If I have a Cancellable implementation can I somehow listen

Question

If I have a Cancellable implementation can I somehow listen for the job cancellation and invoke cancel on Cancellable

Marc Knaup · Answer

Maybe ```coroutineContext job invokeOnCompletion onCancelling = true ``` thinking face

Adam Powell · Answer

iirc that api is considered private internal and it s probably not what you want anyway Can you explain a bit more about what you re trying to do

Adam Powell · Answer

generally things with this pattern are cases where you want to suspend and await completion but you want to propagate cancellation So it ends up being something like ```suspendCancellableCoroutine continuation gt val cancellable = startAsyncWork Callback result gt continuation resume result invokeOnCancellation cancellable cancel ```

Marc Knaup · Answer

It s experimental not internal But yeah more context would be helpful to find a better approach

Adam Powell · Answer

``` InternalCoroutinesApi public fun invokeOnCompletion onCancelling Boolean = false invokeImmediately Boolean = true handler CompletionHandler DisposableHandle```

Adam Powell · Answer

so optin internal rather than keyword `internal` but not really experimental in the sense of this is on track to become public API in the future either

Adam Powell · Answer

the variant that runs when the job is complete children have joined rather than just point where cancellation is initiated is public though

bezrukov · Answer

Another option to achieve this via public api is to launch undispatched coroutine + infinity susped call + try catch

Marc Knaup · Answer

< Adam Powell> you re right I ve mixed it up with something else It s internal

Adam Powell · Answer

< bezrukov> yes but if you re going to do that you might as well do it via `suspendCancellableCoroutine` and `CancellableContinuation invokeOnCancellation` Using try catch means your continuation needs to resume on its dispatcher before the cleanup code runs Generally there s no reason to wait for that unless cancellation requires you to be on a particular dispatcher to perform it

Adam Powell · Answer

e g main UI thread requirements or similar

bezrukov · Answer

`suspendCancellableCoroutine` works for cold Closeable disposable future whatever If you need to close hot closeable once job cancelled suspendCancellableCoroutine won t work because it checks cancellation before

louiscad · Answer

try catch finally + awaitCancellation is also an alternative

Eugen Martynov · Answer

Sorry for the delay and thank you for the answers

Eugen Martynov · Answer

I have a runtime class that accept messages runs update of the state from that message and executes effect that will produce another message

Eugen Martynov · Answer

It is run on three different contexts runtime render and effect

Eugen Martynov · Answer

The runtime by itself is coroutine scope

Eugen Martynov · Answer

and all launch is happening as a child of it

Eugen Martynov · Answer

but it is not important

Eugen Martynov · Answer

The runtime accepts listeners and I want to remove listener as soon as context is cancelled

Eugen Martynov · Answer

But looks from you answers it is not possible

bezrukov · Answer

no it s possible Here is ext ```fun CoroutineScope invokeOnCancel block gt Unit launch start = CoroutineStart UNDISPATCHED try suspendCancellableCoroutine lt Unit gt finally block ``` then do ```scope invokeOnCancel cleanup code here ```

Eugen Martynov · Answer

Interesting let me digest this code first Can you recommend something good about coroutines mechanics where is callback put on which thread it operated I also never saw ```suspendCancellableCoroutine```

Eugen Martynov · Answer

So if I understand this code correctly we launch some empty coroutine that will be cancelled only when scope is cancelled

Eugen Martynov · Answer

and then we can hock to it and run some code that we want

bezrukov · Answer

yes `suspendCancellableCoroutine lt Unit gt ` can be changed to `delay Long MAX VALUE ` if it more descriptive

bezrukov · Answer

gt where is callback put on which thread it operated If the scope was already cancelled callback will be invoked immediately on the caller thread Otherwise it will be invoked on scope s dispatcher If you want it to be invoked on scope s dispatcher even if it s already cancelled use `CoroutineStart ATOMIC`