Imagine I have a `suspend` function that feeds a `StateFlow` with one or two different values, in qu...

# coroutines

[dead.fish]

Imagine I have a suspend function that feeds a StateFlow with one or two different values, in quick succession. How would I test this with Coroutines 1.6? Issue is that during the test I only see the second emitted value and the first is already gone. I tried different things using different dispatches, but nothing worked out unfortunately.

[Dmitry Khalanskiy [JB]]

Hi! You could try collecting the StateFlow in an UnconfinedTestDispatcher. This will work unless the emissions themselves happen in an unconfined dispatcher.

However, could you share why you set SharedFlow to two values in quick succession? In production, the first value will typically be lost as well, because SharedFlow conflates values. If you rely on both elements being received in production, this is very tricky to actually ensure.

[dead.fish]

It’s basically a network follows cache-response scenario. The disk is quickly read and emitted, the network call could take up to 20 seconds. In my test of course I don’t have such delays, but still want to see that both, the cached and the real value are emitted in proper order.

[dead.fish]

Hrm… this blocks for 60s and then bails out, dammit, this API: https://gist.github.com/realdadfish/25cb4d950cec6bfd6f6626cb32af1911

[dead.fish]

(This uses FlowTurbine for collecting / asserting)

[Dmitry Khalanskiy [JB]]

In my test of course I don’t have such delays

The recommended solution is to mock the network call with delay(20.seconds). In the test framework, delays are performed using a virtual source of time. So, in fact, the delay will finish instantly, but the test will "think" that 20 seconds passed.

[dead.fish]

Ah ok, sounds reasonable, will try that!

[dead.fish]

(I used a CompletableDeferred solution for this, which did not work)

[dead.fish]

Many thanks, this worked like a charm! I updated the gist accordingly and made it a bit simpler even: https://gist.github.com/realdadfish/25cb4d950cec6bfd6f6626cb32af1911

[Casey Brooks]

Turbine makes testing Flows much nicer https://github.com/cashapp/turbine