Foundation — the essential types

How it works

• URL = parsed, immutable. URL(string:) is nil on invalid input (iOS 17+: percent-encodes it instead, RFC 3986 — differs by OS). Files: URL(filePath:) (iOS 16), not URL(string: "/p").
• URLComponents = builder: set scheme/host/path/queryItems, read .url; never interpolate input into a URL string. percentEncodedQueryItems (iOS 11) when you encode yourself.
• Data — value type, COW, Hashable by content; base64 built in, hex not. Data(contentsOf:) is synchronous — files only, never network. Write with [.atomic, .completeFileProtection].
• Directories (iOS 16 statics) — URL.documentsDirectory, .applicationSupportDirectory, .cachesDirectory, .temporaryDirectory; before: FileManager.urls(for:in:). App Support may not exist yet: createDirectory. Extensions share an App Group container.
• NSCache — thread-safe; evicts under memory pressure; limits are hints; keys/values must be classes; keys not copied; not enumerable, not Codable — a memory tier, never storage. Dictionary: deterministic, value keys, but unbounded and needs a lock/actor.
• Measurement — Measurement(value: 5, unit: UnitLength.kilometers) .converted(to: .miles); .formatted(.measurement(width: .abbreviated, usage: .road)) picks km/mi by locale.
• Numbers — money is Decimal; .formatted(.currency(code: "EUR")); 0.25.formatted(.percent) → “25%”. Locale decides layout, not which currency — set currencyCode.
• UUID — v4, 122 random bits: idempotency keys, local ids; uuidString is uppercase. identifierForVendor resets when the vendor’s last app goes.
• Bundle — read-only; SPM: Bundle.module; test resources: Bundle(for: Self.self), not .main (the host app).
• NotificationCenter vs KVO — broadcast by name vs one @objc dynamic property; keep and remove the token either way.

Example

var c = URLComponents(string: "https://api.example.com/search")!
c.queryItems = [URLQueryItem(name: "q", value: "a b+c")]
c.percentEncodedQuery = c.percentEncodedQuery?   // form servers
  .replacingOccurrences(of: "+", with: "%2B")    // read + as space
let url = c.url!                  // ...search?q=a%20b%2Bc

let tail = data[5...]             // startIndex 5, not 0
let head = tail[tail.startIndex]  // or Data(tail)[0]

var dir = URL.applicationSupportDirectory          // iOS 16
var rv = URLResourceValues(); rv.isExcludedFromBackup = true
try dir.setResourceValues(rv)                      // needs var
let price = Decimal(string: "19.99")!              // not 19.99
price.formatted(.currency(code: "EUR"))  // "19,99 €" in de_DE

Interview traps

• + in a query value (base64, phone numbers) — URLComponents leaves it; a form-decoding server reads a space. Encode it as %2B.
• slice[0] on Data/ArraySlice — indices are the parent’s.
• Storing an absolute file path — the container UUID changes on update/restore.
• Images “vanish” from Caches/ — that is the contract; persistent data belongs in Application Support.
• Decimal(19.99) goes through Double: already inexact; use Decimal(string:) or integer minor units.
• numberStyle = .currency alone shows $ for a euro amount on a US phone — bind currencyCode.
• Relying on NSCache keeping a value or honouring countLimit exactly.

Remember

Build, don’t concatenate. Slices keep indices. Caches and tmp are borrowed. Money is Decimal, currency is explicit.

Likely questions

1. NSCache vs Dictionary? — auto-evicting + thread-safe vs deterministic + unbounded.
2. Downloaded images go where? — Caches on disk, NSCache in memory.
3. Show a price? — Decimal + .currency(code:).
4. Stable device id? — none; identifierForVendor, or your own UUID in Keychain.