Hi, I have a callback when you click cancel in an...

# android

[Fredrik Larsen]

Hi,

I have a callback when you click cancel in an AlertDialog.

I'm trying to figure out why the exception is not caught in this case. Can anyone shed some light on it? https://gist.github.com/frel/38aaa8a05790a401dd45317c527b85eb

I assume it's because of the stack trace, but I'm not sure what the alternative would be. This is the simplest version I could think of where I'm able to reproduce the issue. I'm actually trying to make an rx stream error, but instead the app crashes.

[Casey Brooks]

It’s all due to the scoping. At the point that the onCancel callback is fired, it’s being dispatched from the Android Looper, not from your MainActivity. The exception bubbles up to the Looper which sent the click event and triggered your callback, not the onCreate() which declared the callback.

Even though the exception looks like it’s in the scope of that try-catch, it’s not because it’s inside a callback that’s passed to some other scope and invoked asynchronously. Generally-speaking, all async callbacks will have strange effects like this, and that’s where coroutines and RxJava starts to play a role, by making this kind of async data flow more natural to how we think of variable scoping.

[Fredrik Larsen]

Thank you, that makes a lot of sense. The next question is how could you do something like this then?

I have a stream, that starts a download and opens a dialog (that observes on the progress etc).

My thought was to throw CancellationException to cancel it, but do you have any other suggestions?

[Casey Brooks]

You can throw a CancellationException to cancel, but you’d need to handle it from the onError callback when you subscribe to the stream, wherever that is

[Fredrik Larsen]

But that is my problem. It doesn't trigger onError, it crashes the app.

[Fredrik Larsen]

So it's the same issue as this, where the try catch doesn't work

[Fredrik Larsen]

I guess this becomes a bit off topic, so I'll see if I can implement this differently. Thanks for the input

[Casey Brooks]

Then the callback isn’t being fired as part of the stream. You would have to set up the dialog as a part of the stream (inside Observable.create { ... }), catch the exception from the callback, and pass that into the stream’s Emitter

Observable.create { emitter ->
        AlertDialog.Builder(requireActivity())
            .setTitle("Downloading...")
            .setView(view)
            .setNegativeButton("Cancel") {
                try {
                    onCancel()
                }
                catch(e: CancellationException) {
                    emitter.onError(e)
                }
            }
        }
        .create()
    }

[dead.fish]

You should never, ever rely on (Dialog)Fragments keeping your callback references around btw, one stop-start cycle is enough to get everything completely bonkers. Better let the calling component (Activity or Fragment) implement an interface that the DialogFragment attaches to in onActivityCreated. This also means that you have the stream on the outside of the dialog and under your control. If you want to keep the stream inside of the AlertDialog, you must put it into a ViewModel (which naturally survives restarts) or any other retaining (headless) fragment.

[Fredrik Larsen]

Thanks for the feedback. I'm aware of the lifecycle, and in this case it's actually not supported (instance state is set to null so the whole activity would be created from a clean state) but your solution would potentially fix my issue! Thank you. I guess I would need to use a Subject/Processor to push an error into the stream then.