Hey guys, I’m trying to understand `Job`’s `cancel...

# coroutines

[Robert Menke]

Hey guys, I’m trying to understand `Job`’s cancel method. I’m working on creating a class that implements CoroutineScope and I’m following the example here https://github.com/Kotlin/kotlinx.coroutines/blob/master/core/kotlinx-coroutines-core/test/guide/example-context-10.kt.

I would expect that job.cancel() would return true in this case since the invocation of cancel is what cancels the job, however, when I print the result of job.cancel() I get false.

fun destroy() {
     val temp = job.cancel()
     print(temp.toString()) //prints "false" even though I would expect it to print "true"
}

Am I misunderstanding cancel?

[elizarov]

Documentation on cancel result is wrong. In fact cancel on Job() always returns false at them moment. It might change.

[Robert Menke]

@elizarov Thanks for the clarification! Any thoughts on writing tests for coroutine cancelation scenarios?

[elizarov]

We have tons of test, but they are all useless, since we don’t have any real use-cases for that boolean. It is just a left-over from earlier work. The proposal is to remove it: https://github.com/Kotlin/kotlinx.coroutines/issues/713

[Derek Seroky]

Thanks @elizarov for the information! The only (current) use we had for the boolean was in unit testing the coroutine to ensure that it was cancelled properly, but we'll figure out another way.