Schedulers and Threading

A scheduler in Combine defines when and where work runs — which thread or queue executes the operators and delivers values to subscribers.

Controlling schedulers is essential for keeping UI updates on the main thread and heavy work off it.

Combine gives you two operators to control threading:

• subscribe(on:) — controls where the subscription and upstream work begins
• receive(on:) — controls where downstream values are delivered

They affect different ends of the pipeline.