Hello! In android I am using `StateKeeperProvider`...

# mvikotlin

[Skeptick]

Hello! In android I am using StateKeeperProvider to keep Store during configuration change. I do it as in the example:
stateKeeperProvider.retainStore (lifecycle) {...

But when I close the fragment and reopen it, I get the error “_The supplier is already register with this key: class com.example.MyStore_”. Shouldn’t the supplier unregister in such cases? And what should I do to avoid getting this error?

[Arkadii Ivanov]

Hello! Could you please elaborate how do you close/reopen the fragment?

[Arkadii Ivanov]

And in what place you are calling retainStore(lifecycle)?

[Skeptick]

Just popBackStack. I tried this with navigation-library as well as simple fragmentManager. retainStore I call in controller, just like in the example.

[Arkadii Ivanov]

It's hard to say without code sample. But in general the error means that there is already a registered key for the StateKeeperController. In case of retainStore(lifecycle) the key is YourStore::class.toString() . Please make sure the StateKeeperController is not reused by the new instance of the fragment.

[Skeptick]

Do you mean StateKeeperContainer?

[Arkadii Ivanov]

StateKeeperContainer is a new thing, it is not yet released. It will go next release.

[Arkadii Ivanov]

The next version will be a stable one "2.0.0", and it will bring breaking changes for StateKeeper API unfortunately.

[Arkadii Ivanov]

Sorry, I confused. Yes I mean StateKeeperContainer

[Arkadii Ivanov]

And StateKeeperController is a replacement.

[Arkadii Ivanov]

But nothing fundamentally new. Just reorganized interfaces a bit.

[Skeptick]

Hmm. Do I need to create a new StateKeeperContainer every time I want to reopen a fragment?

[Arkadii Ivanov]

Please give me some time to double check

[Skeptick]

I thought it should exist in a single instance for the entire application and stored in the MainActivity 🤔

[Skeptick]

(Of course, speaking of SingleActivityApplication)

[Arkadii Ivanov]

I will check, thanks for raising the question

[Arkadii Ivanov]

Yes, you are right. There is no way to unregister from the StateKeeper. I will add it.
Regarding the StateKeeperContainer instance - I think it still should be per fragment. Because the fragments have their own onSaveInstanceState callback and we should definitely use it, not the Activity's one.

[Arkadii Ivanov]

I have filed an issue: https://github.com/arkivanov/MVIKotlin/issues/112

[Skeptick]

The Activity has lastCustomNonConfigurationInstance, which we make as instance of StateKeeperContainer, and which allows Stores to be saved during a configuration change. I don’t understand how we can achieve the same in a fragment without involving Activity 🤔

[Skeptick]

It might be necessary to detect when a fragment dies permanently and call unregister on the supplier?

[Arkadii Ivanov]

Yes, for lastCustomNonConfigurationInstance you have to use Activity, and here you will need the upcoming fix.
For state preservation over process death we should use Fragment's onSaveInstanceState()

[Skeptick]

Okay thanks 🙂 I will wait for this fix 🙃 I will also be grateful for an example of use. Probably need to call unregister manually? As far as I know, at the lifecycle callbacks level, it is impossible to distinguish between closing a fragment and re-creating it during a configuration change.

[Skeptick]

Although, I think using the navigation-library and its callbacks, it will be possible to bring this into one place without unnecessary inheritance or another boilerplate)

[Arkadii Ivanov]

The retainInstance and so the retainStore methods will take care of unregistering.

[Arkadii Ivanov]

As of distinguishing, you can check how MVIKotlin handles this: https://github.com/arkivanov/MVIKotlin/blob/0a1e0aef128ed4252c64cad42c2f65c52f86513d/mvikotlin/src/commonMain/kotlin/com/arkivanov/mvikotlin/core/statekeeper/StateKeeperProviderExt.kt#L49

[Skeptick]

Oh, interesting. I checked in the debugger, it distinguishes recreating from returning, great :)

[Arkadii Ivanov]

I also came up with a solution using ViewModel. In this case there is a StateKeeperProvider instance per fragment instance. Actually the custom ViewModel implements StateKeeperProvider. Need to think about it.

[Arkadii Ivanov]

Hey @Skeptick! I have submitted the PR with a solution. The idea is that both StateKeeperController and StateKeeperProvider should have separate instances per Fragment instance, so there is no need to unregister. I added some extensions which make everything automatically. Could you please check and let me know your thouths? https://github.com/arkivanov/MVIKotlin/pull/114

[Arkadii Ivanov]

I will also add the ability to unregister in a separate PR.

[Skeptick]

Hi! Looks good and understandable! I am glad to get rid of the need to pass StateKeeperProvider from the Activity 😁