New.swift

import Observation
import SwiftUI

struct ContentView2: View {
    @Bindable private var viewModel = Content2ViewModel()

    var body: some View {
        VStack {
            Text(viewModel.title)
            TextField("Username", text: $viewModel.username)
                .textFieldStyle(.roundedBorder)
        }
        .padding()
    }
}

@Observable class Content2ViewModel {
    var username: String = ""

    var title: String {
        if username.isEmpty {
            return "Hello, world!"
        }
        else {
            return "Hello \(username)!"
        }
    }
}