iOS Patterns

// In a View: LocalizedStringKey is inferred
Text("welcome_message")
Text("greeting \(username)")

// In a ViewModel or non-View context, use String(localized:)
let message = String(localized: "error_network")
let formatted = String(localized: "items_count \(count)")

Text("Welcome")                                    // ❌ hard-coded
let title: LocalizedStringKey = "welcome"          // ❌ only works in Views
let title = String(localized: "welcome")           // ✅ works anywhere

enum LocalizedDateStyle {
    case monthDayNumeric         // "Md"       → 12/13 or 13/12
    case monthDayAbbreviated     // "MMMd"     → Dec 13 / 13 Dec / 12月13日
    case yearMonthDayAbbreviated // "yMMMd"    → Dec 13, 2025
    case yearMonthDayFull        // "yMMMMd"   → December 13, 2025
    // ...
}

let text = LocalizedDateFormatter.string(from: date, style: .monthDayAbbreviated)

2025-12-08T10:00:00Z           ✅ supported
2025-12-08T10:00:00.123Z       ❌ fails
2025-12-08T10:00:00.123456Z    ❌ fails

extension JSONDecoder {
    static var withFlexibleDateDecoding: JSONDecoder {
        let decoder = JSONDecoder()
        decoder.dateDecodingStrategy = .custom { decoder in
            let container = try decoder.singleValueContainer()
            let s = try container.decode(String.self)
            let f = ISO8601DateFormatter()

            f.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
            if let d = f.date(from: s) { return d }

            f.formatOptions = [.withInternetDateTime]
            if let d = f.date(from: s) { return d }

            // Normalize PostgreSQL "yyyy-MM-dd HH:mm:ss.SSSSSS" → ISO8601
            let normalized = s
                .replacingOccurrences(of: " ", with: "T")
                .replacingOccurrences(of: #"(\.\d{3})\d+"#, with: "\$1",
                                      options: .regularExpression)
            f.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
            if let d = f.date(from: normalized) { return d }

            throw DecodingError.dataCorruptedError(
                in: container,
                debugDescription: "Unrecognized date format: \(s)")
        }
        return decoder
    }
}

#Preview("Default")       { MyView() }
#Preview("With Data")     { MyView(items: .mock) }
#Preview("Empty State")   { MyView(items: []) }

#Preview("Dark Mode") {
    MyView().preferredColorScheme(.dark)
}

#Preview("Large Text") {
    MyView().environment(\.dynamicTypeSize, .accessibility3)
}

#Preview("繁體中文") {
    MyView().environment(\.locale, .init(identifier: "zh-Hant"))
}

extension User {
    static let mock = User(id: UUID(), name: "Alice", email: "a@example.com")
    static let mockList: [User] = [.mock, /* ... */]
}

@State private var isExpanded = false            // local view state
@Binding var selectedItem: Item?                 // mutable state from parent
@StateObject private var vm = MyViewModel()      // owner of a VM
@ObservedObject var vm: MyViewModel              // passed-in VM
@Environment(\.dismiss) private var dismiss

@MainActor
final class MyViewModel: ObservableObject {
    @Published private(set) var items: [Item] = []
    @Published private(set) var isLoading = false
    @Published var error: Error?

    func loadItems() async {
        isLoading = true
        defer { isLoading = false }
        do { items = try await service.fetchItems() }
        catch { self.error = error }
    }
}

Text("Hello")
    .font(.headline)
    .frame(maxWidth: .infinity)
    .padding()
    .background(.blue)
    .cornerRadius(8)
    .shadow(radius: 4)
    .onTapGesture { }

// Edit sheet — must commit via explicit Save/Cancel
.sheet(isPresented: $showEdit) {
    EditFormView(onSave: save, onCancel: { showEdit = false })
        .interactiveDismissDisabled()
}