Need local mutable state?
├── YES → Is it a value type?
│   ├── YES → Use @State
│   └── NO → Use @StateObject
└── NO → Shared from parent?
    ├── YES → Is it value type?
    │   ├── YES → Use @Binding
    │   └── NO → Use @ObservedObject
    └── NO → Use @Environment

// Local state
struct CounterView: View {
    @State private var count = 0
    
    var body: some View {
        Button("Count: \(count)") { count += 1 }
    }
}

// Shared state (parent to child)
struct ParentView: View {
    @State private var isOn = false
    
    var body: some View {
        ChildView(isOn: $isOn)
    }
}

struct ChildView: View {
    @Binding var isOn: Bool
    
    var body: some View {
        Toggle("Enable", isOn: $isOn)
    }
}

// Observable object
class ViewModel: ObservableObject {
    @Published var items: [Item] = []
}

struct ContentView: View {
    @StateObject private var viewModel = ViewModel()
    
    var body: some View {
        List(viewModel.items) { item in
            Text(item.name)
        }
    }
}

// Environment values
struct EnvironmentView: View {
    @Environment(\.colorScheme) var colorScheme
    @Environment(\.dismiss) var dismiss
    
    var body: some View {
        Text(colorScheme == .dark ? "Dark" : "Light")
    }
}