Hoisted state for forms in iOS (with UI Model)

ContentView.Swift

import SwiftUI

struct ContentView: View {

    @ObservedObject var viewModel = FormViewModel()

    var body: some View {
        VStack {
            SimpleInput($viewModel.screenState.firstInput) {
                print("CUTIKO_TAG FIRST focus lost")
                viewModel.validateFirstInput()
            }
            Text(viewModel.screenState.firstInputError)
            SimpleInput($viewModel.screenState.secondInput) {
                print("CUTIKO_TAG SECOND focus lost")
                viewModel.validateSecondInput()
            }
            Text(viewModel.screenState.secondInputError)
            SimpleInput($viewModel.screenState.thirdInput) {
                print("CUTIKO_TAG THIRD focus lost")
            }
            Text(verbatim: "isEnabled: \(viewModel.screenState.isValid)")
            Button(
                action: {
                    print("CUTIKO_TAG UI MODEL IS: \(viewModel.screenState)")
                },
                label: {
                    Text("Validate UI MODEL in logs")
                }
            )
        }
        .padding()
    }
}

private struct SimpleInput: View {
    var text: Binding<String>
    @FocusState private var isFocused: Bool
    private let onFocusLost: () -> Void
    @State private var previousFocus = false

    init(_ text: Binding<String>, onFocusLost: @escaping () -> Void) {
        self.text = text
        self.onFocusLost = onFocusLost
    }


    var body: some View {
        TextField(
            "INPUT:",
            text: self.text
        )
        .focused($isFocused)
        .onChange(of: isFocused) { newFocus in
            if (previousFocus && !newFocus) {
                self.onFocusLost()
            }
            previousFocus = newFocus
        }
    }
}

FormState.Swift

import Foundation

struct FormState {

    private static let noError = "NO ERROR"

    private var firstInputLostFocus = false
    var firstInput: String = "" {
        didSet {
            if firstInputLostFocus { validateFirstInput() }
        }
    }
    private var secondInputLostFocus = false
    var firstInputError = FormState.noError
    var secondInput = "" {
        didSet {
            if secondInputLostFocus { validateSecondInput() }
        }
    }
    var secondInputError = FormState.noError
    var thirdInput = ""
    var isValid: Bool {
        return isFirstInputValid() && isSecondInputValid() && !thirdInput.isEmpty
    }

    private func isFirstInputValid() -> Bool {
        return !firstInput.isEmpty
    }

    private func isSecondInputValid() -> Bool {
        return secondInput.count >= 3
    }

    mutating func validateFirstInput() {
        firstInputLostFocus = true
        if isFirstInputValid() {
            firstInputError = FormState.noError
        } else {
            firstInputError = "PLEASE WRITE THE FIRST INPUT"
        }
    }

    mutating func validateSecondInput() {
        secondInputLostFocus = true
        if isSecondInputValid() {
            secondInputError = FormState.noError
        } else {
            secondInputError = "Second input is VERY important, 3 char min"
        }
    }
}

extension FormState {
    static let initialValue = FormState()
}

FormViewModel.Swift

import Foundation

class FormViewModel: ObservableObject {

    @Published var screenState = FormState.initialValue

    func validateFirstInput() {
        screenState.validateFirstInput()
    }

    func validateSecondInput() {
        screenState.validateSecondInput()
    }
}