Kotlin on Android: How to use LiveData from a database in a fragment?

Issue

I use MVVM and have a list of data elements in a database that is mapped through a DAO and repository to ViewModel functions.

Now, my problem is rather banal; I just want to use the data in fragment variables, but I get a type mismatch.

The MVVM introduces a bit of code, and for completeness of context I'll run through it, but I'll strip it to the essentials:

The data elements are of a data class, "Objects":

@Entity(tableName = "objects")
data class Objects(
    @ColumnInfo(name = "object_name")
    var objectName: String
) {
    @PrimaryKey(autoGenerate = true)
    var id: Int? = null
}

In ObjectsDao.kt:

@Dao
interface ObjectsDao {
    @Query("SELECT * FROM objects")
    fun getObjects(): LiveData<List<Objects>>
}

My database:

@Database(
    entities = [Objects::class],
    version = 1
)
abstract class ObjectsDatabase: RoomDatabase() {
    abstract fun getObjectsDao(): ObjectsDao
    companion object {
        // create database
    }
}

In ObjectsRepository.kt:

class ObjectsRepository (private val db: ObjectsDatabase) {
    fun getObjects() = db.getObjectsDao().getObjects()
}

In ObjectsViewModel.kt:

class ObjectsViewModel(private val repository: ObjectsRepository): ViewModel() {
    fun getObjects() = repository.getObjects()
}

In ObjectsFragment.kt:

class ObjectsFragment : Fragment(), KodeinAware {

    private lateinit var viewModel: ObjectsViewModel

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

        viewModel = ViewModelProvider(this, factory).get(ObjectsViewModel::class.java)

        // I use the objects in a recyclerview; rvObjectList
        rvObjectList.layoutManager = GridLayoutManager(context, gridColumns)
        val adapter = ObjectsAdapter(listOf(), viewModel)

        // And I use an observer to keep the recyclerview updated
        viewModel.getObjects.observe(viewLifecycleOwner, {
            adapter.objects = it
            adapter.notifyDataSetChanged()
        })
    }
}

The adapter:

class ObjectsAdapter(var objects: List<Objects>,
                     private val viewModel: ObjectsViewModel):
    RecyclerView.Adapter<ObjectsAdapter.ObjectsViewHolder>() {
// Just a recyclerview adapter
}

Now, all the above works fine - but my problem is that I don't want to use the observer to populate the recyclerview; in the database I store some objects, but there are more objects that I don't want to store.

So, I try to do this instead (in the ObjectsFragment):

var otherObjects: List<Objects>
// ...
if (condition) {
    adapter.objects = viewModel.getObjects()
} else {
    adapter.objects = otherObjects
}
adapter.notifyDataSetChanged()

And, finally, my problem; I get type mismatch for the true condition assignment:

Type mismatch: inferred type is LiveData<List> but List was expected

I am unable to get my head around this. Isn't this pretty much what is happening in the observer? I know about backing properties, such as explained here, but I don't know how to do that when my data is not defined in the ViewModel.

Solution

We need something to switch data source. We pass switching data source event to viewModel.

mySwitch.setOnCheckedChangeListener { _, isChecked ->
    viewModel.switchDataSource(isChecked)
}

In viewModel we handle switching data source

(To use switchMap include implementation "androidx.lifecycle:lifecycle-livedata-ktx:2.4.0")

class ObjectsViewModel(private val repository: ObjectsRepository) : ViewModel() {
    // Best practice is to keep your data in viewModel. And it is useful for us in this case too.
    private val otherObjects = listOf<Objects>()
    private val _loadDataFromDataBase = MutableLiveData<Boolean>()

    // In case your repository returns liveData of favorite list
    // from dataBase replace MutableLiveData(otherObjects) with repository.getFavorite()
    fun getObjects() = _loadDataFromDataBase.switchMap {
        if (it) repository.getObjects() else MutableLiveData(otherObjects)
    }

    fun switchDataSource(fromDataBase: Boolean) {
        _loadDataFromDataBase.value = fromDataBase
    }
}

In activity/fragment observe getObjects()

viewModel.getObjects.observe(viewLifecycleOwner, {
    adapter.objects = it
    adapter.notifyDataSetChanged()
})

Answered By - Jeenbek Khydyr