In <https://medium.com/androiddevelopers/a-safer-way-to-collect-flows-from-android-uis-23080b1f8bda>...

# android

[ceedee]

In https://medium.com/androiddevelopers/a-safer-way-to-collect-flows-from-android-uis-23080b1f8bda Google recommends using repeatOnLifecycle for Flow collection within onViewCreated of Fragments.
But there’s a catch to it: when I navigate to another Fragment, the previous Fragment goes to ON_DESTROY_VIEW but returns to ON_VIEW_CREATED state when I navigate back to it.
Since repeatOnLifecycle will only fully terminate itself when the Fragment reached ON_DESTROY state, the call to repeatOnLifecycle in onViewCreated will spawn a DUPLICATE Flow collection. When I repeat this navigation / back navigation, even more Flow collections will add up.

The solutions I considered are:
a) using asLiveData() in the ViewModel
b) moving repeatOnLifecycle calls to onCreate of the Fragment

But I wonder why Google recommends using repeatOnLifecycle in onViewCreated. Can anyone shed some light on this?

[zsmb]

If you call repeatOnLifecycle on viewLifecycleOwner as described in the article, then the collection should be tied to the lifecycle of the Fragment's view, which means that it will be terminated when onDestroyView happens.

[Robert Williams]

Something you may have missed from that article (was briefly mentioned but not really explictly) is that the Flow from the ViewModel should (always?) be a StateFlow (either directly from MutableStateFlow or via another flow using stateIn)

[Robert Williams]

In this way you are correct that the flow will get recollected but that just means the last value will get replayed and no new work will happen

[Albert Chang]

The flow doesn't necessarily have to be a state flow (hot flow). If you have a flow of location information or a flow that starts a web socket, you may want to use a cold flow to avoid unnecessary work when the screen is in the back stack.

[ceedee]

Thank you all for your feedback! I will answer separately:

@zsmb The problem is not that the collection doesn’t stop (which it actually does!) but that due to the Fragment lifecycle (after onDestroyView we can get back to onViewCreated) multiple repeatOnLifecycle calls are issued which add up one after the other.

@Robert Williams Even if the Flow from the ViewModel is a StateFlow , still multiple collections will be launched which lead to multiple UI updates which is a bad thing since my application will become slower and slower after each navigation back to that Fragment.

I still think, that onCreateView is the wrong place to call repeatOnLifecycle.

[Robert Williams]

This is already handled by repeatOnLifecycle. From the docs:

* Runs the given [block] in a new coroutine when this [Lifecycle] is at least at [state] and

* suspends the execution until this [Lifecycle] is [Lifecycle.State.DESTROYED].

[Robert Williams]

So as long as you're using the viewLifecycleOwner scope the old repeatOnLifecycle will just return and the coroutine will be cleaned up on destroy view and the newly started one will be the only collection

[ceedee]

Have a look at the Fragment lifecycle

https://miro.medium.com/v2/resize:fit:952/1*UWTA4pWxMjz32Kisy0sjhQ.png▾

: when you navigate to another Fragment, the current Fragment will enter onDestroyView but NOT onDestroy. From there, it can re-enter onViewCreated on back navigation. Thus, active repeatOnLifecycle calls will add up. So still, I believe the documentation is wrong in this point.

[Robert Williams]

That's why you use the viewLifecycleOwner scope: https://cs-ibrahimyilmaz.medium.com/viewlifecycleowner-vs-this-a8259800367b

[Robert Williams]

You are correct in that it would also work with the Fragment's lifecycle scope and registering in onCreate but this will keep listening when there's no active UI being displayed (and potentially try to update a UI which doesn't exist) which is why viewLifecycleOwner is the recommended way

[ceedee]

Alright, took some time, but now I got it 😉
Thanks for the patience, somehow I all the time missed the viewLifecycleOwner part because I had lifecycleOwner in front of my eyes…
All explanations totally make sense, thank you!