Develop Reference

How to update a TextView after an API response with Kotlin coroutines?

I am completely new to Kotlin, Coroutines and API calls, and I am trying to make an app based on this API.
My intention is to display the information of a game in my MainActivity, so I need ot fill some TextView for that purpose.
My API call and response system works perfectly well: the responses are OK and there are no errors, but the call is made using Kotlin's coroutines which won't let me update my UI after getting the response.
For the sake of simplicity, I am only attaching my MainActivity code, which is the source of the problem.
class MainActivity : AppCompatActivity() {
private lateinit var b: ActivityMainBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
b = ActivityMainBinding.inflate(layoutInflater)
setContentView(R.layout.activity_main)
listAllGames()
}
private fun listAllGames() {
CoroutineScope(Dispatchers.IO).launch {
val call = getRetrofit().create(APIService::class.java).listAllGames("")
val games = call.body()
runOnUiThread {
if (call.isSuccessful) {
b.gameTitle.text = games?.get(0)?.title ?: "Dummy"
b.gameDesc.text = games?.get(0)?.short_description ?: "Dummy"
b.gameGenre.text = games?.get(0)?.genre ?: "Dummy"
}
else {
Toast.makeText(applicationContext, "ERROR", Toast.LENGTH_SHORT).show()
Log.d("mydebug", "call unsuccessful")
}
}
}
}
private fun getRetrofit(): Retrofit {
return Retrofit.Builder()
.baseUrl("https://www.freetogame.com/api/games/")
.addConverterFactory(GsonConverterFactory.create())
.build()
}
}
Concretely, the listAllGames() method is the problem here: the app will build successfully, if a breakpoint is added inside the if (call.isSuccessful) block, it'll show the data correctly; but when running the app the display will be left blank forever.
Thanks to all in advance!

In order to use view binding you need to pass the inflated view from the binding to the setContentView method. Otherwise you inflate the view with the binding but display the XML layout without the binding.
Check the documentation here:
private lateinit var binding: ResultProfileBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ResultProfileBinding.inflate(layoutInflater)
val view = binding.root
setContentView(view)
}
(Source: Android Developer documentation - " View Binding Part of Android Jetpack.")
Change your onCreate method as followed:
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
b = ActivityMainBinding.inflate(layoutInflater)
setContentView(b.root)
listAllGames()
}

The other answer explains your problems with your view, but you also have some issues with the coroutine.
You should use lifecycleScope instead of creating a new scope. lifecycleScope will automatically cancel itself to avoid leaking the Activity if the activity is destroyed, such as during a screen rotation.
You should use Retrofit's call.await() suspend function instead of directly blocking a thread and having to specify a dispatcher. This also lets you leave things on the main dispatcher and directly update UI without having to use runOnUiThread.
private fun listAllGames() {
lifecycleScope.launch {
val call = getRetrofit().create(APIService::class.java).listAllGames("")
try {
val games = call.await()
b.gameTitle.text = games?.get(0)?.title ?: "Dummy"
b.gameDesc.text = games?.get(0)?.short_description ?: "Dummy"
b.gameGenre.text = games?.get(0)?.genre ?: "Dummy"
} catch (e: Exception) {
Toast.makeText(applicationContext, "ERROR", Toast.LENGTH_SHORT).show()
Log.d("mydebug", "call unsuccessful", e)
}
}
}
And actually, you should move API calls like this into a ViewModel so they can keep running during a screen rotation. If you do that, the function in the ViewModel should update a LiveData or SharedFlow instead of UI elements. Then your Activity can observe for changes. If the call is started and the screen is rotated, the API call will keep running and still publish its changes to the new Activity's UI without having to restart.

.setActivity() method in fragment

I'm creating firebase phone-auth application. (I am learning firebase and navigation controller side by side).
I've 1 activity having NavHostFragment in it, I have 2 fragments (1. for getting phone number, 2. for entering and validating OTP)
There is nothing in my activity. I've crated navigation graph etc. perfectly, and everything (related with navigation controller) is working fine.
So after I added Firebase phone-auth in my fragment-1, I released that the activity does nothing other than controlling fragments.(Question is in the end. See que(2))
Also what about
.setActivity(this) // Activity (for callback binding)
We can't use this in fragment.
My questions are:
Alternative of .setActivity(this) in fragments (kotlin)
Is this correct way, or I should implement it in activity by sending values from fragment to activity.
For detailed code:
class IdentityFragment : Fragment() {
private var _binding: FragmentIdentityBinding? = null
private val binding get() = _binding!!
lateinit var auth: FirebaseAuth
lateinit var storedVerificationId:String
lateinit var resendToken: PhoneAuthProvider.ForceResendingToken
private lateinit var callbacks: PhoneAuthProvider.OnVerificationStateChangedCallbacks
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
auth=FirebaseAuth.getInstance()
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
binding.etPhone.requestFocus()
binding.btnSendotp.setOnClickListener {
sendOTP()
}
}
private fun sendOTP() {
var phone=binding.etPhone.text.toString().trim()
if(!phone.isEmpty()) {
phone = "+91" + phone
sendVerificationCode(phone)
}
}
private fun sendVerificationCode(phone: String) {
val options = PhoneAuthOptions.newBuilder(auth)
.setPhoneNumber(phone) // Phone number to verify
.setTimeout(60L, TimeUnit.SECONDS) // Timeout and unit
.setActivity()// problem is here
.setCallbacks(callbacks) // OnVerificationStateChangedCallbacks
.build()
PhoneAuthProvider.verifyPhoneNumber(options)
}
override fun onDestroyView() {
super.onDestroyView()
_binding = null
}
}

In a Fragment, You can use the getActvity() or requireActivity() method to get the activity instance to which this fragment is attached currently.
setActivity(this.requireActivity())//for kotlin getActivity() available in property access as activity
You can still have the callback in the fragment, where you will get the callback of success or failure.

When to call livedata observer?

On a button click i have to get some value from API call and then launch one screen. I have two options:
Call the observer each time when user will click on button.
Call the observer on fragment onActivityCreated() and store the value in variable and act accordingly on button click.
So which approach I should follow?

Actually it's up to you. But i always prefer to call it in Activity's onCreate() function, so activity only has 1 observer. If you call it in button click, it will give you multiple observers as much as button clicking
Here is some example :
class HomeProfileActivity: BaseActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
initObserver()
initView()
}
private fun initObserver() {
viewModel.profileWorkProccess.observe(this, {
swipeRefreshLayout.isRefreshing = it
})
viewModel.isLoadingJobs.observe(this, {
layoutProgressBarJobs.visibility = View.VISIBLE
recyclerViewJobs.visibility = View.GONE
dotsJobs.visibility = View.GONE
})
//other viewmodel observing ......
}
private fun initView() {
imageProfile.loadUrl(user.image, R.drawable.ic_user)
textName.text = identity.user?.fullName
textAddress.text = identity.user?.city
buttonGetData.setOnClickListener { viewModel.getData(this) }
}
}

If the button is placed on the Activity, and data is displayed in the Fragment, you need to store variable in Activity ViewModel and observe it in Fragment
You only need to call observe one time when fragment is created.
For example:
class MyActivity : AppCompatActivity() {
val viewModel: MyActViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
myButton.setOnClickListener { view ->
viewModel.getData()
}
}
}
class MyActViewModel: ViewModel {
val data: LiveData<String> = MutableLiveData()
fun getData() {}
}
class MyFragment: Fragment {
val actViewModel: MyActViewModel by activityViewModels()
override fun onActivityCreated(...) {
....
actViewModel.data.observe(viewLifecycleOwner, Observer { data ->
...
}
}
}

State Management with ViewModel on Android when on a flow

Basically I have a state management system using ViewModel that looks like this:
class ViewModelA: ViewModel() {
private val repository: RepositoryA by inject()
private val _stateLiveData = MutableLiveData<ViewState>()
val stateLiveData: LiveData<ViewState> get() = _stateLiveData
private val _eventLiveData = SingleLiveEvent<ViewEvent>()
val eventLiveData: LiveData<ViewEvent> get() = _eventLiveData
private val exceptionHandler = CoroutineExceptionHandler { _, _ ->
_stateLiveData.postValue(ViewState.Error)
}
fun loadList() {
if (_stateLiveData.value is ViewState.Loading) return
launch(exceptionHandler) {
_stateLiveData.run {
value = ViewState.Loading
value = repository.getDocumentList().let {
if (it.isEmpty()) ViewState.Error
else ViewState.Data(it)
}
}
}
}
}
But whenever I am sharing a ViewModel with several Fragments, it becomes bigger and bigger. I am looking for a solution for this, because I don't want to centralize all the logic for an entire application flow inside a ViewModel and I also don't want to pass arguments here and there all the time.
PS: Sorry about my bad english.
Edit: Clarify a bit the question.

I didn't quite understand your question. However, if your question was as follows:
How can I share the same ViewModel Object and use it inside multiple Fragments.
You can check the documentation of ViewModelProvider which is a utility class that provides ViewModels for a specific scope like Activity.
Following is an example code of the usage of ViewModelProvider within two Fragments that will be created and used in the same Activity object:
// An example ViewModel
class SharedViewModel : ViewModel() {
val intLiveData = MutableLiveData<Int>() // an example LiveData field
}
// the first fragment
class Fragment1 : Fragment() {
private lateinit var viewModel: SharedViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewModel = requireActivity().let { activity ->
ViewModelProvider(activity).get(SharedViewModel::class.java)
}
}
}
// the other fragment
class Fragment2 : Fragment() {
private lateinit var viewModel: SharedViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewModel = requireActivity().let { activity ->
ViewModelProvider(activity).get(SharedViewModel::class.java)
}
}
}

RecyclerView loses state on rotation

I am making a note-taking app using MVVM design pattern, Room for persisting data and paging
The problem is when I rotate my device it appears like it keeps its state for less than one second, then the RecyclerView scrolls up
I've debugged my code and found that onChanged() is called multiple times
here is my code
NoteRepository
override fun loadPagedNotes() : LiveData<PagedList<Note>> {
val factory : DataSource.Factory<Int, Note> = mNotesDao.getNotes()
val mNotesList = MutableLiveData<PagedList<Note>>()
val notesList = RxPagedListBuilder(
factory, PagedList.Config
.Builder()
.setPageSize(20)
.setEnablePlaceholders(true)
.build()
).buildFlowable(BackpressureStrategy.LATEST)
mDisposables.add(
notesList.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe {
mNotesList.value = it
}
)
return mNotesList
}
NoteViewModel
fun loadPagedNotes() : LiveData<PagedList<Note>> {
return mNoteRepository.loadPagedNotes()
}
HomeActivity
class HomeActivity : AppCompatActivity() {
private lateinit var mBinding : ActivityHomeBinding
private val mViewModel : NoteViewModel by viewModel()
override fun onCreate(savedInstanceState : Bundle?) {
super.onCreate(savedInstanceState)
mBinding = DataBindingUtil.setContentView(this, R.layout.activity_home)
loadNotes()
}
private fun loadNotes() {
mViewModel.loadPagedNotes()
.observe(this,
Observer {
if (it.isNotEmpty()) {
addNotesToRecyclerView(it)
}
})
}
private fun addNotesToRecyclerView(list : PagedList<Note>?) {
showRecyclerView()
val adapter = PagedNoteListAdapter(this#HomeActivity)
adapter.submitList(list)
mBinding.notesRecyclerView.adapter = adapter
}
}

When you rotate your device, the view (fragment) is destroyed and re-created, but the ViewModel remains. You're running into trouble because your ViewModel never leverages this feature: instead, on (re-)creation of your view, it causes the ViewModel to re-fetch the data all over again. Each invocation of the VM's loadPagedNotes will create a new LiveData and return it. You really want to have just one and return a reference to that one every single time.
Why don't you change your code to be something like this
class NoteViewModel: ViewModel() {
val pagedNotes = mNoteRepository.loadPagedNotes()
}
and in your activity
override fun onCreate(/*...*/) {
mViewModel.pagedNotes.observe(this, Observer { addNotesToRecyclerView(it)
})
Also, consider moving this val adapter = PagedNoteListAdapter(this#HomeActivity) to an instance member of the Activity: val mAdapter = .... That way, your addNotesToRecyclerView code will not create and bind a new adapter every time there is an update from your repository.
class HomeActivity: /*...*/ {
val mAdapter = PagedNoteListAdapter(this)
override fun onCreate(...) {
...
mBinding.notesRecyclerView.adapter = mAdapter
}
private fun addNotesToRecyclerView(list: PagedList<Note>) // You don't need the question mark since you're never calling the function except when you already know `list` isn't null) {
mAdapter.submitList(list)
}
That could be another reason you're experiencing this.
Also if you still have undesirable refreshing, look into DiffUtil. It enables you to update items of an adapter if they've individually changed, so even if the overall list changes but some elements stay the same, you'll only redraw the changed elements without redrawing the ones that haven't changed (e.g., if you've added one new item it won't redraw everything but just draw the one new item)

I'm not a RxJava expert so I can't speak to that too much but have you tried getting your ViewModel from the ViewModelProviders class instead:
myViewModel = ViewModelProviders.of(this)[MyViewModel::class.java]
I believe creating a ViewModel instance this way allows your ViewModel to live outside of your Activities lifecycle and may help with your state issue.