Hello ! Is it possible to unit test for job cancel...

# coroutines

[Nicolas Verinaud]

Hello ! Is it possible to unit test for job cancellation for a class like below ?

class Sample(private val scope: CoroutineScope) {
    
    private var currentJob: Job? = null
    
    fun doSomeAsyncWork() {
        currentJob?.cancel() // how to test this line ?
        currentJob = scope.launch { 
            delay(5000)
        }
    }
}

[mkrussel]

In the example shown the job does nothing, so you cannot test the method at all.

How to test it would depend on what it is actually doing.

If it calls a method in a injectable dependency, you could replace the dependency with some sort of fake that waits for cancelation on the first call and works on the second call.

Use a unconfined type of scope so the launch happens immediately.

Then call doSomeAsyncWork twice and see that the first job was canceled and the second job was started.

[Robert Williams]

Just to add to @mkrussel’s answer, one of the key principles of testing is to test behaviours and not implementation details.

[Robert Williams]

Here the fact that doSomeAsyncWork is creating the job at all is an implementation detail

[Robert Williams]

You need to work out how that should affect its result or collaborators and test that which will indirectly test the cancellation behaviour

[Nicolas Verinaud]

Thanks for your help. I thought I could use the coroutine API directly without the need of an abstraction over it.

[Nicolas Verinaud]

I successfully tested everything but the cancellation.