Swift

--- name: Swift slug: swift version: 1.0.1 description: Write safe Swift code avoiding memory leaks, optional traps, and concurrency bugs. metadata: {"clawdbot":{"emoji":"🦅","requires":{"bins":["swift"]},"os":["darwin","linux"]}} ---

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