SwiftUI.swift

import SwiftUI


/// If _anything_ changes in the `MainView` (e.g. Timer update), its `var body` is re-created and therefore a new instance of `DetailsView(presenter: DetailsViewPresenter())` is created (even if it is already pushed)

/// A new instance of `DetailsViewPresenter` has `isChecked` set to false so the checkbox is unchecked even if view is already presented
struct MainView: View {
    let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()
    let formatter:  DateFormatter = {
        let f = DateFormatter()
        f.dateFormat = "HH:mm:ss"
        return f
    }()

    @State var currentTime = "00:00:00"

    var body: some View {
        NavigationView {
            VStack(spacing: 20) {
                Text("\(currentTime)")
                           .onReceive(timer) { input in
                            currentTime = formatter.string(from: input)
                           }

                NavigationLink(destination: DetailsView(presenter: DetailsViewPresenter()),
                               label: { Text("@ObservedObject") })

                NavigationLink(destination: DetailsViewStateObject(presenter: DetailsViewPresenter()),
                               label: { Text("@StateObject") })

                NavigationLink(destination: DetailsViewWithState(),
                               label: { Text("@State") })
            }
            .navigationTitle("Main")
        }
    }
}


// MARK: - Child view

class DetailsViewPresenter: ObservableObject {
    @Published var isChecked = false

    deinit {
        print("deinit \(self)")
    }
}

struct DetailsView: View {
    @ObservedObject var presenter: DetailsViewPresenter
    var body: some View {
        VStack {
            Toggle(isOn: $presenter.isChecked) {
                Text("Hello World")
            }
            .padding()
        }
        .navigationTitle("@ObservedObject")
    }
}

struct DetailsViewStateObject: View {
    @StateObject var presenter: DetailsViewPresenter
    var body: some View {
        VStack {
            Toggle(isOn: $presenter.isChecked) {
                Text("Hello World")
            }
            .padding()
        }
        .navigationTitle("@StateObject")
    }
}

struct DetailsViewWithState: View {
    @State var isChecked = false
    var body: some View {
        VStack {
            Toggle(isOn: $isChecked) {
                Text("Hello World")
            }
            .padding()
        }
        .navigationTitle("@State")
    }
}


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