In testing - Scope cannot be cancelled because it does not have a job

[joancolmenerodev]

We are trying to check that the scope is canceled on a Test.
We have injected via constructor the scope and dispatcher, for instance MainScope() and Dispatchers.IO, this works good, now we want to ensure that when we call one method the scope is canceled.

Example code

class ExampleClass @Inject constructor(val scope: CoroutineScope, val dispatcher: CoroutineDispatcher, val service: MyService){
    fun example(){
         scope.launch{withContext(dispatcher){service.getExamples()}.fold{..},{..})
    }

    fun methodToTest(){
        scope.cancel()
    }
}

Note: this service is a fake API call, which requires network operations, we can test the example, but when trying to test the methodToTest which contains the scope.cancel() is not working for us.

Problem?

On test, we are using

val dispatcher = TestCoroutineDispatcher()
val scope = TestCoroutineScope(dispatcher)

It works, but when trying to use this

scope.coroutineContext.isCanceled

It throws

Scope cannot be canceled because it does not have a Job

Investigating a little bit, we noticed that TestCoroutineScope is creating a default context EmptyCoroutineContext and the problem is that This CoroutineContext doesn't have a Job() so it's not possible to check if it's canceled or not.

What we can do and actually fixes the problem is to initialize the scope as

val scope = TestCoroutineScope(Job() + dispatcher)

It solves the problem, but we think that it's not the proper way because is using the "real" implementation of a Job() and there are a lot of unnecessary things that we don't need.

Is there any way to test it without passing a real Job()?

[neworld]

Also, #2312 is not working there, because TestCoroutineScope() does not have a job. But it is expected in the description of the issue #2159

This !! cast always succeeds, but it draws the developer's attention and brings in uncertainty about the correctness of the code.

Also, I found CoroutineScope(EmptyCoroutineContext) has a job.