TimerTroubleInNavigationStack.swift

// Problem with Timer.publisher not work in navigation stack
//
// Timer.publisher *do not* fire reliably if the View is shown through NavigationLink()
// Work fine if it's not inside navigation stack
//
// The Pointfree Publisher.Timer do not fire at all when navigate in
// but start receiving timer when pop back out navigation stack
// run debug preview and watch print message in .onReceive()

import SwiftUI
import Combine
import CombineSchedulers


struct RootView: View {
    @State var currentDate = Date()
    let timer = Timer.publish(every: 0.5, on: .main, in: .common).autoconnect()

    var body: some View {
        NavigationView {
            VStack {
                Text("\(currentDate)")
                
                Group {
                    Text("Timer.publisher do not work reliably if navigated to the view")
                    Text("Tap the button below to view the same DetaisView")
                    NavigationLink(destination: DetailsTimerPublisherView()) {
                        Text("Navigate to DetailsView")
                    }
                }
                
                Group {
                    Text("But .scheduleTimer works!")
                    NavigationLink(destination: DetailsScheduleTimerView()) {
                        Text("Navigate to DetailsScheduleTimerView")
                    }
                }

                Group {
                    Text("Pointfree Publishers.Timer do not work")
                    Text("When navigate into")
                    Text("But it start firing when pop back Nav stack")
                    Text("Run Debug to see print out from .onReceive")
                    NavigationLink(destination: DetailsPointfreePublisherTimerView()) {
                        Text("Navigate to DetailsPointfreePublisherTimerView")
                    }
                }
            }
            .onReceive(timer) { now in
                self.currentDate = now
            }
            .navigationBarTitle("Root")
        }
    }
}


// Timer.Publisher do not work reliably
struct DetailsTimerPublisherView: View {
    @State var currentDate = Date()
    
    // if we do this, doesn't work at all
//    let timer = Timer.publish(every: 0.5, on: .main, in: .common).autoconnect()

    // so declare let, instantiate in init
    // timer fire once in a while, not on "every"
    private let timer: Combine.Publishers.Autoconnect<Timer.TimerPublisher>
    
    init() {
        self.timer = Timer.publish(every: 0.5, on: .main, in: .common).autoconnect()
    }
    
    
    var body: some View {
        Text("\(currentDate)")
            .onReceive(timer) { now in
                self.currentDate = now
            }
            .navigationBarTitle("Details")

    }
}

struct DetailsPointfreePublisherTimerView: View {
    @State var currentDate = Date()
    let timer = Publishers.Timer(every: .seconds(1), scheduler: DispatchQueue.main).autoconnect()
    
    var body: some View {
        Text("\(currentDate)")
            .onReceive(timer) { timer in           // SchedulerTimeType
                
                print(type(of: timer))             // run "Debug Preview" to see print out when view pop back up nav stack
                
                self.currentDate = Date()
            }
            .navigationBarTitle("Details")

    }
}


// Timer.scheduledTimer() arrange this way works
struct DetailsScheduleTimerView: View {
    @State var currentDate = Date()

    @State private var timer: Timer?
    
    
    var body: some View {
        Text("\(currentDate)")
            .onAppear {
                print("Appear")
                self.timer = Timer.scheduledTimer(
                    withTimeInterval: 0.5, repeats: true
                ) { timer in
                    self.currentDate = timer.fireDate
                }
            }
            .onDisappear {
                print("     Disappear")
                if let timer = self.timer {
                    timer.invalidate()
                    self.timer = nil
                }
            }
            .navigationBarTitle("Details")

    }
}



struct KeepTimerAliveThroughNavigation: View {
    var body: some View {
        VStack {
            RootView()
            
            Text("Timers all works here at the root of NavView")
            Text("DetailsView Displayed Outside of NavigationView")
            DetailsTimerPublisherView()
            
            DetailsScheduleTimerView()
            
            // works here, uncomment and see
            // comment out to not get confuse with print message inside
//            DetailsPointfreePublisherTimerView()
        }
    }
}

struct KeepTimerAliveThroughNavigation_Previews: PreviewProvider {
    static var previews: some View {
        KeepTimerAliveThroughNavigation()
    }
}