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:

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

References

Load these based on what you need:

Concurrency Essentials — Load when writing new async code or converting completion handlers
Swift 6 Concurrency — Load when using @concurrent, nonisolated(unsafe), or actor patterns
Task Groups — Load when running multiple async operations in parallel
Task Cancellation — Load when implementing long-running or cancellable operations
Strict Concurrency — Load when enabling Swift 6 strict mode or fixing Sendable errors
Macros — Load when using or understanding Swift macros like @Observable
Modern Attributes — Load when migrating legacy code or using @preconcurrency, @backDeployed
Migration Patterns — Load when modernizing delegate patterns or UIKit views

Common Mistakes

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

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