SwiftUI #State not triggering update when value changed by separate method - ios

Question

I have this (simpilied) section of code for a SwiftUI display:
struct ContentView: View {
private var errorMessage: String?
#State private var showErrors: Bool = false
var errorAlert: Alert {
Alert(title: Text("Error!"),
message: Text(errorMessage ?? "oops!"),
dismissButton: .default(Text("Ok")))
}
init() {}
var body: some View {
VStack {
Text("Hello, World!")
Button(action: {
self.showErrors.toggle()
}) {
Text("Do it!")
}
}
.alert(isPresented: $showErrors) { errorAlert }
}
mutating func display(errors: [String]) {
errorMessage = errors.joined(separator: "\n")
showErrors.toggle()
}
}
When the view is displayed and I tape the "Do it!" button then the alert is displayed as expected.
However if I call the display(errors:...) function the error message is set, but the display does not put up an alert.
I'm guessing this is something to do with the button being inside the view and the function being outside, but I'm at a loss as to how to fix it. It should be easy considering the amount of functionality that any app would have that needs to update a display like this.

Answer 1

Ok, some more reading and a refactor switched to using an observable view model like this:
class ContentViewModel: ObservableObject {
var message: String? = nil {
didSet {
displayMessage = message != nil
}
}
#Published var displayMessage: Bool = false
}
struct ContentView: View {
#ObservedObject private var viewModel: ContentViewModel
var errorAlert: Alert {
Alert(title: Text("Error!"), message: Text(viewModel.message ?? "oops!"), dismissButton: .default(Text("Ok")))
}
init(viewModel: ContentViewModel) {
self.viewModel = viewModel
}
var body: some View {
VStack {
Button(action: {
self.viewModel.displayMessage.toggle()
}) {
Text("Do it!")
}
}
.alert(isPresented: $viewModel.displayMessage) { errorAlert }
}
}
Which is now working as expected. So the takeaway from this is that using observable view models is more useful even in simpler code like this.