Question

我正在尝试按顺序获取深层嵌套的程序化导航堆栈。手动完成导航（即：按下链接）后，以下代码将按预期工作。当您按下Set Nav按钮时，导航堆栈的确发生了变化-但没有按预期进行-最终导致堆栈[start -> b -> bbb]损坏，视图之间发生了很多翻转

class NavState: ObservableObject {
    @Published var firstLevel: String? = nil
    @Published var secondLevel: String? = nil
    @Published var thirdLevel: String? = nil
}

struct LandingPageView: View {

    @ObservedObject var navigationState: NavState

    func resetNav() {
        self.navigationState.firstLevel = "b"
        self.navigationState.secondLevel = "ba"
        self.navigationState.thirdLevel = "bbb"
    }

    var body: some View {

        return NavigationView {
            List {
                NavigationLink(
                    destination: Place(
                        text: "a",
                        childValues: [ ("aa", [ "aaa"]) ],
                        navigationState: self.navigationState
                    ).navigationBarTitle("a"),
                    tag: "a",
                    selection: self.$navigationState.firstLevel
                ) {
                    Text("a")
                }
                NavigationLink(
                    destination: Place(
                        text: "b",
                        childValues: [ ("bb", [ "bbb"]), ("ba", [ "baa", "bbb" ]) ],
                        navigationState: self.navigationState
                    ).navigationBarTitle("b"),
                    tag: "b",
                    selection: self.$navigationState.firstLevel
                ) {
                    Text("b")
                }

                Button(action: self.resetNav) {
                    Text("Set Nav")
                }
            }
            .navigationBarTitle("Start")
        }
        .navigationViewStyle(StackNavigationViewStyle())
    }
}


struct Place: View {
    var text: String
    var childValues: [ (String, [String]) ]

    @ObservedObject var navigationState: NavState

    var body: some View {
        List(childValues, id: \.self.0) { childValue in
            NavigationLink(
                destination: NextPlace(
                    text: childValue.0,
                    childValues: childValue.1,
                    navigationState: self.navigationState
                ).navigationBarTitle(childValue.0),
                tag: childValue.0,
                selection: self.$navigationState.secondLevel
            ) {
                Text(childValue.0)
            }
        }
    }
}

struct NextPlace: View {
    var text: String
    var childValues: [String]

    @ObservedObject var navigationState: NavState

    var body: some View {
        List(childValues, id: \.self) { childValue in
            NavigationLink(
                destination: FinalPlace(
                    text: childValue,
                    navigationState: self.navigationState
                ).navigationBarTitle(childValue),
                tag: childValue,
                selection: self.$navigationState.thirdLevel
            ) {
                Text(childValue)
            }
        }
    }
}

struct FinalPlace: View {
    var text: String
    @ObservedObject var navigationState: NavState

    var body: some View {
        let concat: String = "\(navigationState.firstLevel)/\(navigationState.secondLevel))/\(navigationState.thirdLevel)/"

        return VStack {
            Text(text)
            Text(concat)
        }
    }
}

我最初试图将导航过渡动画作为问题源，但是How to disable NavigationView push and pop animations表示这是不可配置的

这里有没有健全的> 1级程序化导航示例？

编辑：我想要到达这里的一部分也是初始状态，以使导航正常工作-如果我来自具有导航状态的外部环境，则希望反映（即：从通知（从某个应用内上下文开始或从保存到磁盘编码状态开始），那么我希望能够加载顶部View，并正确指向正确的孩子导航视图。本质上-用实际值替换nil中的NavState。 Qt的QML和ReactRouter都可以声明性地做到这一点-SwiftUI也应该能够做到。

Answer 1

这是因为完成动画后会形成新的堆栈级别，这就是为什么在手动点击的情况下它可以工作的原因。

经过以下修改，它可以工作：

func resetNav() {
    self.navigationState.firstLevel = "b"
    DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
        self.navigationState.secondLevel = "ba"
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            self.navigationState.thirdLevel = "bbb"
        }
    }
}

Answer 2

最简单的方法是删除后续级别的@Published。

 class NavState: ObservableObject {
  @Published var firstLevel: String? = nil
    var secondLevel: String? = nil
    var thirdLevel: String? = nil
 }

这还告诉您如何有条件地更改变量可能会影响导航行为。

Answer 3

如果有人遇到NavigationView似乎无法满足的相同导航要求，请在此处提出自己的解决方案。在我的书中-功能比图形元素和动画要难得多。受https://medium.com/swlh/swiftui-custom-navigation-view-for-your-applications-7f6effa7dbcf

的启发

它从根本上解决了单个声明性变量的想法，即在根目录级别决定了导航堆栈-当像从a链跳转到b链这样的操作时，绑定到各个变量似乎很奇怪。我需要上面的演示或从特定位置开始

我将诸如URI之类的字符串化路径的概念用作变量概念-可以用更具表现力的模型（例如enum s的向量）代替它

大提示：非常粗糙，没有动画，根本看起来不是原生的，使用AnyView，您不能多次使用相同的节点名称，只能反映一个{{1} }等-如果我将其转换为更漂亮，理智和通用的名称，我会在Gist中提出。

StackNavigationViewStyle

一些基本用法代码

struct NavigationElement {
    var tag: String
    var title: String
    var viewBuilder: () -> AnyView
}

class NavigationState: ObservableObject {
    let objectWillChange = ObservableObjectPublisher()

    var stackPath: [String] {
        willSet {
            if rectifyRootElement(path: newValue) {
                _stackPath = newValue
                rectifyStack(path: newValue, elements: _stackElements)
            }
        }
    }

    /// Temporary storage for the most current stack path during transition periods
    private var _stackPath: [String] = []

    @Published var stack: [NavigationElement] = []

    var stackElements: [String : NavigationElement] = [:] {
        willSet {
            _stackElements = newValue
            rectifyStack(path: _stackPath, elements: newValue)
        }
    }

    /// Temporary storage for the most current stack elements during transition periods
    private var _stackElements: [String : NavigationElement] = [:]

    init(initialPath: [String] = []) {
        self.stackPath = initialPath
        rectifyRootElement(path: initialPath)
        _stackPath = self.stackPath
    }

    @discardableResult func rectifyRootElement(path: [String]) -> Bool {
        // Rectify root state if set from outside
        if path.first != "" {
            stackPath = [ "" ] + path
            return false
        }
        return true
    }

    private func rectifyStack(path: [String], elements: [String : NavigationElement]) {
        var newStack: [NavigationElement] = []
        for tag in path {
            if let elem = elements[tag] {
                newStack.append(elem)
            }
            else {
                print("Path has a tag '\(tag)' which cannot be represented - truncating at last usable element")
                break
            }
        }
        stack = newStack
    }
}

struct NavigationStack<Content: View>: View {
    @ObservedObject var navState: NavigationState
    @State private var trigger: String = "humperdoo"    //HUMPERDOO! Chose something that would not conflict with empty root state - could probably do better with an enum

    init(_ navState: NavigationState, title: String, builder: @escaping () -> Content) {
        self.navState = navState
        self.navState.stackElements[""] = NavigationElement(tag: "", title: title, viewBuilder: { AnyView(builder()) })
    }

    var backButton: some View {
        Button(action: { self.navState.stackPath.removeLast() }) {
            Text("Back")
        }
    }

    var navigationHeader: some View {
        HStack {
            ViewBuilder.buildIf( navState.stack.count > 1 ? backButton : nil )
            Spacer()
            ViewBuilder.buildIf( navState.stack.last?.title != nil ? Text(navState.stack.last!.title) : nil )
            Spacer()
        }
        .frame(height: 40)
        .background(Color(.systemGray))
    }

    var currentNavElement: some View {
        return navState.stack.last!.viewBuilder()
    }

    var body: some View {
        VStack {
            // This is an effectively invisible element which primarily serves to force refreshes of the tree
            Text(trigger)
            .frame(width: 0, height: 0)
            .onReceive(navState.$stack, perform: { stack in
                self.trigger = stack.reduce("") { tag, elem in
                    return tag + "/" + elem.tag
                }
            })
            navigationHeader
            ViewBuilder.buildBlock(
                navState.stack.last != nil
                ? ViewBuilder.buildEither(first: currentNavElement)
                : ViewBuilder.buildEither(second: Text("The navigation stack is empty - this is odd"))
            )
        }
    }
}

struct NavigationDestination<Label: View, Destination: View>: View {

    @ObservedObject var navState: NavigationState
    var tag: String
    var label: () -> Label

    init(_ navState: NavigationState, tag: String, title: String, destination: @escaping () -> Destination, label: @escaping () -> Label) {
        self.navState = navState
        self.tag = tag
        self.label = label
        self.navState.stackElements[tag] = NavigationElement(tag: tag, title: title, viewBuilder: { AnyView(destination()) })
    }

    var body: some View {
        label()
        .onTapGesture {
            self.navState.stackPath.append(self.tag)
        }
    }
}

深度编程SwiftUI NavigationView导航

3 个答案: