = TestApp.kIndex0
   }


   /*
       Place to buttons which should select on of the the two
       tabs of TestUI
    */
   struct First: View {
       @EnvironmentObject var appState: AppState

       var body: some View {
           VStack(alignment: .leading, spacing: 20) {
               AButton(tabIndex: TestApp.kIndex0, iconName: "1.circle", text: "This first screen")
                   .environmentObject(appState)

               AButton(tabIndex: TestApp.kIndex1, iconName: "2.circle", text: "Second screen")
                   .environmentObject(appState)

                  AButton(tabIndex: TestApp.kIndex1, iconName: "3.circle", text: "Third screen")
                   .environmentObject(appState)
           }
       }
   }


   struct AButton: View {
       let tabIndex: Int
       let iconName: String
       let text: String

       @EnvironmentObject var appState: AppState

       var body: some View {
           Button(action: {
               appState.selectedTab = tabIndex
           }) {
               HStack() {
                   Image(systemName: iconName)
                       .imageScale(.large)
                       .frame(minWidth: 50)
                   Text(text)
               }
           }
       }
   }

Answer 1

你需要让appState被观察，你根本不需要selection（它只是一个重复）。

我已经把所有东西都放在了单独的ContentView 中（只为场景离开场景）

使用 Xcode 12 / iOS 14 测试

struct ContentView: View {

    @StateObject var appState = AppState()

    var body: some View {
        // this code is required to detect a repeated selection of
        // the same tab to trigger a special action
        let index = Binding<Int>(
            get: { self.appState.selectedTab },
            set: {
                if $0 == TestApp.kIndex1  &&  self.appState.selectedTab == $0 {
                    print("Trigger special action for index 1")
                }
                print("Pressed tab: \($0) app.selectedTab: \(appState.selectedTab)")
                appState.selectedTab = $0
            })

        TabView(selection: index) {
            First()
                .environmentObject(appState)
                .tabItem {
                    Image(systemName: "1.circle")
                    Text("Home")
                }.tag(TestApp.kIndex0)

            Text("Second Content View")
                .tabItem {
                    Image(systemName: "2.circle")
                    Text("Screen Two")
                }.tag(TestApp.kIndex1)

            Text("Third Content View")
                .tabItem {
                    Image(systemName: "3.circle")
                    Text("Screen Three")
                }.tag(TestApp.kIndex2)
        }
    }
}


class AppState: ObservableObject {
    @Published var selectedTab = TestApp.kIndex0
}


struct First: View {
    @EnvironmentObject var appState: AppState

    var body: some View {
        VStack(alignment: .leading, spacing: 20) {
            AButton(tabIndex: TestApp.kIndex0, iconName: "1.circle", text: "This first screen")

            AButton(tabIndex: TestApp.kIndex1, iconName: "2.circle", text: "Second screen")

            AButton(tabIndex: TestApp.kIndex2, iconName: "3.circle", text: "Third screen")
        }
    }
}


struct AButton: View {
    let tabIndex: Int
    let iconName: String
    let text: String

    @EnvironmentObject var appState: AppState

    var body: some View {
        Button(action: {
            appState.selectedTab = tabIndex
        }) {
            HStack() {
                Image(systemName: iconName)
                    .imageScale(.large)
                    .frame(minWidth: 50)
                Text(text)
            }
        }
    }
}