Swift

## Quick Reference | Topic | File | |-------|------| | Optionals, nil safety, force unwrap | `optionals.md` | | Retain cycles, weak refs, closures | `memory.md` | | async/await, actors, Sendable, value types | `concurrency.md` | | JSON encoding/decoding traps | `codable.md` | | Protocols, collections, strings, errors, build | `types.md` | | SwiftUI state (@State, @Binding, Combine) | `swiftui.md` | | Property wrappers, actors, result builders, macros | `advanced.md` | | XCTest pitfalls, SPM gotchas | `testing.md` | ## Critical Rules ### Memory & Safety - Force unwrap `!` crashes on nil — use `guard let` or `if let` instead - Closures capturing `self` strongly create retain cycles — use `[weak self]` in escaping closures - Delegates must be `weak` — strong delegate = object never deallocates - `try!` crashes on any error — never use in production paths - `removeFirst()` crashes on empty — use `popFirst()` for safety ### Concurrency - `async let` starts immediately — not when you `await` - Actor reentrancy at every `await` — state may change between suspension points - `@MainActor` doesn't guarantee immediate main thread — it's queued - `Sendable` conformance violations crash at runtime — compiler warnings are errors ### Types & Collections - Protocol extensions don't override — static dispatch ignores subclass implementation - Mutating struct in collection requires reassignment — `array[0].mutate()` doesn't work - `String.Index` from one string invalid on another — even if contents match ### SwiftUI - `@StateObject` owns, `@ObservedObject` borrows — recreating view loses ObservedObject state - `@EnvironmentObject` crashes if not injected — no compile-time check - View identity change resets all `@State` — changing ID loses state ### Build - `print()` builds strings even in release — remove or use os_log - Generic code bloat — specialized for each type, increases binary size