SwiftUI NavigationLink push in onAppear 使用@ObservableObject 时立即弹出视图

Question

I want to programmatically be able to navigate to a link within a List of NavigationLinks when the view appears (building deep linking from push notification). I have a string -> Bool dictionary which is bound to a custom Binding<Bool> inside my view. When the view appears, I set the bool property, navigation happens, however, it immediately pops back. I followed the answer in SwiftUI NavigationLink immediately navigates back and made sure that each item in the List has a unique identifier, but the issue still persists.

Two questions:

1. Is my binding logic here correct?
2. How come the view pops back immediately?

import SwiftUI

class ContentViewModel: ObservableObject {
    @Published var isLinkActive:[String: Bool] = [:]
}

struct ContentViewTwo: View {
    @ObservedObject var contentViewModel = ContentViewModel()
    @State var data = ["1", "2", "3"]
    @State var shouldPushPage3: Bool = true

    var page3: some View {
        Text("Page 3")
            .onAppear() {
                print("Page 3 Appeared!")
        }
    }

    func binding(chatId: String) -> Binding<Bool> {
        return .init(get: { () -> Bool in
            return self.contentViewModel.isLinkActive[chatId, default: false]
        }) { (value) in
            self.contentViewModel.isLinkActive[chatId] = value
        }
    }

    var body: some View {
        return
            List(data, id: \.self) { data in
                NavigationLink(destination: self.page3, isActive: self.binding(chatId: data)) {
                    Text("Page 3 Link with Data: \(data)")
                }.onAppear() {
                    print("link appeared")
                }
            }.onAppear() {
                print ("ContentViewTwo Appeared")
                if (self.shouldPushPage3) {
                    self.shouldPushPage3 = false
                    self.contentViewModel.isLinkActive["3"] = true
                    print("Activating link to page 3")
                }
        }
    }
}

struct ContentView: View {
    var body: some View {
        return NavigationView() {
            VStack {
                Text("Page 1")
                NavigationLink(destination: ContentViewTwo()) {
                    Text("Page 2 Link")
                }
            }
        }
    }
}

Answer 1

The error is due to the lifecycle of the ViewModel, and is a limitation with SwiftUI NavigationLink itself at the moment, will have to wait to see if Apple updates the pending issues in the next release.

Update for SwiftUI 2.0 :

Change:

@ObservedObject var contentViewModel = ContentViewModel()

to:

@StateObject var contentViewModel = ContentViewModel()

@StateObject means that changes in the state of the view model do not trigger a redraw of the whole body .

You also need to store the shouldPushPage3 variable outside the View as the view will get recreated every time you pop back to the root View .

enum DeepLinking {
    static var shouldPushPage3 = true
}

And reference it as follows:

if (DeepLinking.shouldPushPage3) {
    DeepLinking.shouldPushPage3 = false
    self.contentViewModel.isLinkActive["3"] = true
    print("Activating link to page 3")
}

The bug got fixed with the latest SwiftUI release . But to use this code at the moment, you will need to use the beta version of Xcode and iOS 14 - it will be live in a month or so with the next GM Xcode release.

Answer 2

I was coming up against this problem, with a standard (not using 'isActive') NavigationLink - for me the problem turned out to be the use of the view modifiers: .onAppear{code} and.onDisappear{code} in the destination view . I think it was causing a re-draw loop or something which caused the view to pop back to my list view (after approx 1 second). I solved it by moving the modifiers onto a part of the destination view that's not affected by the code in those modifiers.