Traversing a LiveData List in the ViewModel

Question

I'm a rookie Android developer, and could use a little guidance regarding traversing a LiveData List in the ViewModel.

I am basing my app on the MVVM design, and it is simply scanning folders for images, and adding some folders to a favourites list I store in a database. During the scans, I need to check with the stored favourites to see if any of the scanned folders are favourites.

It is the "check against the stored favourites" part that gives me trouble.

Here are the relevant bits from my fragment:

class FoldersFragment : Fragment(), KodeinAware {

    override val kodein by kodein()
    private val factory: FoldersViewModelFactory by instance()

    private var _binding: FragmentFoldersBinding? = null
    private val binding get() = _binding!!
    private lateinit var viewModel: FoldersViewModel

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
        _binding = FragmentFoldersBinding.inflate(inflater, container, false)
        val root: View = binding.root
        return root
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        viewModel = ViewModelProvider(this, factory).get(FoldersViewModel::class.java)
        binding.rvFolderList.layoutManager = GridLayoutManager(context, gridColumns)
        val adapter = FolderItemAdapter(listOf(), viewModel)
        binding.rvFolderList.adapter = adapter

        viewModel.getFolderList().observe(viewLifecycleOwner, {
            adapter.folderItems = it
            binding.rvFolderList.adapter = adapter // Forces redrawing of the recyclerview
        })
        ...
    }

Now, that observer work just fine - it picks up changes and my RecyclerView responds with delight; all is well.

Here are the relevant bits from my RecyclerView adapter:

class FolderItemAdapter(var folderItems: List<FolderItem>, private val viewModel: FoldersViewModel):
    RecyclerView.Adapter<FolderItemAdapter.FolderViewHolder>() {

    private lateinit var binding: FolderItemBinding

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): FolderViewHolder {
        binding = FolderItemBinding.inflate(LayoutInflater.from(parent.context))
        val view = binding.root
        return FolderViewHolder(view)
    }

    override fun onBindViewHolder(holder: FolderViewHolder, position: Int) {
        val currentItem = folderItems[position]
        ...
        if (viewModel.isFavourite(currentItem)) {
            // do stuff
        }
        ...
    }
}

And with that, my problem; the check viewModel.isFavourite(currentItem)always returns false. The implementation in my ViewModel is:

class FoldersViewModel(private val repository: FoldersRepository) : ViewModel() {

   fun getImageFolders() = repository.getImageFolders()

   fun isFavourite(item: FolderItem): Boolean {
        var retval = false
        getImageFolders().value?.forEach {
            if (it.path == item.path) {
                retval = true
            }
        }
    }
}

The `getImageFolders() function is straight from the repository, which again is straight from the Dao:

@Dao
interface FoldersDao {
    @Query("SELECT * FROM image_folders")
    fun getImageFolders(): LiveData<List<FolderItem>>
}

My problem is that I simply can't traverse that list of favourites in the ViewModel. The isFavourite(item: FolderItem) function always returns false because getImageFolders().value always is null. When I check getImageFolders() it is androidx.room.RoomTrackingLiveData@d0d6d31.

And the conundrum; the observer is doing the exact same thing? Or isn't it?

I suspect I am not understanding something basic here?

Answer 1

Your getImageFolders() function retrieves something asynchronously from the database, because you specified that it returns a LiveData. When you get the LiveData back, it will not immediately have a value available. That's why your .value?.forEach is never called. value is still null because you're trying to read it immediately. A LiveData is meant to be observed to obtain the value when it arrives.

There are multiple ways to make a DAO function return something without blocking the current thread. (Handy table here.) Returning a LiveData is one way, but it's pretty awkward to use if you only want one value back. Instead, you should use something from the One-shot read row in the linked table.

If you aren't using RxJava or Guava libraries, that leaves a Kotlin coroutines suspend function as the natural choice.

That would make your Dao look like:

@Dao
interface FoldersDao {
    @Query("SELECT * FROM image_folders")
    suspend fun getImageFolders(): List<FolderItem>
}

And then your ViewModel function would look like:

suspend fun isFavourite(item: FolderItem): Boolean {
    return getImageFolders().any { it.path == item.path }
}

Note that since it is a suspend function, it can only be called from a coroutine. This is necessary to avoid blocking the main thread. If you're not ready to learn coroutines yet, you can replace this function with a callback type function like this:

fun isFavoriteAsync(item: FolderItem, callback: (Boolean)->Unit) {
    viewModelScope.launch {
        val isFavorite = getImageFolders().any { it.path == item.path }
        callback(isFavorite)
    }
}

and at the call site use it like

viewModel.isFavoriteAsync(myFolderItem) { isFavorite ->
    // do something with return value when it's ready here
}

Answer 2

your getImageFolder() is an expensive function so

getImageFolders().value?.forEach {
        if (it.path == item.path) {
            retval = true
        }
    }

in this part the value is still null that is why it returns false. the solution is to make sure the value is not null. Do not check null inside isFavorite function instead call isFavorite() function only when getImageFolder() is done the operation.

What you should do is something like this

observe the liveData of imageFolders 
ondatachange check if the data is null or not
if it is not null update UI and use isFavourite() function