SwiftUI - Subclassed viewModel doesn't trigger view refresh - swiftui

Question

I have this situation where I have a a BaseView containing some common elements and a BaseViewModel containing some common functions, but when its #Published var get updated no BaseView refresh occurs.
The setup is this:
class BaseViewModel: ObservableObject {
#Published var overlayView: AnyView = EmptyView().convertToAnyView()
func forceViewRefresh() {
self.objectWillChange.send()
}
func setOverlayView(overlayView: AnyView) {
self.overlayView = overlayView
}
}
This view model subclasses BaseViewModel:
class FirstViewModel: BaseViewModel {
func showOverlayView() {
self.setOverlayView(overlayView: OverlayView().convertToAnyView())
}
}
also I have a BaseView where I use the overlayView
struct BaseView<Content: View>: View {
let content: Content
#ObservedObject var viewModel = BaseViewModel()
init(content: () -> Content) {
self.content = content()
}
var body: some View {
ZStack {
Color.green.edgesIgnoringSafeArea(.vertical)
content
viewModel.overlayView
}
}
}
The first view that gets displayed is FirstView, which conforms to a BaseViewProtocol and has a FirstViewModel that extends BaseViewModel.
struct FirstView: BaseViewProtocol {
#ObservedObject var viewModel = FirstViewModel()
var body: some View {
BaseView() {
Button("Show overlay") {
viewModel.showOverlayView()
}
}
}
}
Clicking the Show overlay button in First View calls the showOverlayView() func on FirstViewModel which in turn calls setOverlayView on the BaseViewModel. The value of overlayView in BaseViewModel changes as expected, but no view refresh on FirstView is called.
What am I doing wrong?
Thanks a lot.

Answer 1

I have just tested this code sample and works fine on Xcode 12 beta 6 & iOS 14 beta 8
struct ContentView: View {
#StateObject private var viewModel = FirstViewModel()
var body: some View {
ZStack {
Button(action: { viewModel.showOverlayView() }) {
Text("Press")
}
viewModel.overlayView
}
}
}
class BaseViewModel: ObservableObject {
#Published var overlayView: AnyView = AnyView(EmptyView())
func forceViewRefresh() {
self.objectWillChange.send()
}
func setOverlayView(overlayView: AnyView) {
self.overlayView = overlayView
}
}
class FirstViewModel: BaseViewModel {
func showOverlayView() {
self.setOverlayView(
overlayView: AnyView(
Color.blue
.opacity(0.2)
.allowsHitTesting(false)
)
)
}
}

Answer 2

Generally in SwiftUI you don't create views in outside the body. The view creation should be left to SwiftUI - instead you can define some other controls telling SwiftUI how and when to create a view.
Here is a simplified demo how to present different overlays for different views.
You can create a basic OverlayView:
enum OverlayType {
case overlay1, overlay2
}
struct OverlayView: View {
let overlayType: OverlayType
#ViewBuilder
var body: some View {
if overlayType == .overlay1 {
Text("Overlay1") // can be replaced with any view you want
}
if overlayType == .overlay2 {
Text("Overlay1")
}
}
}
and use it in your BaseView (if overlayType is nil the overlay will not be shown):
struct BaseView<Content>: View where Content: View {
let overlayType: OverlayType?
let content: () -> Content
var body: some View {
ZStack {
Color.green.edgesIgnoringSafeArea(.vertical)
content()
if overlayType != nil {
OverlayView(overlayType: overlayType!)
}
}
}
}
Now in the ContentView you can use the BaseView and specify its OverlayType.
struct ContentView: View {
#State var overlayType: OverlayType?
var body: some View {
BaseView(overlayType: overlayType) {
Button("Show overlay") {
overlayType = .overlay1
}
}
}
}
Some considerations:
For simplicity I used #State variables to control overlays. If there are other use cases for your ViewModels you may want to move the logic there.
Note that instead of AnyView it's preferred to use #ViewBuilder.
Also if you want to observe an ObservableObject inside a view, you need to use #ObservedObject, not #ObservableObject.