Swift

Write safe Swift code avoiding memory leaks, optional traps, and concurrency bugs.

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