How to reliably await a change to an Observable type in tests

[vanvoorden (Rick van Voorden)]

@Andropov

Sometimes you do want to test whether a fire-and-forget function will eventually trigger a new value in an async sequence.

One option at that point is to build your own custom AsyncSequence type on a continuation to "deadlock" your test code. Your production code can continue to run from the regular AsyncStream or AsyncChannel but the test code injects in this custom dependency.

Writing testable UI related code with modern Swift

We ran into something similar when testing for the ImmutableData [full discolsure: self promotion] project. What we were looking for was something like an AsyncSequence with backpressure that also blocked on yield. The AsyncAlgorithms.AsyncChannel came close… but yield was async backpressure was applied after the for await was already started. We ended up building our own AsyncSequence just for testing and then injecting that AsyncSequenceTestDouble at compile time with dependency injection in…