Difference between synchronous and asynchronous tasks

Junior PRO

The essence

The terms are about whether the caller waits for the task to finish.

• Synchronous (queue.sync { }) — the calling thread blocks until the task has run, and only then continues. As an optimization GCD usually runs the block right on the calling thread (no new thread is created), but that is not a contract: DispatchQueue.main.sync from a background thread runs the block on the main thread.
• Asynchronous (queue.async { }) — the task is enqueued and the caller continues immediately. When and on which thread the block runs is up to the queue: serial — after all previously enqueued tasks, concurrent — possibly in parallel with them.