/modern-swift

name: modern-swift
description: Use when writing async/await code, enabling strict concurrency, fixing Sendable errors, migrating from completion handlers, managing shared state with actors, or using Task/TaskGroup for concurrency.

Modern Swift (6.2+)

Swift 6.2 introduces strict compile-time concurrency checking with async/await, actors, and Sendable constraints that prevent data races at compile time instead of runtime. This is the foundation of safe concurrent Swift.

Overview

Modern Swift replaces older concurrency patterns (completion handlers, DispatchQueue, locks) with compiler-enforced safety. The core principle: if it compiles with strict concurrency enabled, it cannot have data races.

Quick Reference

| Need | Use | NOT | |------|-----|-----| | Async operation | `async/await` | Completion handlers | | Main thread work | `@MainActor` | `DispatchQueue.main` | | Shared mutable state | `actor` | Locks, serial queues | | Parallel tasks | `TaskGroup` | `DispatchGroup` | | Thread safety | `Sendable` | `@unchecked` everywhere |

Core Workflow

When writing async Swift code: 1. Mark async functions with `async`, call with `await` 2. Apply `@MainActor` to view models and UI-updating code 3. Use `actor` instead of locks for shared mutable state 4. Check `Task.isCancelled` or call `Task.checkCancellation()` in loops 5. Enable strict concurrency in Package.swift for compile-time safety

Reference Loading Guide

**ALWAYS load reference files if there is even a small chance the content may be required.** It's better to have the context than to miss a pattern or make a mistake.

| Reference | Load When | |-----------|-----------| | **[Concurrency Essentials](references/concurrency-essentials.md)** | Writing async code, converting completion handlers, using `await` | | **[Swift 6 Concurrency](references/swift6-concurrency.md)** | Using `@concurrent`, `nonisolated(unsafe)`, or actor patterns | | **[Task Groups](references/task-groups.md)** | Running multiple async operations in parallel | | **[Task Cancellation](references/task-cancellation.md)** | Implementing long-running or cancellable operations | | **[Strict Concurrency](references/strict-concurrency.md)** | Enabling Swift 6 strict mode or fixing Sendable errors | | **[Macros](references/macros.md)** | Using or understanding Swift macros like `@Observable` | | **[Modern Attributes](references/modern-attributes.md)** | Migrating legacy code or using `@preconcurrency`, `@backDeployed` | | **[Migration Patterns](references/migration-patterns.md)** | Modernizing delegate patterns or UIKit views |

Common Mistakes

1. **`@unchecked Sendable` as a quick fix** — Using `@unchecked Sendable` to silence compiler errors means you've opted out of safety. If the error persists after `@unchecked`, your code has a potential data race. Fix the underlying issue instead.

2. **Missing `await` at call sites** — Forgetting `await` when calling async functions is a compiler error, but checking `Task.isCancelled` in a loop without calling `Task.checkCancellation()` silently ignores cancellation.

3. **Capturing `self` in async blocks without `weak`** — Holding a strong reference to `self` in a long-running async task prevents deinit. Always use `[weak self]` in closures or use `.task` which auto-manages the lifecycle.

4. **Not checking task cancellation** — Long-running operations should regularly check `Task.isCancelled` or call `Task.checkCancellation()`, otherwise cancellation signals are ignored.

5. **Forgetting `@MainActor` on UI code and test suites** — Main test struct and view models that update `@Published` properties need `@MainActor`. Forgetting it silently allows cross-thread mutations. Apply `@MainActor` to: view models, view structs, main test structs, and any type that touches UI.

6. **Actor re-entrancy surprises** — `await` inside an actor method can release the lock temporarily. Another task may modify actor state. Design actor methods assuming state can change between `await` points.