[Serialization] Support Compose's `MutableState` in `saved` [378895074]

Fixed

Feature

Status Update

No update yet.

Description

mg...@google.com

created issue #1

Nov 13, 2024 05:12PM

saveable has built-in support for Compose's MutableState, supporting this use case:

val state by saveable { mutableStateOf(...) }

It does this by providing an overloaded function that handles MutableState.value and serializes it when needed.

To help people migrate from saveable to saved, we want to support MutableState in our KotlinX serialization support.

While working on getMutableState, I experimented with this and created a naive KSerializer implementation to handle MutableState.

Here’s what it could look like:

private class StateSerializer<T : Any>(val valueSerializer: KSerializer<T>)

public inline fun <reified T : Any> stateSerializer(): KSerializer<MutableState<T>>

For serialization, we'd use (value as MutableState<T>).value + valueSerializer to serialize the content. Deserialization works in the opposite direction.

Here’s a usage example:

val state by saved(serializer = stateSerializer<Int>()) { mutableStateOf(1) }

However, calling this without the serializer would cause an exception.

Our variant should work like saveable: if T (the generic type in saved) is serializable, it should work without needing the serializer.

The goal is to find the best way to support MutableState in saved (using a KSerializer, overloads, or another method).

Related to b/378895070.

Comments

mg...@google.com <mg...@google.com> #2Nov 13, 2024 05:13PM

Yigit, do you have time to fix it?

reemission of the same liveData is racy

ap...@google.com <ap...@google.com> #3Jan 22, 2025 11:15AM

yea i'll take it.

ap...@google.com <ap...@google.com> #4Jan 22, 2025 11:15AM

Thanks for the detailed analysis. This may not be an issue anymore since we've started using Main.immediate there but I' not sure; I'll try to create a test case.

mg...@google.com <mg...@google.com> Jan 22, 2025 05:34PM

Marked as fixed.

ap...@google.com <ap...@google.com> #5Jan 29, 2025 05:37PM

just emitting same live data reproduces the issue.

@Test
fun raceTest() {
val subLiveData = MutableLiveData(1)
val subject = liveData(testScope.coroutineContext) {
emitSource(subLiveData)
emitSource(subLiveData) //crashes
}
subject.addObserver().apply {
testScope.advanceUntilIdle()
}
}

pr...@google.com <pr...@google.com> #6Feb 12, 2025 06:40PM

With 2.2.0-alpha04 (that use Main.immediate), the issue seems to be still there (I tested it by calling emitSource() twice, like your test case)

ap...@google.com <ap...@google.com> #7Feb 24, 2025 07:24PM

yea sorry immediate does not fix it.
I actually have a WIP fix for it:

https://android-review.googlesource.com/c/platform/frameworks/support/+/1112186

if your case is the one i found (emitting same LiveData multiple times, as shown in #5) you can work around it by adding a dummy transformation.

val subLiveData = MutableLiveData(1)
val subject = liveData(testScope.coroutineContext) {
emitSource(subLiveData.map {it })
emitSource(subLiveData.map {it} )
}

ap...@google.com <ap...@google.com> #8Feb 24, 2025 07:24PM

Project: platform/frameworks/support
Branch: androidx-master-dev

commit af12e75e6b4110f48e44ca121466943909de8f06
Author: Yigit Boyar <yboyar@google.com>
Date: Tue Sep 03 12:58:11 2019

Fix coroutine livedata race condition

This CL fixes a bug in liveData builder where emitting same
LiveData source twice would make it crash because the second
emission registry could possibly happen before first one is
removed as source.

We fix it by using a suspending dispose function. It does feel
a bit hacky but we cannot make DisposableHandle.dispose async
and we do not want to block there. This does not mean that there
is a problem if developer disposes it manually since our emit
functions take care of making sure it disposes (and there is
no other way to add source to the underlying MediatorLiveData)

Bug: 140249349
Test: BuildLiveDataTest#raceTest_*

Change-Id: I0b464c242a583da4669af195cf2504e2adc4de40

M lifecycle/lifecycle-livedata-ktx/api/2.2.0-alpha05.txt
M lifecycle/lifecycle-livedata-ktx/api/current.txt
M lifecycle/lifecycle-livedata-ktx/api/public_plus_experimental_2.2.0-alpha05.txt
M lifecycle/lifecycle-livedata-ktx/api/public_plus_experimental_current.txt
M lifecycle/lifecycle-livedata-ktx/api/restricted_2.2.0-alpha05.txt
M lifecycle/lifecycle-livedata-ktx/api/restricted_current.txt
M lifecycle/lifecycle-livedata-ktx/src/main/java/androidx/lifecycle/CoroutineLiveData.kt
M lifecycle/lifecycle-livedata-ktx/src/test/java/androidx/lifecycle/BuildLiveDataTest.kt

https://android-review.googlesource.com/1112186

https://goto.google.com/android-sha1/af12e75e6b4110f48e44ca121466943909de8f06

ap...@google.com <ap...@google.com> #9Feb 24, 2025 07:24PM

Project: platform/frameworks/support
Branch: androidx-main
Author: Marcello Galhardo <mgalhardo@google.com>
Link: https://android-review.googlesource.com/3500349

Clean-up MutableStateSerializer

Expand for full commit details

Clean-up `MutableStateSerializer` 
 
Bug: 378895074 
Test: MutableStateSerializerTest 
Change-Id: I429cff7fb9947da4cfd3c12b4411773f89151e65

Files:

M savedstate/savedstate-compose/src/commonMain/kotlin/androidx/savedstate/compose/serialization/serializers/MutableStateSerializer.kt

Hash: 74e185a819ed398f68d21f68d41c69efaca97105
Date: Thu Feb 20 20:27:08 2025

ap...@google.com <ap...@google.com> #10Feb 24, 2025 07:25PM

Project: platform/frameworks/support
Branch: androidx-main
Author: Marcello Galhardo <mgalhardo@google.com>
Link: https://android-review.googlesource.com/3501338

Clean-up MutableStateFlowSerializer