How to test async code?

[tmillr]

...for example, testing that an autocmd triggered as expected?

Or, testing that a timer fires sometime in the future as expected?

and how to keep the plenary test harness from returning/exiting while inside a test fn that I need to wait for a timer result in?

Or as another example, let's say that I have to test a function which creates/loads some ui (e.g. creates a new internal/nvim window) and I need to wait until the next turn of the event loop until the window has completely finished being created and displayed, without the test function (i.e. the fn passed to it()) returning in the meantime?

It's not clear how to do these kinds of tests from the docs/README alone, so help or guidance on this would be immensely appreciated as well as would potentially help any others who are similarly stuck on this. Thanks!

P.S. I think it's kinda unfortunate that lua doesn't already come with an easy-to-use async framework built-in, unlike say, javascript, which has had easy-to-use promises and foolproof async functionality built into the language for some time now. I kinda wish Neovim had just went with js and embedded v8 instead of using Lua lol. I feel like alot more people already know how to use js and it has alot more really great features (e.g. Promises/async, TypeScript and a plethora of tools and packages, highly optimized/advanced engines) compared to Lua. Anyway sorry for the rant lol.

[idanarye]

I came here to ask about async tests, only to see it has already been asked. Lua actually does have something for async - coroutines - and Neovim has a relatively easy way to utilize them - just make the callback resume the coroutine, and then yield. It has some rough edges (e.g. - error handling needs special treatment), but I think plenary.async does enough wrapping to make this the "easy-to-use async framework" you are looking for.

tests/plenary/async has some tests for plenary's async modules, but from a quick glance I think they are only doing fake awaits (e.g. - wait for a channel but send value to it in the same iteration of the Neovim main loop) because plenary.test_harness does not support it.

I'm going to try making a PR that makes plenary tests work with async coroutines.

[Conni2461]

closed in 1252cb3

[tmillr]

closed in 1252cb3

@Conni2461 So I can simply yield now (using Lua's native coroutine.yield()) inside of the test fn passed to it()? This sounds really nice! Is there anything else that needs to be done on my end to enable this functionality?

[idanarye]

@tmillr See the discussion in the PR (#426) - there is a discussion about making this opt-in, and a PR for it (#447)

Also, it should go without saying but worth mentioning anyway - just doing coroutine.yield() is going to get your test stuck (until a timeout? Is there a timeout?). You need to ensure something resumes the coroutine before yielding.

[tmillr]

@idanarye oh I see, the feature flag is an additional feature under discussion but is not currently implemented

And I understand that, thanks. This is exactly what I was looking for (mainly for tests involving autocommands or timers) because I haven't fully wrapped my head around plenary's async utils (while I have a fairly solid understanding of generators and Lua's native coroutines). I think there was technically support for async testing before, but it wasn't documented (you had to go digging through the code to try to figure it out) and you had to use plenary's async impl/module for it to work (i.e. it didn't seem to support vanilla Lua coroutines) along with alternative describe/it functions (that I think were also undocumented, technically speaking). So I think this is a nice addition.