swift - SwiftUI应用程序的NavigationView中的onAppear和onDisappear是否表现正常？

我想知道SwiftUI(在我撰写本文时为Xcode 11 beta 6)中onAppear和onDisappear的行为是开发人员会发现更有用的东西，还是仅仅是一个问题而不是功能。

现在，如果我们使用级联导航，就像您在我附加的示例代码中发现的那样(在Xcode 11b6中编译并运行良好)，则来回导航的用户的控制台输出只会在出现以下情况时触发onAppear:向前的新 View 加载(意味着更深入)。

在导航中:
根-> NestedView1-> NestedView2-> NestedView3，
在每个 View 阶段添加调试助手时，

  .onAppear(perform: {print("onAppear level N")})
  .onDisappear(perform: {print("onDisappear level N")})

调试控制台将显示
onAppear root level 0onAppear level 1onAppear level 2onAppear level 3
(没有onDisappear触发)

但是回去
根
调试控制台将不显示任何内容

(没有onAppear或onDisappear触发)

struct NestedViewLevel3: View {

    @Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>

    var body: some View {
        VStack {
            Spacer()
            Text("Level 3")
            Spacer()
            Button(action: {
                self.presentationMode.wrappedValue.dismiss()
            }) {
                Text("Back")
                    .padding(.horizontal, 15)
                    .padding(.vertical, 2)
                    .foregroundColor(Color.white)
                    .clipped(antialiased: true)
                    .background(
                        RoundedRectangle(cornerRadius: 20)
                            .foregroundColor(Color.blue)
                            .frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
                )
            }
            Spacer()
        }
        .navigationBarBackButtonHidden(false)
        .navigationBarTitle("Level 3", displayMode: .inline)
        .onAppear(perform: {print("onAppear level 3")})
        .onDisappear(perform: {print("onDisappear level 3")})

    }
}

struct NestedViewLevel2: View {

    @Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>

    var body: some View {
        VStack {
            Spacer()
            NavigationLink(destination: NestedViewLevel3()) {
                Text("To level 3")
                    .padding(.horizontal, 15)
                    .padding(.vertical, 2)
                    .foregroundColor(Color.white)
                    .clipped(antialiased: true)
                    .background(
                        RoundedRectangle(cornerRadius: 20)
                            .foregroundColor(Color.gray)
                            .frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
                )
                    .shadow(radius: 10)
            }
            Spacer()
            Text("Level 2")
            Spacer()
            Button(action: {
                self.presentationMode.wrappedValue.dismiss()
            }) {
                Text("Back")
                    .padding(.horizontal, 15)
                    .padding(.vertical, 2)
                    .foregroundColor(Color.white)
                    .clipped(antialiased: true)
                    .background(
                        RoundedRectangle(cornerRadius: 20)
                            .foregroundColor(Color.blue)
                            .frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
                )
            }
            Spacer()
        }
        .navigationBarBackButtonHidden(false)
        .navigationBarTitle("Level 2", displayMode: .inline)
        .onAppear(perform: {print("onAppear level 2")})
        .onDisappear(perform: {print("onDisappear level 2")})
    }
}

struct NestedViewLevel1: View {

    @Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>

    var body: some View {
        VStack {
            Spacer()
            NavigationLink(destination: NestedViewLevel2()) {
                Text("To level 2")
                    .padding(.horizontal, 15)
                    .padding(.vertical, 2)
                    .foregroundColor(Color.white)
                    .clipped(antialiased: true)
                    .background(
                        RoundedRectangle(cornerRadius: 20)
                            .foregroundColor(Color.gray)
                            .frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
                )
                    .shadow(radius: 10)
            }
            Spacer()
            Text("Level 1")
            Spacer()
            Button(action: {
                self.presentationMode.wrappedValue.dismiss()
            }) {
                Text("Back")
                    .padding(.horizontal, 15)
                    .padding(.vertical, 2)
                    .foregroundColor(Color.white)
                    .clipped(antialiased: true)
                    .background(
                        RoundedRectangle(cornerRadius: 20)
                            .foregroundColor(Color.blue)
                            .frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
                )
            }
            Spacer()
        }
        .navigationBarBackButtonHidden(false)
        .navigationBarTitle("Level 1", displayMode: .inline)
        .onAppear(perform: {print("onAppear level 1")})
        .onDisappear(perform: {print("onDisappear level 1")})
    }
}

struct RootViewLevel0: View {

    var body: some View {
        NavigationView {
        VStack {
            Spacer()
            NavigationLink(destination: NestedViewLevel1()) {
            Text("To level 1")
                .padding(.horizontal, 15)
                .padding(.vertical, 2)
                .foregroundColor(Color.white)
                .clipped(antialiased: true)
                .background(
                    RoundedRectangle(cornerRadius: 20)
                    .foregroundColor(Color.gray)
                    .frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
                )
                .shadow(radius: 10)
        }

            Spacer()

            }
    }

        .navigationBarTitle("Root level 0", displayMode: .inline)
        .navigationBarBackButtonHidden(false)
        .navigationViewStyle(StackNavigationViewStyle())
        .onAppear(perform: {print("onAppear root level 0")})
        .onDisappear(perform: {print("onDisappear root level 0")})

    }

}


struct ContentView: View {
    var body: some View {
        RootViewLevel0()
    }
}


}

现在，开发人员宁愿拥有onAppear和onDisappear:

1)被触发以启动仅需一次且仅当用户向前行驶时才执行的 Action ，例如当前观察到的行为。

2)每次 View 出现时都会触发，更像是 Action 名称的意思，即向后，向前和任意次数。

我会选择选项2，简单而又残酷(以及当前需要什么)，但是我在NavigationView上还是个天真的新手，而选项2可能会打破很多我没有考虑的既定范例。

您的反馈将帮助我了解SwiftUI的相应Feedback Assistant案例是否有正当理由。

最佳答案

这是苹果公司的错误。.onAppear()现在可以在iOS 13.1和Xcode 11 Beta 7中正常工作。
前后导航到NavigationView时，.onAppear()将触发。