Hi All! 👋 I'm wondering if this is OK

you don't have a

Job

in the coroutineContext so trying to cancel the context won't do very much for you

FYI, it's not recommended to implement

CoroutineScope

on your own because it can lead to easily launch in the wrong scope later on. Best to have it be a property for the cases where you need it.

yeah that too, in particular a private property so that other code doesn't get into the habit of launching arbitrary jobs into your scopes.

Should've been

protected

, right?

perhaps, but arguably I think it shouldn't have existed at all. creating a scope manually and cancelling it in

onCleared

is a pretty small inconvenience in those cases where you have an AAC ViewModel that really needs to own a CoroutineScope; many don't. The existence of

viewModelScope

terminates further thought about whether an operation should be scoped to the UI or to a repository.

If you don't have any

ViewModel

subclasses in your app, you don't have this problem though :think-smart:

which sort of circles back to the point from above: don't create objects capable of acting autonomously within a lifecycle of their own when it's not necessary, View, ViewModel, or otherwise.

As a side note; I strongly discourage having ViewModels in your (custom) Views. Let Views be Views and just let them have a plain API that deals with their state modifications.

agreed. Kotlin extension functions are a great tool for providing a bridge/convenience for binding the contents of a viewmodel-like object to a view without the actual view subclass having to know anything about the viewmodel class's existence.

If you want testable behavior associated with a (custom) View, create a Controller, a Delegate, whatever you want to call it that is coupled to your custom View, much like a super simple, light-weight Fragment that wraps around your (custom) View. That Controller/Delegate/Wrapper could have a ViewModel, if need be. Still, consider that a ViewModel may not be necessary at all in these cases.

Thanks for the replies! but I'm still not sure how to proceed

The question of how to proceed is very open-ended and subject to a lot of opinion 🙂

Maybe you want a "view controller" that has a reference to the views, the repository(ies) or whatever, plus gets the right coroutine scope, and runs the right code.

I see... So this cast can potentially fail

More or less. If you don't want "Fragments" that's fine, but fragments do fill a particular niche in the stack that View subclasses are not well-suited to do on their own.

I can't use fragment in this scenario since this view needs to be placed inside an already existing RecyclerView

Modify your Fragment/Activity that hosts the recycler-view and let its ViewModel emit the proper data to fill the recycler-view's adapter and ViewHolders

This might be what you're looking for: https://github.com/yigit/suspend-bind

Modify your Fragment/Activity that hosts the recycler-view

Right now this is a custom view that I can put anywhere, I'd like to keep it that way 🙂

Still, your Fragment/Activity that hosts your recycler-view needs to send the recycler-view the data (to its adapter) that it needs to show. This data is emitted by the Fragment/Activity's ViewModel. If your custom View in your recycler-view needs to be showing new data, update the recycler-view's adapter's data with that new data. (Tightly) coupling ViewModels with (custom) Views can get troublesome.

This view works on its own, does not need to be bound or provided with data. It gets injected with a Service that has Application Scope and reacts to this service stream of events (a StateFlow)

binding is what assigns the position. If you're doing nothing in an onBindViewHolder you're going to have issues when RV rebinds the same view instance to a different position.

I thought binding is what changes the contents of the view

binding is what assigns the position

😱 what do you mean by this?

If your custom View gets its own data and it gets bound to a different adapter-position, your custom View may then show the incorrect data if it doesn't get updated in the onBindViewHolder callback This happens after an recycler item-view gets recycled and re-bound to a new adapter position.

I think I'm good because

onViewAttached

subscribes to a stateflow that updates the data

Not if it's subscribed to the wrong data because assigning the data source was skipped during binding

Using the right coroutine scope, combined with the suspend-bind library by Yigit (who worked on RecyclerView itself) is most likely the safest approach, despite the library using internal APIs in RecyclerView. I say it's safe because these APIs are quite unlikely to ever change, for backwards compatibility reasons and the fact that Compose APIs are the ones being focused on when it comes to scrolling large data sets and UI in general.

Not if it's subscribed to the wrong data because assigning the data source was skipped during binding

this is my view

class CustomView @JvmOverloads constructor(...) : FrameLayout {

@Inject 
lateinit var service : MyService;

overrideOnAttach() {
 job = (context as LifecycleOwner).lifecycleScope.launch {
   service.stateFlow.collect { ... }
}
}
}

You mean it's the same data for all similar items in the RecyclerView?

Yes.. More specifically There's only one instance of this view type in the recyclerview

Then, it's safe if you're 100% certain there will never be another instance that expects different data in the same RecyclerView. You can skip its code in the onBind altogether.

🙂 I thought so, I was afraid I had been using adapters wrong for a while LOL

tbh what you started the thread with was better than that

interesting... 🤔 why?

/me looks back at the whole thread

It'd still be better to use the suspend-bind thing with a coroutine scope injected into the adapter @frankelot

but I'm not doing anything in bind 😄 no need to suspend @louiscad

To get the lifecycle-owner from the view's context:

// top level val
private val Context?.lifecycleOwner: LifecycleOwner? get() = when (this) {
  is LifecycleOwner -> this
  is ContextWrapper -> this.baseContext.lifecycleOwner
  else -> null
}

@frankelot What you'd do in that suspending bind is running the code you currently run in the

launch { … }

, i.e. collecting the

StateFlow

.

@streetsofboston there's a great big, "it depends" around that. That won't catch the

LifecycleOwner

of any containing fragments, for example.

ViewTreeLifecycleOwner.get()

is probably more reliable but even that has its edge cases

Yup; this'll only work if the recycler-view is hosted by the Activity directly.... But it's safer than just casting

context as LifecycleOwner

🙂 I never heard of ViewTreeLifecycleOwner... interesting

which is why this whole approach is unnecessarily brittle. It's not simpler to do things this way and there are edge cases that you have to have a lot of domain knowledge to be aware of.

@louiscad that would work! but I see two drawbacks. 1 - it requires adding a library just for this one use case, 2 - The view is not reusable anymore. It requires its parent to know where to get data to feed it with

The view is still reusable, you just also need to attach the right "data provider" (the stateFlow). FYI, I personally copy pasted the code of the library into my project to integrate it with some extension I made, but even adding the dependency should not be a big deal, it's a quite small library, made by someone not that random (the early leader of Android architecture components, which includes androidx.lifecycle)

you just also need to attach the right "data provider"

Ideally I'd like this not be the responsibility of the adapter. The adapter just knows about what views need to be laid out and in what order.

The adapter has two main responsibilities constrained by its API: giving new views of a given type, and enabling/disabling views for a given position (the binding, which is cancellable with suspend-bind)

the best solution for this case, imho is: find another way. (for example, as some1 suggested, dispatch state changes to the view. Do not give it the responsibility to observe and modify state)