In SwiftUI how do I update a Published property inside ViewModel1 from ViewModel2? - ios

Question

Fairly new to SwiftUI and trying to figure out how to use ViewModels. Coming from UIKit I tend to like binding button presses to view model events, then apply the business logic and return a new value.
I am trying this in SwiftUI:
struct MainView: View {
#ObservedObject private var viewModel: MainViewModel
#State private var isShowingBottomSheet = false
var body: some View {
VStack {
Text("Hello \(viewModel.username)")
.font(.title)
Button("Show bottom sheet") {
isShowingBottomSheet = true
}
.sheet(isPresented: $isShowingBottomSheet) {
let viewModel = SheetViewModel()
viewModel.event.usernameUpdated
.assign(to: &$viewModel.username)
SheetView(viewModel: viewModel)
.presentationDetents([.fraction(0.15), .medium])
}
}
}
// MARK: - Initializers
init(viewModel: MainViewModel) {
self.viewModel = viewModel
}
}
With the view model:
final class MainViewModel: ObservableObject {
// MARK: - Properties
#Published var username = "John"
}
And SheetView:
struct SheetView: View {
#ObservedObject private var viewModel: SheetViewModel
var body: some View {
VStack {
Text("Some Sheet")
.font(.title)
Button("Change Name") {
viewModel.event.updateUsernameButtonTapped.send(())
}
}
}
// MARK: - Initializers
init(viewModel: SheetViewModel) {
self.viewModel = viewModel
}
}
And SheetViewModel:
final class SheetViewModel: ObservableObject {
// MARK: - Events
struct Event {
let updateUsernameButtonTapped = PassthroughSubject<Void, Never>()
let usernameUpdated = PassthroughSubject<String, Never>()
}
// MARK: - Properties
let event = Event()
private var cancellables = Set<AnyCancellable>()
// MARK: - Binding
private func bindEvents() {
event.updateUsernameButtonTapped
.map { "Sam" }
.sink { [weak self] name in
self?.event.usernameUpdated.send(name)
}
.store(in: &cancellables)
}
}
I am getting the error Cannot convert value of type 'Binding<String>' to expected argument type 'Published<String>.Publisher'. I want my SheetViewModel to update the value of #Published var username in the MainViewModel. How would I go about this?

Answer 1

We usually don't need view model objects in SwiftUI which has a design that benefits from value semantics, rather than the more error prone reference semantics of UIKit. If you want to move logic out of the View struct you can group related state vars and mutating funcs in their own struct, e.g.
struct ContentView: View {
#State var config = SheetConfig()
var body: some View {
VStack {
Text(config.text)
Button(action: show) {
Text("Edit Text")
}
}
.sheet(isPresented: $config.isShowing,
onDismiss: didDismiss) {
TextField("Text", $config.text)
}
}
func show() {
config.show(initialText: "Hello")
}
func didDismiss() {
// Handle the dismissing action.
}
}
struct SheetConfig {
var text = ""
var isShowing = false
mutating func show(initialText: String) {
text = initialText
isShowing = true
}
}
If you want to persist/sync data, or use Combine then you will need to resort to the reference type version of state which is #StateObject. However if you use the new async/await and .task then it's possible to still not need it.