Swift UI Navigator API (can only push and pop to Root)

comments.md

Problem

This implementation is a proof that I can't pop to a specific view using the isActive property of the navigation link. I can easily push, pop and pop to the root. But I can't pop to a specific view.

Discussion

I could not find anything to make the NavigationView work with a proper popToView behavior. Here's a discussion on the Apple Forums: https://developer.apple.com/forums/thread/134078.

Solutions

1. Abort popToView, support only popToRoot.
2. Use multiple navigators to create multiple roots and instead of popToView, implement popToPinned, which pops to the last pinned route.
3. It looks like there's a lib that does exactly what we need, it might be interesting to take a look: https://github.com/johnpatrickmorgan/FlowStacks

ContentView.swift

import SwiftUI

func safeAt<T>(array: [T], position: Int) -> T? {
    return (position <= array.count - 1) ? array[position] : nil
}

protocol Navigator {
    func push(url: String) -> Void
    func pop() -> Void
}

class CoreNimbusView {
    let navigator: Navigator
    private var listener: ((String) -> Void)?
    private var lateUrl: String?
    
    init(navigator: Navigator) {
        self.navigator = navigator
    }
    
    func onChange(listener: @escaping (String) -> Void) {
        self.listener = listener
        if (lateUrl != nil) {
            listener(lateUrl!)
            lateUrl = nil
        }
    }
    
    func paint(url: String) {
        if (listener == nil) {
            lateUrl = url
        } else {
            listener!(url)
        }
    }
}

func pushActionHandler(url: String, view: CoreNimbusView) {
    view.navigator.push(url: url)
}
func popActionHandler(view: CoreNimbusView) {
    view.navigator.pop()
}

class HistoryItem: ObservableObject {
    let url: String
    var linkActive = false
    let coreView: CoreNimbusView
    
    init(url: String, coreView: CoreNimbusView) {
        self.url = url
        self.coreView = coreView
    }
}

class History: ObservableObject {
    @Published var pages: [HistoryItem] = []
    @Published var current = -1
}

struct NimbusView: View, Identifiable {
    let id: Int
    
    @State var url = ""
    @EnvironmentObject var history: History

    var body: some View {
        let page = safeAt(array: history.pages, position: id)
        if (page == nil) {
            EmptyView()
        } else {
            let page = page!
            VStack {
                // navigation logic
                UseMemo(key: page.linkActive) { linkActive in
                    VStack {
                        NavigationLink(
                            destination: NimbusView(id: id + 1),
                            isActive: $history.pages[id].linkActive
                        ) {
                            EmptyView()
                        }
                    }
                }
                
                // view content
                UseMemo(key: url) { keys in
                    VStack {
                        if (url.isEmpty) {
                            Text("No URL yet")
                        } else {
                            Text(url)
                            Button("Next") { pushActionHandler(url: "url \(id)", view: page.coreView) }
                            Button("Previous") { popActionHandler(view: page.coreView) }
                            //Button("Previous") { history.forceUpdate.toggle() }
                        }
                    }
                }
            }.onAppear() {
                page.coreView.onChange(listener: { content in url = content })
            }
        }
    }
}

struct NimbusNavigator: View, Navigator {
    let initialUrl: String
    @StateObject var history = History()
    
    func push(url: String) {
        if (history.current >= 0) {
            history.pages[history.current].linkActive = true
        }
        let newPage = HistoryItem(url: url, coreView: CoreNimbusView(navigator: self))
        history.pages.removeLast(history.pages.count - 1 - history.current)
        history.pages.append(newPage)
        newPage.coreView.paint(url: url)
        history.current += 1
    }
    
    func pop() {
        if (history.current <= 0) {
            return print("Can't pop single view")
        }
        history.pages[history.current - 1].linkActive = false
        history.current -= 1
    }
    
    var body: some View {
        NavigationView { NimbusView(id: 0) }
            .environmentObject(history)
            .onAppear() {
                push(url: initialUrl)
            }
    }
}

struct ContentView: View {
    var body: some View {
        NimbusNavigator(initialUrl: "initial")
    }
}

UseMemo.swift

import SwiftUI

struct UseMemo<KeyType: Equatable, ViewType: View>: View, Equatable {
    let key: KeyType
    let content: (KeyType) -> ViewType

    var body: some View {
        content(key)
    }
    
    static func ==(lhs: UseMemo, rhs: UseMemo) -> Bool {
        return lhs.key == rhs.key
    }
}