I always thought that the following code would pas...

# coroutines

[Jelle Fresen [G]]

I always thought that the following code would pass (🧵), but I just discovered it does not. Can some explain to me why?

[Jelle Fresen [G]]

val ctx1: CoroutineContext = StandardTestDispatcher()
val ctx2: CoroutineContext = MyContinuationInterceptor()
val ctx3 = ctx1 + ctx2
assert(ctx3[ContinuationInterceptor] == ctx2)

[Jelle Fresen [G]]

In other words, I thought that adding a ContinuationInterceptor to a context that has a TestDispatcher would replace that dispatcher, because a dispatcher is a ContinuationInterceptor

[ross_a]

It's hard to tell without knowing your MyContinuationInterceptor implementation

[ross_a]

e.g. this passes for me:

@Test
    fun thing() {
        val ctx1: CoroutineContext = StandardTestDispatcher()
        val ctx2: CoroutineContext = object : ContinuationInterceptor {
            override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> {
                TODO("Not yet implemented")
            }

            override val key: CoroutineContext.Key<*>
                get() = ContinuationInterceptor
        }
        val ctx3 = ctx1 + ctx2
        assert(ctx3[ContinuationInterceptor] == ctx2)
    }

[ross_a]

I assume your problem is because they keys are different, i.e. ContinuationInterceptor.Key is different to CoroutineDispatcher.Key

[Jelle Fresen [G]]

I'm in the process of setting up a Kotlin Playground snippet

[Jelle Fresen [G]]

https://pl.kotl.in/lM7O3B8Nq

[Jelle Fresen [G]]

It is probably what you say. I can't remember now why 2 years ago I decided I needed an AbstractCoroutineContextElement, but that might be the problem?

[ross_a]

So to be retrieved when calling ctx3[ContinuationInterceptor] your implementation should return something like

override val key: CoroutineContext.Key<*>
get() = ContinuationInterceptor.Key`

or, since it's a delegate anyway I think you could do

class DelegatingContinuationInterceptor(private val delegate: ContinuationInterceptor) :
    AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor by delegate

and don't override the key property

[ross_a]

hmm though I didn't look at the impl of AbstractCoroutineContextKey - it does seem like baseKey should match as well

[ross_a]

It looks like you just end up with two Keys - the Dispatcher and your Interceptor - which both are ContinuationInterceptors and ends up picking the Dispatcher first. It doesn't work so well as a Key in this regard.

e.g. this works:

fun main() {
    val ctx1: CoroutineContext = kotlinx.coroutines.Dispatchers.Default
    val ctx2 = DelegatingContinuationInterceptor(ctx1[ContinuationInterceptor]!!)
	val ctx3 = ctx1 + ctx2    
	assert(ctx3.minusKey(kotlinx.coroutines.CoroutineDispatcher)[ContinuationInterceptor] == ctx2)
}

[ross_a]

so when attaching your Interceptor you could do something like:

val ctx2 = DelegatingContinuationInterceptor(ctx1[ContinuationInterceptor]!!)
	val ctx3 = ctx1.minusKey(ContinuationInterceptor) + ctx2

which I'm sure you could wrap in a nice extension method