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Use Circuit's Retained state for back stack support #1551
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…the back stack (#888) This PR contains a bunch of subtle changes, to enable retained state to be retained whilst UIs and Presenters are on the back stack. To see this in action, you can see this [draft PR](chrisbanes/tivi#1551) on Tivi. The videos attached to it highlight the benefits pretty clearly 🎉. This follows on from a big discussion on Slack about the best way to keep state around so that presenter cold starts don't mean that we're showing empty states when coming back from the back stack. ## So what are the changes? ### Nested RetainedStateRegistry Currently `RetainedStateRegistry` is designed to be used single-y, having a global registry used at the root of the content. Since we need to manage the lifecycle of retained state differently per back stack entry, we need a way to be able to 'save' retained state at different times. This PR implements this by making `RetainedStateRegistry` nested-aware, such that registries practically become a tree structure. In practice, this just means a small change to `RetainedStateRegistry` so as `saveAll()` is proxied to any child registries. ### CanRetainChecker changes We use `CanRetainChecker` as the way to control the lifecycle of retained state. This PR changes the default checker to either `CanRetainChecker.Always` (on non-Android platforms), or makes it nested registry aware on Android. ### collectAsRetainedState I found this super useful in Tivi. I initially thought about retaining the `Presenter`s last emitted `CircuitUiState` (a bit like `rememberUpdatedState`), but the UI state typically contains some transient properties (`isLoading`, `errorMessage`, etc) which don't make sense to retain. Using `rememberRetainedState` or `collectAsRetainedState` allows retaining of the more long-lived properties only. ### Other - `continuityRetainedStateRegistry()`. Moved this to `commonMain` with simple implementations for non-Android platforms. - `SaveableHolder` -> `RetainableHolder` - Updated Star sample to use a `continuityRetainedStateRegistry` ## Future There's a few things that this PR doesn't tackle: - Retaining state for different navigation roots. If the user switches the root navigation entry (i.e via a bottom navigation bar), it would be nice to (optionally) retain the state of each root, similar to AndroidX Navigations `saveState` flag. Should be fairly easy, just need to update the `CanRetainChecker` to retain `if screen was root` (or something). --------- Co-authored-by: Zac Sweers <pandanomic@gmail.com>
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Current
Note: the content changing size and/or wrong scroll position when popping back
Current.mp4
Retained
The content stays stable whilst popping back
Retained.mp4
Future
Still need to work out how (or if) I should get this working with Paging, but that can wait.