使用 MVVM 视图模型和实时数据时，如何在配置更改后保留 RecyclerView 的滚动位置？

Question

READ FIRST: Apologies, it seems I have played myself. I was using RecyclerView in my xml earlier, but switched it over for CardStackView (it still uses the exact same RecyclerView adapter). If I switch back to RecyclerView, the original code below works - the scroll position is saved and restored automatically on configuration change.

I'm using a MVVM viewmodel class which successfully retains list data for a RecyclerView after a configuration change. However, the previous RecyclerView position is not restored. Is this expected behaviour? What would be a good way to solve this?

I saw a blog post on medium briefly mentioning you can preserve scroll position by setting the adapter data before setting said adapter on the RecyclerView. From what I understand, after a configuration change the livedata that was being observed earlier gets a callback. That callback is where I set my adapter data. But it seems this callback happens after the onCreate() function finishes by which point my RecyclerView adapter is already set.

class MainActivity : AppCompatActivity() {
    private val adapter = MovieAdapter()
    private lateinit var viewModel: MainViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        // Create or retrieve viewmodel and observe data needed for recyclerview
        viewModel = ViewModelProvider(this).get(MainViewModel::class.java)
        viewModel.movies.observe(this, {
            adapter.items = it
        })

        binding.recyclerview.adapter = adapter

        // If viewmodel has no data for recyclerview, retrieve it
        if (viewModel.movies.value == null) viewModel.retrieveMovies()
    }
}

class MovieAdapter :
    RecyclerView.Adapter<MovieAdapter.MovieViewHolder>() {

    var items: List<Movie> by Delegates.observable(emptyList()) { _, _, _ ->
        notifyDataSetChanged()
    }

    class MovieViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        private val binding = ItemMovieCardBinding.bind(itemView)

        fun bind(item: Movie) {
            with(binding) {
                imagePoster.load(item.posterUrl)
                textRating.text = item.rating.toString()
                textDate.text = item.date
                textOverview.text = item.overview
            }
        }
    }


    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MovieViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.item_movie_card, parent, false)

        return MovieViewHolder(view)
    }

    override fun onBindViewHolder(holder: MovieViewHolder, position: Int) {
        holder.bind(items[position])
    }

    override fun getItemCount() = items.size
}

class MainViewModel : ViewModel() {
    private val _movies = MutableLiveData<List<Movie>>()
    val movies: LiveData<List<Movie>> get() = _movies

    fun retrieveMovies() {
        viewModelScope.launch {
            val client = ApiClient.create()
            val result: Movies = withContext(Dispatchers.IO) { client.getPopularMovies() }

            _movies.value = result.movies
        }
    }
}

Answer 1

Set adapter only after its items are available.

    viewModel.movies.observe(this, {
        adapter.items = it
        binding.recyclerview.adapter = adapter
    })