From the general testing approach, testing callbacks seems to be incredibly bad idea.
When writing unit test, one should understand what is unit. From the point of OTP view, single GenServer is single unbreakable unit - a kind of black-box, which is receiving and sending messages, is holding state and is doing the job.
By the way, it’s the same description as for a class instance in OOP language.
Have you every heard, that instance methods are tested without instance? No, of course. So, why do you think you can break entire GenServer implementation? How can you call this kind of testings - sub-unit, half-unit, under-unit?
Now: when you see that you can’t test GenServer as a black-box -what does that mean?
It means that your implementation is very tight coupled.
While in OOP world, you think a lot about dependency injection, composition, and all this kind of stuff - why you forget about them here? Because it’s functional language?
Now, as you see, your question is not about how to test GebServer but about how to create TESTABLE GenServer. The answer is simple, because OOP guys already know the answer!
- GenServer callbacks IS wrapper code, adapter for OTP stuff. This adapter should adapt another module, who is implementing real business logic and does know nothing about OTP. This module should know nothing about he’s called via messages, callback or anything like this. Think about it as about pure functional module. For sure, you know how to test it?)
- But we still need to test our GenServer communications. So, what to do? We should replace our tight copling with loose coupling. By dependency injection of course! Do you remember pure functional module on the previous step? It has interface that in Elixir world is called behaviour. Make your GenServer call not a specific module, but any module that implements this behaviour.
- Mock all heavy calculations using new defined behaviour. You already know, that they are working - they are tested inside your pure functional module. You just need to test cross-process communication now. Use Mox library from Plataformtec for these needs.
- ???
- Profit! You have fully tested loose coupled system!
Enjoy)
