State & Data Flow

@State — local view data

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

@Binding — share editable state with a child

struct NameField: View {
    @Binding var name: String
    var body: some View { TextField("Name", text: $name) }
}
// parent passes $name (a binding)

ObservableObject for shared data

class Cart: ObservableObject {
    @Published var items: [String] = []
}

struct CartView: View {
    @StateObject private var cart = Cart()   // owns the object
    var body: some View { Text("\(cart.items.count) items") }
}

@EnvironmentObject (app-wide)

// inject once at the top
ContentView().environmentObject(Cart())

// read anywhere below
@EnvironmentObject var cart: Cart

Quick guide: simple value here → @State; passed to a child → @Binding; shared class → @StateObject/@ObservedObject; whole app → @EnvironmentObject.