State Management

State Management

Property Wrapper Quick Reference

@StateObject vs @ObservedObject

Use @StateObject when the view creates and owns the object. Use @ObservedObject when passed from a parent.

// BAD - recreated on every view rebuild
struct ContentView: View {
    @ObservedObject var viewModel = ViewModel()  // Wrong!
}

// GOOD - survives view rebuilds
struct ContentView: View {
    @StateObject var viewModel = ViewModel()
}

// GOOD - received from parent
struct ChildView: View {
    @ObservedObject var viewModel: ViewModel  // Not creating it
}

iOS 17+ @Observable Pattern

With @Observable, use @State at app level, plain properties in children.

@Observable class AppStore { /* ... */ }

@main
struct MyApp: App {
    @State private var store = AppStore()
    var body: some Scene {
        WindowGroup { ContentView(store: store) }
    }
}

// Child receives without wrapper
struct ChildView: View {
    var store: AppStore  // Read-only access
}

// For two-way bindings, use @Bindable
struct EditView: View {
    @Bindable var user: User
    var body: some View {
        TextField("Name", text: $user.name)
    }
}

@State Mistakes

// BAD - @State ignores external updates
struct ChildView: View {
    @State var user: User  // Updates from parent ignored!
}

// BAD - property observers don't work
@State var count = 0 {
    didSet { print("Changed") }  // Never called!
}

// GOOD - use .onChange modifier
.onChange(of: count) { oldValue, newValue in
    print("Changed from \(oldValue) to \(newValue)")
}

Environment Usage (iOS 17+)

// Old syntax
@Environment(\.modelContext) private var context

// New syntax for custom types
@Environment(AuthService.self) private var authService

Critical Anti-Patterns

Review Questions

1. Is @StateObject used when the view creates the object?
2. Is @ObservedObject only used for objects passed from parent?
3. For iOS 17+, is @State with @Observable in a stable parent view?
4. Is @Bindable used where two-way bindings are needed?
5. Is .onChange used instead of didSet on @State?