11.4 · receive(on:), subscribe(on:) and where code runs

Combine does not own threads. A value is delivered on whatever thread produced it, until you say otherwise with a scheduler.

Mental model. A pipeline has no thread of its own. The thread is wherever the value was made; receive(on:) moves everything downstream of it.

Threads and schedulers

Where each stage runs, and how to move it.

1. What do receive(on:) and subscribe(on:) do, and how do you know which thread a stage runs on? 11.4.1 · L3
   A scheduler (DispatchQueue, OperationQueue, RunLoop, ImmediateScheduler) decides when and where a closure runs. Combine does nothing to threads by default: a…

More in Combine

• 11.1 · Publisher, Subscriber, Subscription and demand
• 11.2 · map, scan, combineLatest, flatMap, switchToLatest, debounce, share
• 11.3 · PassthroughSubject, CurrentValueSubject and @Published
• 11.5 · AnyCancellable, subscription lifetime, and failure versus Never
• 11.6 · Combine versus async/await, and testing pipelines
• 11.7 · Operator behaviour, error placement, retry, flatMap limits and debugging
• 11.8 · Future versus Deferred, subject replay, share, and bridging to async
• 11.9 · Which thread each stage runs on, schedulers, throttle versus debounce, and timers
• 11.10 · Cancelling and completing, the cycle that survives a deinit, and failure types