What will happen inside `suspendCancellableCoroutine` if a callback causes calling `continuation.res...

# coroutines

[Lukasz Kalnik]

What will happen inside suspendCancellableCoroutine if a callback causes calling continuation.resume() twice?
E.g.

suspendCancellableCoroutine { continuation ->
            bleManager.startScan(
                onDeviceAppeared = { device ->
                        continuation.resume(Unit.right())
                    }
                },
                onScanError = {
                    continuation.resume(it.left())
                },
             )
        }

If I get a second onDeviceAppeared callback will the coroutine try to resume again?

[streetsofboston]

It'll throw an exception: Continuations can only be resumed once.

[Lukasz Kalnik]

Ok, so the correct solution is to remove the callback before resuming?

[streetsofboston]

Yup, or guard it.

[streetsofboston]

And you probably want to stop the scan when the coroutine has been cancelled as well....

[Lukasz Kalnik]

Oh yes, good point. Thank you!

[Lukasz Kalnik]

Guard would be a check if (!resumed) or something like this?

[streetsofboston]

However, if onDeviceAppeared can occur more than once, you may want to consider using a Flow (callbackFlow { ... }) instead.

[Lukasz Kalnik]

I just want to finish the scan after first device

[Lukasz Kalnik]

I actually simplified the code, I have an if (device.name == myName) check there

[streetsofboston]

Still, maybe a Flow could be good to use as the one that produces the scanned devices and then call devicesFlow.first() on it to get the first one.

[Lukasz Kalnik]

Yes, that's also a good idea

[Lukasz Kalnik]

Would move the whole completion/cancellation logic more into the consumer

[streetsofboston]

Then you can filter the flow, combine, etc

[streetsofboston]

Not really; the caller/consumer of the suspend function or of the Flow would govern the cancellation. Both scenarios are 'cold': Only active while someone is listening/consuming.

[Lukasz Kalnik]

Sorry, I haven't made myself clear.
I meant rather the stopping of the BLE scan

[streetsofboston]

Ah... yup.

[Lukasz Kalnik]

Also Flow would guarantee the resume cannot be called twice

[streetsofboston]

You mean 'suspend' would guarantee that?
A Flow's emit can be called many times.

[Lukasz Kalnik]

1. In case of suspendCancellableCoroutine I filter for the specific device inside of the callback. That means, if onDeviceAppeared will be called twice for the same device, I would try to resume twice.
2. In case of a callbackFlow I would just emit the same device twice -> no problem, as the collection on consumer side probably already finished anyway (because we stopped collecting when we got the desired device).

[Lukasz Kalnik]

So a callbackFlow is even easier to implement here.

[streetsofboston]

Yup; and you can filter (eg filter on name) inside the callbackFlow, like you would in a suspendCancellableCoroutine, but also later in a .filter operator.

[Lukasz Kalnik]

Yes, that's also much nicer.

[Lukasz Kalnik]

Thanks for your helpful suggestions!

[streetsofboston]

You're welcome 🙂