swift-patterns

Swift Language Patterns

Modern Swift idioms for clean, safe code.

Optionals

Safe Unwrapping

// ✅ Good: guard let for early exit
func processUser(_ user: User?) {
    guard let user = user else {
        print("User is nil")
        return
    }
    // user is non-optional here
    print(user.name)
}

// ✅ Good: if let for conditional
if let email = user.email {
    sendEmail(to: email)
}

// ✅ Good: nil coalescing
let displayName = user.name ?? "Anonymous"

// ✅ Good: Optional chaining
let street = user.address?.street

// ❌ Bad: Force unwrap (crashes if nil)
let email = user.email!  // Don't do this

Optional Mapping

// ✅ Optional.map — transform the wrapped value (no `?` before .map)
let uppercase = user.name.map { $0.uppercased() }  // String?

// ✅ Optional.flatMap — when the transform itself returns an optional
let domain = user.email.flatMap { $0.split(separator: "@").last }  // Substring?

// ❌ Bad: `user.name?.map { ... }` optional-chains into String's
// Sequence.map — it maps over Characters and yields [String]?, not String?

Closures

Trailing Closures

// ✅ Trailing closure syntax
viewModel.fetchUsers { users in
    self.users = users
}

// Equivalent to the non-trailing form
viewModel.fetchUsers(completion: { users in
    self.users = users
})

// ✅ Shorthand argument names
viewModel.fetchUsers {
    self.users = $0
}

Capture Lists

// ✅ Capture lists to avoid retain cycles
final class UserListViewController: UITableViewController {
    private let viewModel = UserViewModel()
    private var timer: Timer?

    func setupRefresh() {
        viewModel.onRefresh = { [weak self] in
            guard let self else { return }  // shorthand since Swift 5.7
            self.tableView.reloadData()
        }
    }

    // ⚠️ A repeating Timer retains its closure until invalidate() —
    // a strong `[self]` capture there is a retain cycle. Use weak self:
    func setupTimer() {
        timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { [weak self] _ in
            self?.updateUI()
        }
    }

    private func updateUI() { tableView.reloadData() }
}