How do barriers, groups, semaphores and dispatch sources coordinate work?

A barrier block (flags: .barrier) on a concurrent queue waits for all earlier blocks to finish, runs alone, and then lets later blocks resume. That gives the classic reader–writer lock: reads are plain sync or async blocks that may overlap, writes are barriers that get exclusive access. It only works on your own concurrent queues (a barrier on a global queue behaves like a normal block). A dispatch group tracks a set of tasks: group.enter() and leave() (or submitting with group:) count them, notify(queue:) runs a block when the count reaches zero without blocking anyone, and wait() blocks (never on the main thread). Each enter needs exactly one leave, or the group never completes.

A semaphore (DispatchSemaphore(value:)) is a counter: wait() decrements and blocks at zero, signal() increments. Use it to limit concurrency (value: 3 allows three at a time) or, carefully, to turn a callback into a blocking call; it has no priority donation, so a low-priority task holding it can cause priority inversion, and a wait with a timeout returns .timedOut instead of blocking forever. Dispatch sources deliver system events to a queue without polling: timers (makeTimerSource), file descriptor readiness, process events, signals and memory pressure. A source starts suspended, needs resume(), and must be cancelled with cancel(); suspending and resuming must be balanced before you release it. For new code, prefer actors and async sequences over these primitives, but you must read them, and they remain the right tool for low-level event sources.