What is backpressure/demand in Combine?
The essence
Backpressure is the mechanism by which a slow consumer holds back a fast producer. In Combine it is implemented through demand: the subscriber explicitly tells the publisher how many values it is ready to accept (Subscribers.Demand — .none, .max(n), .unlimited), and by contract the publisher may not send more. Demand accumulates: every request(.max(n)) adds to the current amount, and the value returned from receive(_:) buys more. It is a pull model on top of push delivery.
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