it seams that `rememberSaveableStateHolder` is bro...

# compose

[Mikołaj Kąkol]

it seams that rememberSaveableStateHolder is broken in somehow in alpha12. Consider example from here: https://github.com/androidx/androidx/blob/androidx-main/compose/runtime/runtime-sa[…]/compose/runtime/saveable/samples/SaveableStateHolderSamples.kt
and change

Box(modifier) {
            restorableStateHolder.SaveableStateProvider(currentScreen) {
                content(currentScreen)
            }
       }

to

Box(modifier) {
            restorableStateHolder.SaveableStateProvider(currentScreen) {
                Crossfade(targetState = currentScreen) {
                    content(it)
                }
            }
        }

you will notice that animation doesn’t work.

If you will remove that state holder:

Box(modifier) {
            Crossfade(targetState = currentScreen) {
                content(it)
            }
        }

crossfade works.

So either stateholder is broken or crossfade. From what I unsderstand problem is that Crossfade implementation uses remember to store items to render during transition.

What I wanted to achieve is some nice animation between screens. What to do? 🤔 🙀

[Mikołaj Kąkol]

@Adam Powell could you take a look? ☺️

[Mikołaj Kąkol]

and idea like

Crossfade(targetState = currentScreen) {
            Box(modifier) {
                restorableStateHolder.SaveableStateProvider(currentScreen) {
                    content(it)
                }
            }
        }

crashes after screen change with

java.lang.IllegalArgumentException: Key screen2 was used multiple times 
        at androidx.compose.runtime.saveable.SaveableStateHolderImpl$SaveableStateProvider$1.invoke(SaveableStateHolder.kt:91)

[Andrey Kulikov]

you need to first call Crossfade and only then SaveableStateProvider

[Andrey Kulikov]

there is a demo which does exactly this: https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/[…]/animation/demos/CrossfadeDemo.kt?q=crossfadedemo&ss=androidx

[Andrey Kulikov]

in your last sample you did it almost right, but you are ignoring the parameter which Crossfade gives you in a trailing lambda. please see how it is done in the demo I shared

[Albert Chang]

I think your problem here is that when currentScreen changes, instead of recomposing the composable passed to SaveableStateProvider, a new composition is created, which means that the Crossfade is not the same to the old Crossfade thus not working.

[Mikołaj Kąkol]

thank you Andrey, that worked

Crossfade(targetState = currentScreen) {
            Box(modifier) {
                restorableStateHolder.SaveableStateProvider(it) {
                    content(it)
                }
            }
        }

amazing :)