Hi, i have confusion when using `CoroutineScope.ca...

# coroutines

[Jemshit Iskenderov]

Hi, i have confusion when using CoroutineScope.cancel() and CoroutineScope.job.cancel().
Here is the CoroutineScope:

val scope = object : CoroutineScope {
        val parentJob = Job()
        override val coroutineContext: CoroutineContext
            get() = parentJob + <http://Dispatchers.IO|Dispatchers.IO> 
}

Observation 1: when Job of CoroutineScope is cancelled, both seemed not active:

scope.launch {
        withContext(Dispatchers.Default) {
            println("result")
        }
        scope.parentJob.cancel()
        println("isActive:$isActive, job.isActive:${scope.parentJob.isActive}")

        // PRINTS isActive:false, job.isActive:false
}

Observation 2: when CoroutineScope is cancelled, only CoroutineScope.isActive seems false. Job.isActive is true

scope.launch {
        withContext(Dispatchers.Default) {
            println("result")
        }
        cancel()
        println("isActive:$isActive, job.isActive:${scope.parentJob.isActive}")

        // PRINTS isActive:false, job.isActive:true
}

Observation 3 (CoroutineScope for Android Fragment):

protected lateinit var job: Job
override val coroutineContext: CoroutineContext
        get() = Dispatchers.Main + job

launch {
    val result = withContext(Dispatchers.Default) {
          // ...
    }
    [e-mail]()
    // Log.d("$isActive ${[e-mail]}")
    // PRINTS true, false
}

Question: when looking at source code, CoroutineScope.cancel and CoroutineScope.isActive calls methods of its Job. But in above code, CoroutineScope.cancel does not return Job.isActive or vice versa. Why this behavior? Is withContext changing something?

[Dico]

withContext creates a child coroutine, its context contains that child as a Job. The cancellation of said Job is not exceptional, and thus the event is not propagated to the parent job from the coroutine scope that you declared.

[Dico]

I dont think this explains what you are seeing though. I think that's down to Job being threadsafe. Theres no guarantee that its cancellation completes before cancel() returns. It will go through at least 2 state changes before it is finally cancelled/dead.

[Jemshit Iskenderov]

coroutineContext[Job]==scope.parentJob returns false for some reason

[Dico]

And I explained why that is the case in my first comment.

[Jemshit Iskenderov]

You are saying cancel is called for/from coroutineContext of withContext() coroutine, thats why cancellation does not go to parent job, right? But i call cancel() inside scope.launch{}, also i directly cancel parentJob of scope.

[Jemshit Iskenderov]

This returns `true`:
println("${coroutineContext[Job] === scope.job.children.iterator().next()}").

So this means, Job instance inside CoroutineScope is only parent of CoroutineScope.coroutineContext

[Dico]

Yes.

[Jemshit Iskenderov]

CoroutineScope.job.cancel(): cancels parent Job of that scope. Control is done via if(CoroutineScope.job.isActive)


CoroutineScope.cancel(): does not cancel parent Job of that scope. (why does this exists?). Control is done via if(CoroutineScope.isActive)

[Jemshit Iskenderov]

I understand the Observation 1 & 2. I don’t get why cancelling the most parentJob does not cancel job of scope.launch{} in Observation 3.

[Dico]

The job in scope.launch cannot be inactive until the end of the coroutine's code block. It might be in cancelling state internally.

[Jemshit Iskenderov]

Oh, that is confusing. Thanks