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

Question

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.

Answer 1

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()
})

Answer 2

You can do something like this:
var displayDataFromDatabase = true // Choose whatever default fits your use-case
var databaseList = emptyList<Objects>() // List we get from database
val otherList = // The other list that you want to show
toggleSwitch.setOnCheckedChangeListener { _, isChecked ->
displayDataFromDatabase = isChecked // Or the negation of this
// Update adapter to use databaseList or otherList depending upon "isChecked"
}
viewModel.getObjects.observe(viewLifecycleOwner) { list ->
databaseList = list
if(displayDataFromDatabase)
// Update adapter to use this databaseList
}