How to change #State var from Swift class - swiftui

Question

I have SwiftUI ContentView struct from which I call function in standard Swift class. This function may throw an error which I`d like to show via Alert in ContentView. Appearance of Alert is controlled by Bool #State var declared in ContentView.
I was trying to use #Binding property wrapper in the function but it is obviously not correct. Should I rather use ObservableObject or what is the best approach?
Thanks.
Fragment of ContentView with Alert
HStack {
Button("Load data...", action: {
let panel = NSOpenPanel()
panel.title = "Select CSV formatted data file"
panel.canChooseFiles = true
panel.allowedFileTypes = ["csv"]
panel.allowsMultipleSelection = false
panel.begin(completionHandler: {result in
if result == .OK {
getDataset(fromFileURL: panel.url!, withHeaderLine: headerLine)
}
})
})
.padding()
.alert(isPresented: $isError, content: {
Alert(title: Text("Error"), message: Text(errorText), dismissButton: .default(Text("OK")))
})
Toggle("With header line", isOn: $headerLine)
}.toggleStyle(SwitchToggleStyle())
}
Fragment of called function which can throw error
do {
var fromRow = 0
let fileContent = try String(contentsOf: fromFileURL)
let rows = fileContent.components(separatedBy: "\n")
if withHeaderLine { fromRow = 1 }
for i in fromRow...rows.count - 1 {
let columns = rows[i].components(separatedBy: ",")
guard let xValue = Double(columns[0]) else {
throw myError.conversionFailed
}
guard let yValue = Double(columns[1]) else {
throw myError.conversionFailed
}
myDataset.append(Dataset(x: xValue, y: yValue))
}
} catch myError.conversionFailed {
errorText = "Value conversion to Double failed."
isError.toggle()
} catch let error {
errorText = error.localizedDescription
isError.toggle()
}
}

Answer 1

I would suggest creating a ViewModel for that View. Inside that ViewModel you create the two PublishedValues for the errorText and isError. Then you can the function inside ViewModel and update Published value. ViewModel would look like this and then update your other View accordingly.
class ContentViewModel : ObservableObject {
#Published var isError : Bool = false
#Published var errorText : String = ""
func getDataset() {
//Here you call your function and return the result or call it directly inside here
errorText = "Value conversion to Double failed." //<< here you can change published values
isError.toggle()
}
}
Create ViewModel and map to their States
struct ContentView : View {
#ObservedObject var viewModel : ContentViewModel = ContentViewModel()
#State var headerLine : Bool = false
var body : some View {
HStack {
Button("Load data...", action: {
let panel = NSOpenPanel()
panel.title = "Select CSV formatted data file"
panel.canChooseFiles = true
panel.allowedFileTypes = ["csv", "png"]
panel.allowsMultipleSelection = false
panel.begin(completionHandler: {result in
if result == .OK {
viewModel.getDataset()
}
})
})
.padding()
.alert(isPresented: $viewModel.isError, content: {
Alert(title: Text("Error"), message: Text(viewModel.errorText), dismissButton: .default(Text("OK")))
})
Toggle("With header line", isOn: $headerLine)
.toggleStyle(SwitchToggleStyle())
}
}
}
If you still outsourced your function into another view, just return the error String from that function or use closures.

Answer 2

Here is a demo of possible approach (with some simulation of async call, if it matters)
Tested with Xcode 12.1 / iOS 14.1
class DemoClass {
func simulate(isError: Binding<Bool>) {
DispatchQueue.global(qos: .background).async {
sleep(1)
DispatchQueue.main.async {
isError.wrappedValue.toggle()
}
}
}
}
struct ContentView: View {
let demo = DemoClass()
#State private var isError = false
var body: some View {
VStack {
Button("Demo") { demo.simulate(isError: $isError) }
}
.alert(isPresented: $isError, content: {
Alert(title: Text("Error"), message: Text("errorText"), dismissButton: .default(Text("OK")))
})
}
}