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

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

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

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

@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