ObservedObject view-model is still in memory after the view is dismissed

Question

I am having some trouble with memory management in SwiftUI and Combine.

For example, if I have a NavigationView and then navigate to a detail view with a TextField, and enter a value in the TextField and tap on the back button, next time I go to that view the TextField has the previously entered value.

I noticed that the view-model is still in memory after the detail view is dismissed, and that's probably why the TextField still holds a value.

In UIKit, when dismissing a ViewController, the view-model will be deallocated and then created again when the ViewController is presented. This seems to not be the case here.

I attach some minimum reproductible code for this issue.

import SwiftUI
import Combine

struct ContentView: View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: OtherView()) {
                Text("Press Here")
            }
        }
    }
}

struct OtherView: View {

    @ObservedObject var viewModel = ViewModel()

    var body: some View {
        VStack {
            TextField("Something", text: $viewModel.enteredText)
                .textFieldStyle(RoundedBorderTextFieldStyle())

            Button(action: {
                print("Tap")
            }) {
                Text("Tapping")
            }.disabled(!viewModel.isValid)
        }
    }
}

class ViewModel: ObservableObject {

    @Published var enteredText = ""
    var isValid = false

    var cancellable: AnyCancellable?

    init() {
        cancellable = textValidatedPublisher.receive(on: RunLoop.main)
            .assign(to: \.isValid, on: self)
    }

    deinit {
        cancellable?.cancel()
    }

    var textValidatedPublisher: AnyPublisher<Bool, Never> {
        $enteredText.map {
            $0.count > 1
        }.eraseToAnyPublisher()
    }


}

I also noticed that, if for example, I add another view, let's say SomeOtherView after OtherView, then each time I type in the TextField from OtherView, then the deinit from SomeOtherView's view-model is called. Can anyone please also explain why this happens?

Answer 1

Moreover, I noticed that if I to a change in ContetView and the view is reevaluated, then I will have two ViewModels in memory

It is due to cross-reference in ViewModel, here is fixed variant

struct OtherView: View, Constructable {

    @ObservedObject var viewModel = ViewModel()

    var body: some View {
        VStack {
            TextField("Something", text: $viewModel.enteredText)
                .textFieldStyle(RoundedBorderTextFieldStyle())

            Button(action: {
                print("Tap")
            }) {
                Text("Tapping")
            }.disabled(!viewModel.isValid)
        }
        .onDisappear {
            self.viewModel.invalidate()     // << here !!
        }
    }
}

class ViewModel: ObservableObject {

    @Published var enteredText = ""
    var isValid = false

    var cancellable: AnyCancellable?

    init() {
        print("[>>] created")
        cancellable = textValidatedPublisher.receive(on: RunLoop.main)
            .assign(to: \.isValid, on: self)
    }

    func invalidate() {
        cancellable?.cancel()
        cancellable = nil
        print("[<<] invalidated")
    }

    deinit {
//        cancellable?.cancel()     // not here !!!
        print("[x] done")
    }

    var textValidatedPublisher: AnyPublisher<Bool, Never> {
        $enteredText.map {
            $0.count > 1
        }.eraseToAnyPublisher()
    }
}

--

Update:

is there a way to instantiate OtherView when navigating?

Here is a solution (tested with Xcode 11.4 / iOS 13.4), but this is only half-a-deal, because once created it will be alive until navigation link revalidated (ie. on back it remains in memory until next navigate)

struct ContentView: View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: 
               // create wrapper view with type of view which creation
               // is deferred until navigation
               DeferCreatingView(of: OtherView.self)) {
                Text("Press Here")
            }
        }
    }
}

protocol Constructable {
    init()
}

struct DeferCreatingView<T: View & Constructable>: View {
    var ViewType: T.Type

    init(of type: T.Type) {
        ViewType = type
    }

    var body: some View {
        ViewType.init()     // << create only here
    }
}


struct OtherView: View, Constructable {
    // .. not changed code from first part
}

Answer 2

Add navigation view style .navigationViewStyle(StackNavigationViewStyle()) to Navigation View. It will deinit the view model. See your modified code below.

import SwiftUI
import Combine

struct ContentView: View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: OtherView()) {
                Text("Press Here")
            }
        }
        .navigationViewStyle(StackNavigationViewStyle())
        // Added navigation style here.
    }
}

struct OtherView: View {

    @ObservedObject var viewModel = ViewModel()

    var body: some View {
        VStack {
            TextField("Something", text: $viewModel.enteredText)
                .textFieldStyle(RoundedBorderTextFieldStyle())

            Button(action: {
                print("Tap")
            }) {
                Text("Tapping")
            }.disabled(!viewModel.isValid)
        }
    }
}

class ViewModel: ObservableObject {

    @Published var enteredText = ""
    var isValid = false

    var cancellable: AnyCancellable?

    init() {
        cancellable = textValidatedPublisher.receive(on: RunLoop.main)
            .assign(to: \.isValid, on: self)
    }

    deinit {
        cancellable?.cancel()
    }

    var textValidatedPublisher: AnyPublisher<Bool, Never> {
        $enteredText.map {
            $0.count > 1
        }.eraseToAnyPublisher()
    }


}