I.e. that a coroutine launched in onStart will get...

# coroutines

[Paul Woitaschek]

I.e. that a coroutine launched in onStart will get cancelled in onStop, onCreateView -> onDestroyView, onCreate -> onDestroy; etc?

[ribesg]

Usually you basically make your Activity/Fragment implement CoroutineScope by MainScope() and call cancel() in onDestroy()

[Paul Woitaschek]

And how would that cancel onStart jobs in onDestroy?

[ribesg]

If you started your job with just launch { } or something like that in onStart, then it’ll die with the rest if I understood it correctly

[Paul Woitaschek]

My question was how I can make stuff from onStart not die with the rest, but get cancelled in onStop?

[ribesg]

Oh, you want something specific to each lifecycle ‘level’? Then you’ll have to do it yourself

[Paul Woitaschek]

And how could I do that by myself?

[ribesg]

You could define multiple scopes with nested jobs I suppose, but I’m not fluent enough in coroutines

[ribesg]

`Job`s can have parent `Job`s, I think you’ll need to play with that and make a tree of jobs

[ribesg]

I usually just don’t care about things surviving between onStop and onDestroy... You just have to be sure that there’s nothing left after onDestroy. Are you sure the gains are worth the complexity of what you want to do?

[Paul Woitaschek]

Sure I am

[Paul Woitaschek]

I.e. I clear my views in onDestroyView, so I don't want a coroutine I started in onCreateView access null views after onDestroyView

[ribesg]

I usually make things nullable and use calls like view?.something so that it’s automagic

[Paul Woitaschek]

Nah, that hides bugs

[ribesg]

I agree that it’s not perfect but I don’t have the time for the complex task you’re talking about. But I guess you could implement all that in subclasses of Activity and Fragment and then use that

[Paul Woitaschek]

I want my code to be correct and not be constantly checking for conditions I didn't write the code to run in

[ribesg]

I’m still getting lost between scopes, jobs, context, dispatchers from time to time so I don’t feel I’m ready to implement such thing properly. It would take some time and not be guaranteed to work any better. Once I understand everything more I guess I’ll do something like that. Or once everything explodes 😛

[ribesg]

Thinking about it, it seems very complex. For example my Presenter just have a reference to the Fragment/Activity, which is a CoroutineScope, and that’s how my Presenter launches coroutines. But in your case you’ll have to pass 3 or 4 scopes to the Presenter, or maybe make them accessible in the Fragment/Activity, and chose carefully which one to use every time...

[Paul Woitaschek]

Job is just an interface. I can just create a base class and keep a map of <LifecycleEvent, Job> and just return the correct job depending on the lifecycle.

[Paul Woitaschek]

Wouldn't that work?

[ribesg]

Hmmm maybe yeah

[ribesg]

Make sure you look at how Job cancellation works, I think you need `SupervisorJob`s

[ribesg]

Children of a supervisor job can fail independently of each other.

[ribesg]

If you use basic `Job`s any error would cancel everything

[streetsofboston]

I.e. I clear my views in onDestroyView, so I don't want a coroutine I started in onCreateView access null views after onDestroyView

@Paul Woitaschek A Fragment is a Lifecycle owner and its lifecycle is per default tied to the Fragment itself. This my cause problems as you described above.

Instead of this:

viewModel.liveData.observe(this, Observer { ... } )
}

Do this

viewModel.liveData.observe(viewLifecycleOwner, Observer { ... } )
}

where this is replaced by this.viewLifecycleOwner

This makes sure the LiveData observer will not get callbacks on dead Fragments and only when Views have been created properly, ie between onViewCreated and onViewDestroyed.

[Paul Woitaschek]

I don't use LiveData

[streetsofboston]

You then could tie the viewLifecycleOwner to your own life-cycle observers, which could guard against encountering null values for your views in your Fragments.

[Paul Woitaschek]

I'm not talking about any observers, I'm talking about CoroutineScope

[streetsofboston]

You could tie your CoroutineScope to this lifecycle-owner, making your CoroutineScope a life-cycle observer.

[Dico]

I was looking for some kind of "activity observer" the other day to help solve problems like this

[Dico]

It seems like you can't listen for those events given an instance of Activity sadly

[Dico]

someone please correct me because I hope I'm wrong

[streetsofboston]

@Dico Could this help you:
https://developer.android.com/reference/android/app/Application.ActivityLifecycleCallbacks

[Dico]

oh Awesome!

[Jemshit Iskenderov]

You could create multiple coroutineScope properties (not implement them) that gets cancelled on different lifecycle event, then every .launch will use correct version of coroutineScope. But don’t rely on job.cancel(), it will cancel parent job and children, but your “computation” code will still continue to run (null view access might be done after cancelled). So parentJob.isActive check should be done. @Paul Woitaschek

[streetsofboston]

If you use LiveData in your ViewModel, your Activity's or Fragment's observers should never get notified when View references are null.

It's true, Jobs are cancelled only at suspension points, not just anywhere in the code. Either rely on those points or query the isActive property and bail out.