Check `untilOutputFrom`’s demand count.

until_output_from_demand.swift

import Combine
import PlaygroundSupport

var subscriptions = Set<AnyCancellable>()

[1, 2, 3]
	.publisher
	.prefix(
		untilOutputFrom: Just(4)
			.print() /// (1) To log out lifecycle events.
	).sink(receiveValue: { print($0) })
	.store(in: &subscriptions)

PlaygroundPage.current.needsIndefiniteExecution = true

/// (2) Output. Note that only a max of one value is requested from `Just(4)`.
/// ```none
/// receive subscription: (Just)
/// request max: (1)
/// receive value: (4)
/// receive cancel
/// receive finished
/// ```