// iOS 17+ Modern Pattern
@Observable
class FeatureViewModel {
    var items: [Item] = []
    @ObservationIgnored var cache: Cache? // Non-observed
}

struct FeatureView: View {
    @State private var viewModel = FeatureViewModel()
    var body: some View { ... }
}

import SwiftUI

@Observable
class SettingsViewModel {
    var isDarkMode: Bool {
        didSet { UserDefaults.standard.set(isDarkMode, forKey: "darkMode") }
    }
    var notificationsEnabled: Bool {
        didSet { UserDefaults.standard.set(notificationsEnabled, forKey: "notifications") }
    }

    init() {
        isDarkMode = UserDefaults.standard.bool(forKey: "darkMode")
        notificationsEnabled = UserDefaults.standard.bool(forKey: "notifications")
    }
}

struct SettingsView: View {
    @State private var viewModel = SettingsViewModel()

    var body: some View {
        Form {
            Section("Appearance") {
                Toggle("Dark Mode", isOn: $viewModel.isDarkMode)
                    .accessibilityHint("Switches between light and dark appearance")
            }

            Section("Notifications") {
                Toggle("Enable Notifications", isOn: $viewModel.notificationsEnabled)
                    .accessibilityHint("Allows the app to send you notifications")
            }
        }
        .navigationTitle("Settings")
    }
}

import SwiftUI

struct Item: Identifiable {
    let id = UUID()
    let title: String
    let subtitle: String
}

struct ItemListView: View {
    let items: [Item]

    var body: some View {
        List(items) { item in
            NavigationLink(value: item) {
                VStack(alignment: .leading, spacing: 4) {
                    Text(item.title)
                        .font(.headline)
                    Text(item.subtitle)
                        .font(.subheadline)
                        .foregroundStyle(.secondary)
                }
                .padding(.vertical, 4)
            }
            .accessibilityLabel("\(item.title), \(item.subtitle)")
        }
        .navigationTitle("Items")
        .navigationDestination(for: Item.self) { item in
            ItemDetailView(item: item)
        }
    }
}

import SwiftUI

@Observable
class DataViewModel {
    var items: [Item] = []
    var isLoading = false
    var errorMessage: String?

    func loadData() async {
        isLoading = true
        errorMessage = nil

        do {
            items = try await APIService.shared.fetchItems()
        } catch {
            errorMessage = error.localizedDescription
        }

        isLoading = false
    }
}

struct DataView: View {
    @State private var viewModel = DataViewModel()

    var body: some View {
        Group {
            if viewModel.isLoading {
                ProgressView("Loading...")
            } else if let error = viewModel.errorMessage {
                ContentUnavailableView(
                    "Error",
                    systemImage: "exclamationmark.triangle",
                    description: Text(error)
                )
            } else {
                List(viewModel.items) { item in
                    Text(item.title)
                }
            }
        }
        .task {
            await viewModel.loadData()
        }
        .refreshable {
            await viewModel.loadData()
        }
    }
}