Ways to achieve concurrency in iOS/macOS

Middle PRO

The essence

Four levels of abstraction, from low to high:

• Thread / pthread — manual thread management. You decide how many to create, how to synchronize and when to finish. Expensive (a ~512 KB stack, context switches) and scales poorly; almost never used in app code.
• GCD (DispatchQueue) — task queues on top of a system thread pool. You describe a task and put it on a queue; the system picks the number of threads. Serial/concurrent queues, QoS, DispatchGroup, barrier.
• OperationQueue / Operation — an ObjC wrapper over GCD with task objects: dependencies between operations, cancellation, maxConcurrentOperationCount, KVO states. Handy for dependency graphs and cancellable pipelines.
• Swift Concurrency (async/await, Task, actor) — the language-level model since iOS 15. Structured concurrency, cooperative cancellation, compiler-checked data races (Swift 6). The default choice for new code in 2026.