swiftui subview reappear after click the back button and update state data

Question

Very strange behavior.

Click the back button on the subpage (Subview) to return to the main page (ContentView). However, the subpage (Subview) automatically opens again. Why?

import SwiftUI

struct ContentView: View {
    @State var things: [String] = []
    @State var count: Int = 0
    
    var body: some View {
        NavigationView{
            List {
                ForEach(things.indices, id: \.self) { index in
                    Text(things[index])
                }
            }
            .onAppear {
                update()
            }
           
            .navigationTitle("a")
            .toolbar{
                NavigationLink(destination: Subview(count: $count), label: {
                    Text("sub")
                })
            }
        }
        
    }
    
    func update() {
        things = []
        for i in 0...count {
            things.append(String(i))
        }
    }
}

struct Subview: View {
    var count : Binding<Int>

    var body: some View {
        Text("sub")
            .onAppear {
                count.wrappedValue += 1
            }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Answer 1

NavigationLink should always be inside a NavigationView. If you put it in the toolbar or some other place, you might run into weird issues.

Instead, use the init(destination:isActive:label:) initializer. Then set the presentingNextPage property to true when you want to present the next page.

struct ContentView: View {
    @State var things: [String] = []
    @State var count: Int = 0
    @State var presentingNextPage = false
    
    var body: some View {
        NavigationView {
            
            List {
                ForEach(things.indices, id: \.self) { index in
                    Text(things[index])
                }
                
                /// placeholder navigation link
                NavigationLink(destination: Subview(count: $count), isActive: $presentingNextPage) {
                    EmptyView()
                }
            }
            .onAppear {
                self.update()
            }
            .navigationTitle("a")
            .toolbar{
                ToolbarItem(placement: .navigationBarTrailing) {
                    Button("sub") {
                        presentingNextPage = true /// set to true
                    }
                }
                
            }
        }
    }
    
    func update() {
        things = []
        for i in 0...count {
            things.append(String(i))
        }
    }
}

Result:

Answer 2

Put "onAppear{...}" on the NavigationView not the List, like this:

struct ContentView: View {
    @State var things: [String] = []
    @State var count: Int = 0
    
    var body: some View {
        NavigationView{
            List {
                ForEach(things.indices, id: \.self) { index in
                    Text(things[index])
                }
            }
            .navigationTitle("a")
            .toolbar{
                NavigationLink(destination: Subview(count: $count), label: {
                    Text("sub")
                })
            }
        }
        .onAppear {   // <---
            update()
        }
    }