Consider we have a RootView
and a DetailView
. DetailView
has it's own BindableObject, let's call it DetailViewModel
and we have scenario:
RootView
may be updated by some kind of global event e.g. missed
internet connection or by it's own data/view modelRootView
handling event it's
content is updated and this is causes new struct of DetailView
to
be createdDetailViewModel
is created by DetailView
on init,
there would be another reference of DetailViewModel
and it's state (e.g. selected object) will be missedHow can we avoid this situation?
Sorry me for not providing any code. This question is on architecture concept of Swift UI where we trying to combine declarative structs and reference objects with data.
For now I don't see da way to keep references that corresponds to appropriate view only and don't keep them in memory/environment forever in their current states.
Lets add some code to see whats happening if VM is created by it's View
import SwiftUI
import Combine
let trigger = Timer.publish(every: 2.0, on: .main, in: .default)
struct ContentView: View {
@State var state: Date = Date()
var body: some View {
NavigationView {
VStack {
NavigationLink(destination: ContentDetailView(), label: {
Text("Navigation push")
.padding()
.background(Color.orange)
})
Text("\(state)")
.padding()
.background(Color.green)
ContentDetailView()
}
}
.onAppear {
_ = trigger.connect()
}
.onReceive(trigger) { (date) in
self.state = date
}
}
}
struct ContentDetailView: View {
@ObservedObject var viewModel = ContentDetailViewModel()
@State var once = false
var body: some View {
let vmdesc = "View model uuid:\n\(viewModel.uuid)"
print("State of once: \(once)")
print(vmdesc)
return Text(vmdesc)
.multilineTextAlignment(.center)
.padding()
.background(Color.blue)
.onAppear {
self.once = true
}
}
}
class ContentDetailViewModel: ObservableObject, Identifiable {
let uuid = UUID()
}
It seems that if we store ObservableObject as @State in view (not as ObservedObject) View keeps reference on VM
@State var viewModel = ContentDetailViewModel()
Any negative effects? Can we use it like this?
It seems that if ViewModel kept in View's @State:
subs
will be reseted, but properties will be the sameMehhh...
This approach also allows you to keep strong references in bindings closures
import Foundation
import Combine
import SwiftUI
/**
static func instanceInView() -> UIViewController {
let vm = ContentViewModel()
let vc = UIHostingController(rootView: ContentView(viewModel: vm))
vm.bind(uiViewController: vc)
return vc
}
*/
public protocol ViewModelProtocol: class {
static func instanceInView() -> UIViewController
var bindings: Set<AnyCancellable> { get set }
func onAppear()
func onDisappear()
}
extension ViewModelProtocol {
func bind(uiViewController: UIViewController) {
uiViewController.publisher(for: \.parent)
.sink(receiveValue: { [weak self] (parent) in
if parent == nil {
self?.bindings.cancel()
}
})
.store(in: &bindings)
}
}
struct ModelView<ViewModel: ViewModelProtocol>: UIViewControllerRepresentable {
func makeUIViewController(context: UIViewControllerRepresentableContext<ModelView>) -> UIViewController {
return ViewModel.instanceInView()
}
func updateUIViewController(_ uiViewController: UIViewController, context: UIViewControllerRepresentableContext<ModelView>) {
//
}
}
struct RootView: View {
var body: some View {
ModelView<ParkingViewModel>()
.edgesIgnoringSafeArea(.vertical)
}
}
Apple says that we should use ObservableObject for the data that lives outside of SwiftUI. It means you have to manage your data source yourself.
It looks like a single state container fits best for SwiftUI architecture.
typealias Reducer<State, Action> = (inout State, Action) -> Void
final class Store<State, Action>: ObservableObject {
@Published private(set) var state: State
private let reducer: Reducer<State, Action>
init(initialState: State, reducer: @escaping Reducer<State, Action>) {
self.state = initialState
self.reducer = reducer
}
func send(_ action: Action) {
reducer(&state, action)
}
}
You can pass the instance of the store into the environment of your SwiftUI app and it will be available in all views and will store your app state without data losses.
I wrote a blog post about this approach, take a look at it for more information https://swiftwithmajid.com/2019/09/18/redux-like-state-container-in-swiftui/
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