Do people feel that `StateFlow` is a good replacem...

# coroutines

[melatonina]

Do people feel that StateFlow is a good replacement for LiveData on Android?

There is often a noticeable delay in updates, which leads to the UI to represent incorrect information when it first shows up.

That never happened to me with LiveData.

[Zach Klippenstein (he/him) [MOD]]

There shouldn’t be a delay. Are you sure you’re not using the non-immediate Main dispatcher somehow?

[tateisu]

LiveData supports lifecycle, Flow does not. Not simple replacement.

[Albert Chang]

Please don't cross-post.
@tateisu Obviously you haven't read this post.

[gildor]

LiveData supports lifecycle, Flow does not

It’s completely incorrect

Flow was built with structured concurrency from ground up which is much more strict and powerful version than integration with lifecycle on LiveData

it just doesn’t have exactly the same semantics as LiveData by default, but it’s not an issue of Flow, it’s just a matter of different semantics

[melatonina]

@Zach Klippenstein (he/him) [MOD] I checked and I was using Dispatchers.Main, but even changing it to Dispatchers.Main.immediate, the problem remains.
@Albert Chang Yes, cross-posting was a bad idea. Sorry.
I'm attaching a video which illustrates the problem. When the RecyclerView item view appear on the screen, the text is missing and an icon which should be hidden is visible.
The video is slowed down to 1/10 of the original speed.

Android View delay with StateFlow.mp4

[melatonina]

Here is the declaration of the item layout:

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="<http://schemas.android.com/apk/res/android>"
    xmlns:tools="<http://schemas.android.com/tools>"
    xmlns:app="<http://schemas.android.com/apk/res-auto>">
    <data>
        <import type="android.view.View" alias="View" />
        <variable
            name="viewModel"
            type="com.example.ProjectViewModel" />
    </data>

    <com.google.android.material.card.MaterialCardView
        android:id="@+id/card_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginVertical="5dp"
        android:layout_marginHorizontal="10dp"
        android:background="#fff"
        app:cardCornerRadius="4dp">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:padding="10dp"
            android:orientation="vertical">

            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:fontFamily="@font/montserrat"
                android:text="@{viewModel.stateFlow.projectEntity.name}"
                android:textSize="24sp"
                android:textStyle="bold"
                tools:text="Project name" />

            <LinearLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:orientation="horizontal">

                <ImageView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:src="@drawable/ic_baseline_cloud_24"
                    android:visibility="@{viewModel.isRemoteFlow ? View.VISIBLE : View.INVISIBLE}"
                    />
            </LinearLayout>
        </LinearLayout>

    </com.google.android.material.card.MaterialCardView>

</layout>

and here is the ViewModel definition:

class ProjectViewModel(
    initialState: ProjectState2
) : ViewModel() {
    companion object {
        fun remote(projectEntity: ProjectEntity) =
            ProjectViewModel(ProjectState2.Remote.Available(projectEntity))
        fun local(projectEntity: ProjectEntity) =
            ProjectViewModel(ProjectState2.Local.Available(projectEntity))
    }

    private val stateMutableFlow = MutableStateFlow<ProjectState2>(initialState)
    val stateFlow : StateFlow<ProjectState2> get() = stateMutableFlow
    var state by stateMutableFlow
        private set

    val projectId get() = state.projectId

    val isRemoteFlow = state(stateFlow.map { it.isRemote }, false)
    val isRemote by isRemoteFlow
}

where state() is defined as:

fun <T> ViewModel.state(flow : Flow<T>, initialValue: T) =
    flow.stateIn(viewModelScope, SharingStarted.Eagerly, initialValue)

[melatonina]

Note that, so far, state is never updated, so the underlying MutableStateFlow never changes its state after inizialization.

[gildor]

where is code which consuming this view model?

[melatonina]

@gildor The only code interacting with the view model is the view holder:

class ViewHolder(
    private val binding: ProjectItemBinding
) : RecyclerView.ViewHolder(binding.root) {
    fun bindTo(projectViewModel: ProjectViewModel) {
        binding.viewModel = projectViewModel
    }
}

[gildor]

how binding is implemented?

[gildor]

so I’m curious about the code which actually subscribes on Flow

[gildor]

is it DataBinding?

[melatonina]

@gildor The code which subscribes to the StateFlow and updates the view is automatically generated by DataBinding. I posted the layout file.

[melatonina]

@gildor Did I omit anything?

[gildor]

so it’s databindign

[gildor]

looks that binding doesn’t uses default value of stateflow

[melatonina]

What was missing is probably setting the lifecycleOwner of the item binding.

My code, at the time when I reported the problem was:

override fun onCreateViewHolder(viewGroup: ViewGroup, viewType: Int): ViewHolder =
    ViewHolder(
        ProjectItemBinding.inflate(
            LayoutInflater.from(viewGroup.context),
            viewGroup,
            false
        )
    )

now it's:

override fun onCreateViewHolder(viewGroup: ViewGroup, viewType: Int): ViewHolder =
    ViewHolder(
        ProjectItemBinding.inflate(
            LayoutInflater.from(viewGroup.context),
            viewGroup,
            false
        ).apply {
            lifecycleOwner = [e-mail]
        }
    )

[gildor]

huh, but without lifecycleOwner it shouldn’t work a all, isn’ it? Same for livedata, it will not subscribe until you set lifecycleOwner

[melatonina]

Everything is working fine, now.

[gildor]

👍

[gildor]

I just surprised that it had this behaviour before, I would expect that nothing is rendered without lifecycleOwner