PrefixUntilOutputFrom

.prefix(untilOutputFrom:) (Publishers.PrefixUntilOutput) is not precisely a joiner; but it does involve input from two publishers, like a joiner, so it is more like a joiner than its .prefix cousins, which are clearly partitioners. Therefore I’m mentioning it here.

What .prefix(untilOutputFrom:) does is to allow all values from upstream pass through, until some other publisher emits a value. That value is ignored, and the subscription to that other publisher is dropped — and any further values from upstream are now prevented from flowing down the pipeline.

Basically, this is a kind of switch (or gate), where a secondary publisher has the power to throw the switch (or close the gate) and prevent values from flowing.

The output type of the second publisher doesn’t matter; its value is a dummy signal and is disregarded.

The closing of the gate works by default in the simplest possible way: when a value is received from the second publisher, the upstream publisher is cancelled. However, if the second publisher sends a completion rather than a value, regardless of whether that completion is .finished or .failure, the upstream publisher continues to publish (but its output is blocked by the .prefix(untilOutputFrom:) operator).

The complementary operator is .drop(untilOutputFrom).