How to deinit ViewModel with combine framework when views disappears - swiftui

Question

I use Combine in viewModels to update the views. But if I store the AnyCancellable objects into a set of AnyCancellable, the deinit method is never called. I use the deinit to cancel all cancellables objects.
struct View1: View {
#ObservedObject var viewModel:ViewTextModel = ViewTextModel()
#Injected var appActions:AppActions
var body: some View {
VStack {
Text(self.viewModel.viewText)
Button(action: {
self.appActions.goToView2()
}) {
Text("Go to view \(self.viewModel.viewText)")
}
}
}
}
class ViewTextModel: ObservableObject {
#Published var viewText: String
private var cancellables = Set<AnyCancellable>()
init(state:AppState) {
// initial state
viewText = "view \(state.view)"
// updated state
state.$view.removeDuplicates().map{ "view \($0)"}.assign(to: \.viewText, on: self).store(in: &cancellables)
}
deinit {
cancellables.forEach { $0.cancel() }
}
}
Each time the view is rebuilt, a new viewmodel is instantiated but the old one is not destroyed. viewText attribute is updated on each instance with state.$view.removeDuplicates().map{ "view \($0)"}.assign(to: \.viewText, on: self).store(in: &cancellables)
If I don't store the cancellable object in the set, deinit is called but viewText is not updated if the state's changed for the current view.
Do you have an idea of ​​how to manage the update of the state without multiplying the instances of the viewmodel ?
Thanks

Answer 1

You could use sink instead of assign:
state.$view
.removeDuplicates()
.sink { [weak self] in self?.viewText = $0 }
.store(in: &cancellables)
But I question the need for Combine here at all. Just use a computed property:
class ViewTextModel: ObservableObject {
#Published var state: AppState
var viewText: String { "view \(state.view)" }
}
UPDATE
If your deployment target is iOS 14 (or macOS 11) or later:
Because you are storing to an #Published, you can use the assign(to:) operator instead. It manages the subscription for you without returning an AnyCancellable.
state.$view
.removeDuplicates()
.map { "view \($0)" }
.assign(to: &$viewText)
// returns Void, so nothing to store