What is a Scheduler and the receive(on:)/subscribe(on:) operators?

Senior PRO

The essence

Scheduler is a protocol describing where and when work runs: DispatchQueue, RunLoop, OperationQueue, ImmediateScheduler. It is needed by time operators (debounce, delay, throttle, timeout) and by the two context-switching operators.

• receive(on:) — everything downstream (operators and the subscriber) receives values and completion on the given scheduler. This is how a result is delivered to the main thread before updating UI.
• subscribe(on:) — the subscription operations run on the given scheduler: subscribe, request(demand), cancel — i.e. the moment the upstream starts working. Where values are emitted is not guaranteed by the operator: the publisher itself decides. URLSession.dataTaskPublisher delivers on its own queue, Subject.send on the caller's thread, and subscribe(on:) does not change that.