Would it be safe to expose a `Flow` from a custom ...

# android

[bohsen]

Would it be safe to expose a Flow from a custom view or could this cause leaks?

[gildor]

Depends on what this flow does and how it implemented

[bohsen]

It's a custom view with an EditText where a TextWatcher listens for changes and emit when changes are made to the text.

kotlin
@ExperimentalCoroutinesApi
    val textWatcher: Flow<CharSequence> = callbackFlow {
        custom_inputview_text_input_edit_text.addOnTextChangedListener {
            afterTextChanged { s -> Timber.d("AfterTextChanged: $s") }
            beforeTextChanged { text, start, count, after -> Timber.d("BeforeTextChanged: $text") }
            onTextChanged { text, start, before, count -> Timber.d("Within $text, the $count characters beginning at $start have just replaced old text that had length $before.") }
        }
    }

[bohsen]

The listener is a Presenter that is attached to a Fragment in onActivityCreated() and detached in onDestroy()

[gildor]

It will leak

[gildor]

also because flow is lazy you can attempt to consume this flow when view already detached

[gildor]

to avoid it you have to subscribe after onViewCreated and detach onDestroyView

[bohsen]

I thought so.

subscribe after onViewCreated and detach onDestroyView

Afraid this will lead to other issues.

[gildor]

it’s not unique for Flow

[gildor]

but otherwise your view will leak, that’s the problem

[gildor]

actually it’s pretty easy to implement, you should collect your flow only in coroutine launched in viewLifecycleObserver.coroutineScope, which will be cancelled automatically on view destroy

[rkeazor]

To be honest , it's never a good idea to have logic in your views

[bohsen]

So if my Presenter would implement (Default)LifecycleObserver and start observing viewLifecycle of the Fragment it would be possible to avoid leaking the view?

[bohsen]

It would cancel collect when the `Fragment`s view gets destroyed?

[gildor]

No, Presenter cannot implement it

[gildor]

I mean it can, but it's not what I propose, if presenter implements scope it still should know somehow about view lifecycle, but only fragment knows this

[bohsen]

@gildor When you say viewLifecycleObserver.coroutineScope

Do you mean the new lifecycleOwner.lifecycleScope-API that is released in lifecycle-runtime-ktx:2.2.0-alpha01?

[gildor]

Yes, this API, but I mean specifically fragment.viewLifecycleObserver property, which provided by Fragment and has a lifecycle of view, not a lifecycle of Fragment

[bohsen]

Thanks. I'll take a look at it.