An example of using UserDefaults

ContentView.swift

import SwiftUI

struct ContentView: View {
    // stores the input from the text field
    @State private var message = ""
    // stores the saved input
    // note the this variable is initialized from the UserDefaults
    @State private var pesistedMessage = UserDefaults.standard.string(forKey: "UserMessage")

    var body: some View {
        VStack {
            // $message means save the result straight into $message, that's temporary
            TextField("Enter message...", text: $message)
            Button("Save") {
                // overwrite the persisted message so it updates the display
                self.pesistedMessage = self.message
                // update the persissted message in UserDefaults so it loads by default next time
                UserDefaults.standard.set(
                    self.pesistedMessage,
                    forKey: "UserMessage"
                )
            }
            // display the persisted message
            Text("Output: \(pesistedMessage ?? "Default")")
        }
        
    }
}


struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}