LiveData always returns LiveData<Object>? - kotlin

Question

I'm trying to put a recyclerview which get its data from room. My function getAllHomework returns LiveData<List<Homework>>, but when I tried to set the return value to the recyclerview adapter, it will always return this error
Type Mismatch.
Required: List<Homework>
Found: List<Homework>?
Here's my HomeworkViewModel class which has the function getAllHomework looks like:
class HomeworkViewModel : ViewModel() {
private var matrixNumber: String? = null
private var schoolID: Int = 0
lateinit var listAllHomework: LiveData<List<Homework>>
lateinit var homeworkRepository: HomeworkRepository
fun init(params: Map<String, String>) {
schoolID = Integer.parseInt(params["schoolID"])
homeworkRepository = HomeworkRepository()
listAllHomework = homeworkRepository.getAllHomework(1, "2018")
}
fun getAllHomework(): LiveData<List<Homework>>{
return listAllHomework
}
}
And below is the part in my Homework activity that tries to set the value into recyclerview adapter but will always return the type mismatch error.
class Homework : AppCompatActivity(), LifecycleOwner {
lateinit var linearLayoutManager: LinearLayoutManager
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.homework)
linearLayoutManager = LinearLayoutManager(this)
rvHomeworks.layoutManager = linearLayoutManager
var adapter = AdapterHomework(this)
rvHomeworks.adapter = adapter
var homeworkViewModel = ViewModelProviders.of(this).get(HomeworkViewModel::class.java)
homeworkViewModel.init(params)
homeworkViewModel.getAllHomework().observe(this, Observer {
allHomework -> adapter.setHomeworkList(allHomework)
})
}
}
The line allHomework -> adapter.setHomeworkList(allHomework) above will show the Type Mismatch error I mentioned above.
Here's how my AdapterHomework looks like:
class AdapterHomework(context: Context): RecyclerView.Adapter<AdapterHomework.ViewHolder>() {
lateinit var homeworkList: List<Homework>
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder{
val v = LayoutInflater.from(parent.context).inflate(R.layout.rv_homeworks, parent, false)
return ViewHolder(v);
}
#JvmName("functionToSetTheHomeworkList")
fun setHomeworkList(myHomeworkList: List<Homework>){
homeworkList = myHomeworkList
notifyDataSetChanged()
}
}
I could not find where in my code did I ever return List<Homework>? instead of List<Homework>.

Answer 1

This has actually nothing to do with Room, but with how LiveData was designed - specifically Observer class:
public interface Observer<T> {
void onChanged(#Nullable T t);
}
as you can see T (in your case List<Homework>) is marked as #Nullable therefore you will always get it's Kotlin equivalent List<Homework>? as a parameter of onChanged in your Observer implementation.
I would recommend changing setHomeworkList to something like this:
fun setHomeworkList(myHomeworkList: List<Homework>?){
if(myHomeworkList != null){
homeworkList = myHomeworkList
notifyDataSetChanged()
}
}
You can also use let function for that like this:
fun setHomeworkList(myHomeworkList: List<Homework>?){
myHomeworkList?.let {
homeworkList = it
notifyDataSetChanged()
}
}