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null cannot be cast to non-null type RecyclerView with stateFlow

I got some categories from an api and trying to show them on a recycler view but it doesn't work for some reason.
Although the data appears correctly in the logcat, it is sent as null to the Category adapter.
This is the Main Activity (where I'm trying to show the data):
`
#AndroidEntryPoint
class MainActivity : AppCompatActivity() {
private val TAG = "MEALZ"
private lateinit var binding: ActivityMainBinding
private val viewModel:MealsViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
val adapter = CategoryAdapter(this)
binding.categoriesRv.adapter = adapter
viewModel.getMeals()
lifecycleScope.launch {
viewModel.categories.collect {
adapter.setData(it?.categories as List<Category>)
Log.d(TAG, "onCreate: ${it?.categories}")
}
}
}
}
`
This is Recycler Category Adapter :
`
class CategoryAdapter(private val context: Context?) :
RecyclerView.Adapter<CategoryAdapter.CategoryViewHolder>() {
private var categoryList: MutableList<Category?> = mutableListOf<Category?>()
inner class CategoryViewHolder(itemView: CategoryLayoutBinding) :
RecyclerView.ViewHolder(itemView.root) {
val name = itemView.categoryNameTv
val img = itemView.categoryIv
val des = itemView.categoryDesTv
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): CategoryViewHolder {
val binding = CategoryLayoutBinding.inflate(LayoutInflater.from(context), parent, false)
return CategoryViewHolder(binding)
}
override fun onBindViewHolder(holder: CategoryViewHolder, position: Int) {
var category = categoryList[position]
holder.name.text = category?.strCategory
holder.des.text = category?.strCategoryDescription
Glide.with(context as Context).load(category?.strCategoryThumb).into(holder.img)
}
override fun getItemCount(): Int {
return categoryList.size
}
fun setData(CategoryList: List<Category>) {
this.categoryList.addAll(CategoryList)
notifyDataSetChanged() //to notify adapter that new data change has been happened to adapt it
}
}
`
This is the View Model class:
#HiltViewModel
class MealsViewModel #Inject constructor(private val getMealsUseCase: GetMeals): ViewModel() {
private val TAG = "MealsViewModel"
private val _categories: MutableStateFlow<CategoryResponse?> = MutableStateFlow(null)
val categories: StateFlow<CategoryResponse?> = _categories
fun getMeals() = viewModelScope.launch {
try {
_categories.value = getMealsUseCase()
} catch (e: Exception) {
Log.d(TAG, "getMeals: ${e.message.toString()}")
}
}
}

you create your _categories with null as initial value, so first value of categories flow will be null and only second one will contain fetched data. As a workaround, you can check that data is not null:
viewModel.categories.collect {
if (it != null) {
adapter.setData(it?.categories as List<Category>)
Log.d(TAG, "onCreate: ${it?.categories}")
}
}
or introduce some kind of "loading" state

How to fetch edittext value from one activity to recyclerview of next activity?

I'm trying to fetch edittext value from one activity and displaying that text in recycleview and storing using room DB....
Basically the idea is adding the address in activity address when clicks on plus it will redirect to next page where the user gets the address form onsubmit it will fetch address and add to previous activity recycleview.
here is my code for room:
table:---
#Entity(tableName = "address")
class Address {
#PrimaryKey
var id = 0
#ColumnInfo(name = "address")
var address: String? = null
}
Database:-
#Database(entities = [Address::class], version = 1)
abstract class Database : RoomDatabase() {
abstract fun AddressDao(): AddressDao
}
AddressDao:
#Dao
interface AddressDao {
#Insert
suspend fun addData(address: Address)
#Query("select * from address")
fun getAddressesWithChanges() :LiveData<List<Address>>
#Query("SELECT EXISTS (SELECT 1 FROM address WHERE id=:id)")
suspend fun isAddressAdded(id: Int): Int
#Delete
suspend fun delete(address: Address)
}
AddAddressActivity activity:
class AddAddressActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.addaddress)
val editText: EditText = findViewById(R.id.longaddress)
val application = application as CustomApplication
saveaddress.setOnClickListener {
val address = Address()
address.address = editText.getText().toString()
lifecycleScope.launch {
application.addressDao.addData(address)
finish()
}
}
}
}
AddressActivity:-
class AddressActivity : AppCompatActivity() {
private val adapter = AddressAdapter()
private lateinit var data: LiveData<List<Address>>
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.address)
addbutton.findViewById<View>(R.id.addbutton).setOnClickListener {
val intent = Intent(this, AddAddressActivity::class.java)
startActivity(intent)
}
val recyclerView = findViewById<RecyclerView>(R.id.recyclerview)
recyclerView.setHasFixedSize(true)
recyclerView.layoutManager = LinearLayoutManager(this,
LinearLayoutManager.VERTICAL, false)
recyclerView.adapter = adapter
val application = application as CustomApplication
data = application.database.AddressDao().getAddressesWithChanges()
data.observe(this, Observer { words1 ->
// Update the cached copy of the words in the adapter.
words1?.let { adapter.updateData(it) }})
}
}
AddressAdapter activity:-
class AddressAdapter: RecyclerView.Adapter<AddressAdapter.ViewHolder>() {
private var addresses: List<Address> = Collections.emptyList()
override fun onCreateViewHolder(viewGroup: ViewGroup, itemViewType: Int):
ViewHolder =
ViewHolder(LayoutInflater.from(viewGroup.context)
.inflate(R.layout.address_item, viewGroup, false) )
override fun onBindViewHolder(viewHolder: ViewHolder, position: Int) {
val fl: Address = addresses[position]
viewHolder.tv.setText(fl.address)
}
override fun getItemCount(): Int = addresses.size
inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
var tv: TextView
init {
tv = itemView.findViewById(R.id.ftv_name)
} }
fun updateData(addresses:
List<Address>) {
this.addresses = addresses
notifyDataSetChanged() // TODO: use ListAdapter if animations are needed
}
}
CustomApplication:
class CustomApplication: Application() {
lateinit var database: Database
private set
lateinit var addressDao: AddressDao
private set
override fun onCreate() {
super.onCreate()
this.database = Room.databaseBuilder<Database>(
applicationContext,
Database::class.java, "database"
).build()
}
}
list is not displaying recycleview is empty when came back to address activity
please help ..I'm new to kotlin and room db..

abstract fun AddressDao(): AddressDao?
Why is this nullable?
val favoriteData: List<Table>?
Why is this nullable?
fun addData(favoriteList: Table?)
Why is this nullable?
fun delete(favoriteList: Table?)
Why is this nullable?
Typed nullability is to enable you to represent where a given value can be null, in the sense that it makes sense for it to be absent.
Neither of the above cases represent such a scenario. Room will never return you null instead of an actual dao.
Kotlin's null safety is to allow you to eliminate potential error cases by making it part of the type system, it shouldn't be used to create "false" error cases such as these. If you're trying to add null to a database instead of a real object, you've probably got a bug in your code, and Kotlin should be able to help you eliminate that possibility.
As for your actual question, you're misusing Room.
.allowMainThreadQueries() is a way to disable an error check, but that error is there for a reason. You're not supposed to be reading the data on the UI thread, because with enough data, your app will freeze (ANR).
The code is supposed to look something like this:
#Entity(tableName = "address")
class Address {
#PrimaryKey
var id = 0
#ColumnInfo(name = "address")
var address: String? = null
}
Database:-
#Database(entities = [Address::class], version = 1)
abstract class Database : RoomDatabase() {
abstract fun addressDao(): AddressDao
}
AddressDao:
#Dao
interface AddressDao {
#Insert
suspend fun addData(address: Address)
#Query("select * from address")
fun getAddressesWithChanges(): LiveData<List<Address>>
#Query("SELECT EXISTS (SELECT 1 FROM address WHERE id=:id)")
suspend fun isAddressAdded(id: Int): Int
#Delete
suspend fun delete(address: Address)
}
class CustomApplication: Application() {
lateinit var database: Database
private set
lateinit var addressDao: AddressDao
private set
override fun onCreate() {
super.onCreate()
this.database = Room.databaseBuilder<Database>(
applicationContext,
Database::class.java, "database"
).build()
addressDao = database.addressDao()
}
}
AndroidManifest.xml
<application android:name=".CustomApplication"
...
Address activity:
import androidx.lifecycle.observe
class AddressActivity : AppCompatActivity() {
private val adapter = AddressAdapter()
private lateinit var data: LiveData<List<Address>>
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.address_activity)
findViewById<View>(R.id.addButton).setOnClickListener {
val intent = Intent(this, AddAddressActivity::class.java)
startActivity(intent)
}
val recyclerView = findViewById<RecyclerView>(R.id.recyclerview)
recyclerView.setHasFixedSize(true)
recyclerView.layoutManager = LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false)
recyclerView.adapter = adapter
val application = application as CustomApplication
val addressDao = application.addressDao
data = addressDao.getAddressesWithChanges()
data.observe(this) {
adapter.updateData(it)
}
}
}
AddressAdapter:
class AddressAdapter: RecyclerView.Adapter<AddressAdapter.ViewHolder>() {
private var addresses: List<Table> = Collections.emptyList()
override fun onCreateViewHolder(viewGroup: ViewGroup, itemViewType: Int): ViewHolder =
ViewHolder(LayoutInflater.from(viewGroup.context).inflate(R.layout.address_item, viewGroup, false))
override fun onBindViewHolder(viewHolder: ViewHolder, position: Int) {
val address = addresses[position]
viewHolder.tv.setText(address.address)
}
override fun getItemCount(): Int = addresses.size
inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
val tv = itemView.findViewById(R.id.ftv_name)
}
fun updateData(addresses: List<Address>) {
this.addresses = addresses
notifyDataSetChanged() // TODO: use ListAdapter if animations are needed
}
}
AddAddress activity:-
class AddAddressActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.addaddress)
val editText: EditText = findViewById(R.id.longaddress)
val application = application as CustomApplication
val addressDao = application.addressDao
saveaddress.setOnClickListener {
val address = Address()
address.address = editText.getText().toString()
lifecycleScope.launch {
addressDao.addData(address)
finish()
}
}
}
}
You can potentially also refer to https://codelabs.developers.google.com/codelabs/android-room-with-a-view-kotlin/#6

Please replace your line with
val rv=findViewById<RecyclerView>(R.id.recyclerview)
You have passed the wrong id addressrecyclerview that's why it was assigning null

How to show query to database in ListView in kotlin with android studio?

I am learning to develop in kotlin with android studio, so I don't have any experience.
I am wanting to insert data into a local database using room database, so far I think I'm doing fine. Now I need to be able to consult those data but I cannot do it, I have searched the internet but I have not been able to solve my problem.
I attach the code.
Class #Entity
class TablasBdApp {
#Entity(tableName = TblConteo.TABLE_NAME)
data class TblConteo(
#PrimaryKey(autoGenerate = true) #ColumnInfo(name = "linea_id") val Linea_Id: Int = 0,
#ColumnInfo (name = "articulo") val Articulo : String?
)
{
companion object {
const val TABLE_NAME = "TablaConteo"
}
}
}
Class #Dao
#Dao
public interface ItblConteoDao {
#Insert(onConflict = OnConflictStrategy.REPLACE)
fun insertArticulo(taskTblConteo: TablasBdApp.TblConteo);
#Query("SELECT * FROM " + TablasBdApp.TblConteo.TABLE_NAME + " ORDER BY Linea_Id desc")
fun getConteoArticulos(): LiveData<List<TablasBdApp.TblConteo>>
}
Class #DataBase
class BaseDeDatos {
#Database(entities = [TablasBdApp.TblConteo::class], version = 1)
abstract class PortatilDataBase : RoomDatabase() {
abstract fun itblconteoDao () : ItblConteoDao
companion object {
private const val DATABASE_NAME = "portatildb"
#Volatile
private var INSTANCE: PortatilDataBase? = null
fun getInstance(context: Context): PortatilDataBase? {
INSTANCE ?: synchronized(this) {
INSTANCE = Room.databaseBuilder(
context.applicationContext,
PortatilDataBase::class.java,
DATABASE_NAME
).build()
}
return INSTANCE
}
}
}
}
Class #Repository
class ConteoRepository(application: Application) {
private val itblConteoDao: ItblConteoDao? = BaseDeDatos.PortatilDataBase.getInstance(application)?.itblconteoDao()
fun insert (tblconteo: TablasBdApp.TblConteo){
if(itblConteoDao != null) InsertAsyncTask(itblConteoDao).execute(tblconteo)
}
fun getConteo(): LiveData<List<TablasBdApp.TblConteo>> {
return itblConteoDao?.getConteoArticulos() ?: MutableLiveData<List<TablasBdApp.TblConteo>>()
}
private class InsertAsyncTask(private val itblConteoDao: ItblConteoDao) :
AsyncTask<TablasBdApp.TblConteo, Void, Void>() {
override fun doInBackground(vararg tblconteos: TablasBdApp.TblConteo?): Void? {
for (tblconteo in tblconteos) {
if (tblconteo != null) itblConteoDao.insertArticulo(tblconteo)
}
return null
}
}
}
Class #CustomAdapter
class CustomAdapter(context: Context): BaseAdapter() {
private val mContext: Context
init{
mContext = context
}
override fun getView(position: Int, convertView: View?, parent: ViewGroup?): View {
//val repository = ConteoRepository(application = Application())
var textView = TextView(mContext)
val observer = Observer<List<TablasBdApp.TblConteo>> { conteos ->
if (conteos != null) {
var text = ""
for (conteo in conteos) {
text += conteo.Linea_Id.toString() + " " + conteo.Articulo
}
textView.text = text
}
}
//repository.getConteo().observe(this, observer)
return textView
}
override fun getItem(position: Int): Any {
return "Test Articulo"
}
override fun getItemId(position: Int): Long {
return position.toLong()
}
override fun getCount(): Int {
TODO("Not yet implemented")
}
}
Class #Fragment
class CapturaConteoFragment() : Fragment() {
private lateinit var capturaConteoViewModel: CapturaConteoViewModel
#SuppressLint("ResourceType")
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
capturaConteoViewModel =
ViewModelProviders.of(this).get(CapturaConteoViewModel::class.java)
val root = inflater.inflate(R.layout.fragment_capturaconteo, container, false)
//val textView: TextView = root.findViewById(R.id.text_capturaconteo)
capturaConteoViewModel.text.observe(viewLifecycleOwner, Observer {
//textView.text = it
val botonGuardar: Button = root.findViewById(R.id.btn_guardar)
val textoArticulo : EditText = root.findViewById(R.id.edit_art_upc)
var listview: ListView = root.findViewById(R.id.list_conteo)
botonGuardar.setOnClickListener {
if(textoArticulo.getText().toString().trim().isEmpty()){
Toast.makeText(activity, "Captura un articulo", Toast.LENGTH_LONG).show()
textoArticulo.requestFocus()
}
else{
saveConteo(TablasBdApp.TblConteo(Articulo = textoArticulo.text.trim().toString()))
Toast.makeText(activity, "Articulo guardado", Toast.LENGTH_LONG).show()
//var listaconteo = arrayOf(muestraConteo())
var prodAdapter = CustomAdapter(**this**) **<- This is where it marks error.**
listview?.adapter = prodAdapter
}
}
})
return root
}
private fun saveConteo(tblConteo: TablasBdApp.TblConteo) {
val repository = ConteoRepository(application = Application())
repository.insert(tblConteo)
val conteo = repository.getConteo()
}
private fun showConteo (){
val repository = ConteoRepository(application = Application())
repository.getConteo()
}
}
Thanks for your help.
Regards.

In your DAO you are returning LiveData that means you need to observe it and best practice would be through ViewModel which implies you should MVVM architecture, there are plenty of tutorials and courses on it.
Here is something from google Codelabs
Other way would would be to return just list rather than live data of a list. This is bad practice because you will have to refresh it all the time and do a lot of extra work.
PS your adapter needs fixing too my suggestion is follow codelabs or watch complete tutorial on Room + MVVM.
Hope this helps

Why do it still need launch notifyDataSetChanged() when I have used LiveData?

I'm learning Room with the sample project RoomWordsSample at https://github.com/googlecodelabs/android-room-with-a-view/tree/kotlin.
The following code are from the project.
In my mind, the LiveDate will update UI automatically when the data changed if it was observed.
But in the file WordListAdapter.kt, I find notifyDataSetChanged() is added to the function setWords(words: List<Word>)， it's seems that it must notify UI manually when data changed.
Why do it still need launch notifyDataSetChanged() when I have used LiveData ?
MainActivity.kt
class MainActivity : AppCompatActivity() {
private val newWordActivityRequestCode = 1
private lateinit var wordViewModel: WordViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val recyclerView = findViewById<RecyclerView>(R.id.recyclerview)
val adapter = WordListAdapter(this)
recyclerView.adapter = adapter
recyclerView.layoutManager = LinearLayoutManager(this)
wordViewModel = ViewModelProvider(this).get(WordViewModel::class.java)
wordViewModel.allWords.observe(this, Observer { words ->
words?.let { adapter.setWords(it) }
})
}
}
WordViewModel.kt
class WordViewModel(application: Application) : AndroidViewModel(application) {
private val repository: WordRepository
val allWords: LiveData<List<Word>>
init {
val wordsDao = WordRoomDatabase.getDatabase(application, viewModelScope).wordDao()
repository = WordRepository(wordsDao)
allWords = repository.allWords
}
fun insert(word: Word) = viewModelScope.launch {
repository.insert(word)
}
}
WordListAdapter.kt
class WordListAdapter internal constructor(
context: Context
) : RecyclerView.Adapter<WordListAdapter.WordViewHolder>() {
private val inflater: LayoutInflater = LayoutInflater.from(context)
private var words = emptyList<Word>() // Cached copy of words
inner class WordViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
val wordItemView: TextView = itemView.findViewById(R.id.textView)
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): WordViewHolder {
val itemView = inflater.inflate(R.layout.recyclerview_item, parent, false)
return WordViewHolder(itemView)
}
override fun onBindViewHolder(holder: WordViewHolder, position: Int) {
val current = words[position]
holder.wordItemView.text = current.word
}
internal fun setWords(words: List<Word>) {
this.words = words
notifyDataSetChanged()
}
override fun getItemCount() = words.size
}

Actually, livedata will give you updated data in your activity. But now, it is your activity's job to update the ui. So, whenever live data gives you updated data, you will have to tell the ui to update the data. Hence, notifyDataSetChanged().

notifyDataSetChanged has nothing to do with LiveData, it's part of RecyclerView api.
LiveData - is way of receiving data in lifecycle-aware way, RecyclerView simply displays views.

Reload RecyclerView after data change with Room, ViewModel and LiveData

I am trying, without success, to solve a problem for days. I would like to update my recyclerView whenever the records of a particular model change in the Database (DB Room). I use ViewModel to handle the model data and the list of records are stored in LiveData.
Database
#Database(entities = arrayOf(Additive::class), version = ElementDatabase.DB_VERSION, exportSchema = false)
abstract class ElementDatabase() : RoomDatabase() {
companion object {
const val DB_NAME : String = "element_db"
const val DB_VERSION : Int = 1
fun get(appContext : Context) : ElementDatabase {
return Room.databaseBuilder(appContext, ElementDatabase::class.java, DB_NAME).build()
}
}
abstract fun additivesModels() : AdditiveDao
}
Model
#Entity
class Additive {
#PrimaryKey #ColumnInfo(name = "id")
var number : String = ""
var dangerousness : Int = 0
var description : String = ""
var names : String = ""
var notes : String = ""
var risks : String = ""
var advice : String = ""
}
Dao
#Dao
interface AdditiveDao {
#Query("SELECT * FROM Additive")
fun getAllAdditives() : LiveData<List<Additive>>
#Query("SELECT * FROM Additive WHERE id = :arg0")
fun getAdditiveById(id : String) : Additive
#Query("DELETE FROM Additive")
fun deleteAll()
#Insert(onConflict = REPLACE)
fun insert(additive: Additive)
#Update
fun update(additive: Additive)
#Delete
fun delete(additive: Additive)
}
ViewModel
class AdditiveViewModel(application: Application) : AndroidViewModel(application) {
private var elementDatabase : ElementDatabase
private val additivesModels : LiveData<List<Additive>>
init {
this.elementDatabase = ElementDatabase.get(appContext = getApplication())
this.additivesModels = this.elementDatabase.additivesModels().getAllAdditives()
}
fun getAdditivesList() : LiveData<List<Additive>> {
return this.additivesModels
}
fun deleteItem(additive : Additive) {
DeleteAsyncTask(this.elementDatabase).execute(additive)
}
private class DeleteAsyncTask internal constructor(private val db: ElementDatabase) : AsyncTask<Additive, Void, Void>() {
override fun doInBackground(vararg params: Additive): Void? {
db.additivesModels().delete(params[0])
return null
}
}
}
Fragment
class AdditivesFragment : LifecycleFragment() {
private var viewModel : AdditiveViewModel? = null
private var adapter : AdditivesAdapter? = null
companion object {
fun newInstance() : AdditivesFragment {
val f = AdditivesFragment()
val args = Bundle()
f.arguments = args
return f
}
}
override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
return inflater?.inflate(R.layout.fragment_additives, container, false)
}
override fun onActivityCreated(savedInstanceState: Bundle?) {
this.adapter = AdditivesAdapter(ArrayList<Additive>())
this.additives_list.layoutManager = GridLayoutManager(this.context, 2, GridLayoutManager.VERTICAL, false)
this.additives_list.adapter = this.adapter
this.viewModel = ViewModelProviders.of(this).get(AdditiveViewModel::class.java)
this.viewModel?.getAdditivesList()?.observe(this, Observer<List<Additive>> { additivesList ->
if(additivesList != null) {
this.adapter?.addItems(additivesList)
}
})
super.onActivityCreated(savedInstanceState)
}
}
Now, my question is why is the observer called only once (at the start of the fragment) and then is not called back again? How can I keep the observer constantly listening to the changes in the DB (insert, update, delete) so that my recyclerView instantly can be updated? Thanks a lot for any suggestion.

This is where you made a mistake:
this.viewModel = ViewModelProviders.of(this).get(AdditiveViewModel::class.java)
you are passing this while you are inside the fragment which is pretty disturbing for some people cause it is not a syntax error but logical. You have to pass activity!! instead, it will be like this:
this.viewModel = ViewModelProviders.of(activity!!).get(AdditiveViewModel::class.java)
UPDATE:
Pass viewLifecycleOwner while being inside fragment while observing the Data
mainViewModel.data(viewLifecycleOwner, Observer{})
If you're using fragmentKtx, you can init viewModel this way:
private val viewModel by viewModels<MainViewModel>()
If You've viewModelFactory:
private val viewModel by viewModels<MainViewModel>{
viewModelFactory
}
with this approach you don't need to call:
// you can omit this statement completely
viewModel = ViewModelProviders.of(this).get(AdditiveViewModel::class.java)
You can simply just start observing the data..