I'm very new to Kotlin Flows. As the title suggests I basically have 2 Fragments that Shares a ViewModel. I want to send data between them using SharedFlow as a substitute for LiveData without retaining it's state.
Fragment A
class FragmentA: Fragment() {
private lateinit var viewModelShared: SharedViewModel
//Others//
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
uper.onViewCreated(view, savedInstanceState)
viewModelShared = ViewModelProvider(requireActivity())[SharedViewModel::class.java]
someView.setOnClickListener{
viewModelShared.sendData("Hello")}
//Fragment Navigates From Fragment A to B using NavController
navController.navigate(some_action_id)
}
}
}
Fragment B
class FragmentB: Fragment() {
private lateinit var viewModelShared: SharedViewModel
//Others//
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
viewModelShared = ViewModelProvider(requireActivity())[SharedViewModel::class.java]
lifecycleScope.launchWhenCreated {
viewModelMain.sharedFlow.collectLatest {
//Data or the word 'Hello' sent from Fragment A not being Received Here
}
}
}
}
SharedViewModel
class SharedViewModel:ViewModel() {
private val _sharedFlow= MutableSharedFlow<String>()
val sharedFlow= _sharedFlow.asSharedFlow()
fun sendData(data:String){
viewModelScope.launch {
_sharedFlow.emit(data)
}
}
}
Your FragmentB only collects when it is in started state or higher. This is correct because you don't want to be working with views when it's not visible and possibly doesn't currently have views.
However, since your SharedFlow has no replay history, this means there is nothing for FragmentB to collect when it is back on screen. Presumably it is off screen when FragmentA updates the flow.
So, your SharedFlow with no replay has nothing to do when it currently has no collectors, and the emitted value is thrown away. You need to have a replay of at least 1 for this to work.
private val _sharedFlow= MutableSharedFlow<String>(replay = 1)
You mentioned "without retaining its state", but this isn't possible without state if the two fragments are not on screen at the same time.
By the way, there is a simpler way to declare your shared ViewModel:
private val viewModelShared: SharedViewModel by activityViewModels()
Related
I'm facing an issue with my ViewModel that I use to hold user login data.
I update this ViewModel with user data from fragment A after a user logs in, but when I try to access the data from fragment B the data fields I just set are always null.
When fragment B is initialized the user LiveData field is never initially observed, however, when I trigger a change to the user object from fragment B the change is correctly observed within fragment B. It appears that the previous values of the fields in my ViewModel never reach fragment B, but new values do.
For a sanity check I made a simple string variable (not even a LiveData object) that I set to a value from fragment A, then, after navigating to fragment B I printed the value: it is uninitialized every time. It's as if the ViewModel I inject into fragment B is totally separate from the ViewModel I inject into fragment A.
What am I missing that causes the ViewModel observation in fragment B not to initially trigger with the last known value of user set from fragment A?
Fragment A
class FragmentA : Fragment() {
private val viewModel: LoginViewModel by viewModel()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewModel.user.observe(this, {
it?.let {
//Called successfully every time
navigateToFragmentB()
}
})
val mockUserData = User()
viewModel.loginSuccess(mockUserData)
}
}
Fragment B
class FragmentB : Fragment() {
private val viewModel: LoginViewModel by viewModel()
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
...
viewModel.user.observe(viewLifecycleOwner, { user ->
user?.let {
binding.initialsBubble.text = user.getInitials()
} ?: navigateAway()
})
}
}
ViewModel
class LoginViewModel(
private val loginRepo: LoginRepo
) : ViewModel() {
private val _user = MutableLiveData<User?>()
val user: LiveData<User?> = _user
fun loginSuccess(result: AuthenticationResult) {
val user = loginRepo.login(result)
_user.postValue(user)
}
}
You should use sharedViewModel for both fragment.
Use these lines of code in both fragments
private val viewModel: LoginViewModel by activityViewModels()
instead of
private val viewModel: LoginViewModel by viewModel()
An Activity opens fragments A,B,C,D,E,F,G,H,.... in a PageView2, which obviously slides back and forth through the fragments, all the fragments are using binding view layouts they only know about their own layout not each others, the Activity only knows it's own layout binding, so when a user changes widgets within the fragments how does that data get sent back to the Activity, in a collective way that one place can access all changes made to fragments A,B,C,D,E,F,G,H,.... so that the input can be saved.
The way I'd like it to work is;
User Clicks Edit
Makes alterations within the fragments
Chooses Apply or Cancel changes.
Well it works to a point, the problem is if the Fragments haven't been initialized, you get an instant crash, I presume I'm doing this wrong.
class mySharedViewModel : ViewModel() {
lateinit var udetails : FragmentEdcardsDetailsBinding
lateinit var uanswers : FragmentEdcardsAnswersBinding
lateinit var umath : FragmentEdcardsMathBinding
lateinit var uanimimage : FragmentEdcardsMediaAnimimageBinding
lateinit var ufullscreen : FragmentEdcardsMediaFullscreenimageBinding
lateinit var uvideo : FragmentEdcardsMediaVideoBinding
lateinit var uaudio : FragmentEdcardsMediaAudioBinding
fun cardapply() {
mytools.debug("${udetails}" )
mytools.debug("${uanswers}" )
}
}
Edit 2
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
u = FragmentEdcardsDetailsBinding.bind(view)
model.udetails= u
model.udetailsinit = true
Created a workaround, my gut is still telling me this is way wrong! idea being when apply is press it checks if model.udetailinit is true, because testing an uninitialized udetail just results in crash.
This should be done using a shared ViewModel, you should create a ViewModel object in your Activity and then access this ViewModel using Activity scope in your fragments.
Define a ViewModel as
class MyViewModel : ViewModel() {
val action: MutableLiveData<String> = MutableLiveData()
}
In Activity create object of this ViewModel
class SomeActivity: AppCompatActivity(){
// create ViewModel
val model: MyViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
model.action.observe(viewLifecycleOwner, Observer<String> { action->
// Do something with action
})
}
}
And in your Fragments access the ViewModel from Activity Scope
class SomeFragment: Fragment() {
private val model: MyViewModel by activityViewModels()
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
// Change value of action and notify Activity
model.action.value = "SomeAction"
}
}
The problem: I'm facing a problem with fragments and shared viewmodel LiveData ... The problem that is there is a FragmentA that update data in shared viewmodel and observe for it's changes then display the result for the user inside FragmentA, FragmentA can launch new instance of FragmentA to get new data and display it so the fragment launch it self and old instance gets added to the back stack, until here nothing is wrong the new instance updates LiveData in viewModel and displays the new data perfectly the problem that is when i popUpBackstack() return to FragmentA old instance the data displays in it is the data that new FragmentA instance gets it which means that old FragmentA instance still observing the data even if i remove the observers ... this is general overview about the problem now i will show you fragments structure, the code and what the solution's that i'v tried.
Expected behavior: what i want to achieve is that when FragmentA launch it self and gets added to back stack stop observing the data in viewModel and when i back to it displays the old data that's all.
Fragment structure: i use one activity to hold all the fragments ... MainActivity have FindMoviesFragment inside it there is a viewPager which holds FragmentMovies which launches MovieDetailsFragment and inside it there is ViewPager also which holds the fragments that displays the data from MoviesViewModel it will get clear when you see the code below.
This code shows how MovieDetailsFragment initialize MoviesViewModel and updates data in viewmodel:
class MovieDetailsFragment:Fragment(R.layout.movie_details_fragment) {
private val args: MovieDetailsFragmentArgs by navArgs()
private val fragmentList:ArrayList<Fragment> by lazy {
arrayListOf(MovieDetailsOneFragment(args.movieId), MovieDetailsTwoFragment(),MovieDetailsThreeFragment())
}
private lateinit var pagerAdapter: ViewPagerAdapter
private lateinit var moviesViewModel: MoviesViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
moviesViewModel = ViewModelProvider(requireActivity()).get(MoviesViewModel::class.java)
//these two lines updates the data in viewmodel
moviesViewModel.getMovieDetails(args.movieId)
moviesViewModel.changeMovieID(args.movieId)
}
}
//---------------------MoviesViewModel------------------//
class MoviesViewModel constructor(private val repo:Repository):ViewModel() {
private val _currentMovieDetails = MutableLiveData<Resource<MovieDetails>>()
val currentMovieDetails :LiveData<Resource<MovieDetails>>
get() = _currentMovieDetails
fun getMovieDetails(movieID:Int) = viewModelScope.launch {
_currentMovieDetails.postValue(Resource.loading(null))
val result = repo.getMovieDetails(movieID)
if(result.isSuccessful){
_currentMovieDetails.postValue(Resource.success(result.body()))
}
else{
_currentMovieDetails.postValue(Resource.error(result.errorBody().toString(),null))
}
}
}
And inside MovieDetailsOne which is inside viewpager in MovieDetailsFragment i observe the data like this:
class MovieDetailsOneFragment(private val movieId: Int):Fragment(R.layout.movie_details_one) {
private lateinit var moviesViewModel:MoviesViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
//this is how i define viewModel(global scope)
moviesViewModel = ViewModelProvider(requireActivity()).get(MoviesViewModel::class.java)
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
//in this method i observe on changes in currentMovieDetils liveData
subscribeToObservers()
//i try to call this from on create and nothing changes too
}
}
Now What i tried is the following:
-Loacal scope for fragments
//Define viewModel like this in MovieDetilsFragment
moviesViewModel = ViewModelProvider(this).get(MoviesViewModel::class.java)
//And in MovieDetailsOne like this
moviesViewModel = ViewModelProvider(this).get(MoviesViewModel::class.java)
//and this doesn't work MovieDetailsOne don't observe any changes
-Observe once extinction function:
fun <T> LiveData<T>.observeOnce(lifecycleOwner: LifecycleOwner, observer: Observer<T>) {
observe(lifecycleOwner, object : Observer<T> {
override fun onChanged(t: T?) {
observer.onChanged(t)
removeObserver(this)
}
})
}
//and this doesn't work MovieDetailsOne don't observe first time it's lanches
I know i took so long to explain :D but I'm trying to give you clear idea and sorry for you time <3 ... if you want any additional information about the code or the problem comment down below.
Finally i achieve the behavior that i want by doing some steps which is:
Attach ViewPager childs using childFragmentManager which means that android will treat viewpager fragments as childs for viewPager holder:
//instead of doing this
pagerAdapter = ViewPagerAdapter(activity?.supportragmentManager, lifecycle, fragmentList)
//do this
pagerAdapter = ViewPagerAdapter(childFragmentManager, lifecycle, fragmentList)
Define local viewModel for parent fragment like this:
//instead of doing this
moviesViewModel = ViewModelProvider(requireActivity()).get(MoviesViewModel::class.java)
//do this
moviesViewModel = ViewModelProvider(this).get(MoviesViewModel::class.java)
Define parent fragment scope viewModel in childs fragment:
//instead of doing this
moviesViewModel = ViewModelProvider(requireActivity()).get(MoviesViewModel::class.java)
//do this
private val moviesViewModel:MoviesViewModel by viewModels(
{requireParentFragment()}
)
by doing these steps parent fragment will stop observing data when it's destroyed and child fragments also.
I have the single activity with several fragments on top, as Google recommends. In one fragment I wish to place a switch, and I wish to still know it's state when I come back from other fragments. Example: I am in fragment one, then I turn on the switch, navigate to fragment two or three, go back to fragment one and I wish to load that fragment with that switch in the on position as I left it.
I have tried to copy the examples provided by google advocates, just to see the code to fail hard and do nothing.
/////////////////////////////////////////////////////////////////
//Inside the first fragment:
class myFragment : Fragment() {
companion object {
fun newInstance() = myFragment()
}
private lateinit var viewModel: myViewModel
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
return inflater.inflate(R.layout.my_fragment, container, false)
}
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
**viewModel = ViewModelProvider(this, SavedStateVMFactory(this)).get(myViewModel::class.java)
//Here I was hoping to read the state when I come back.
switch_on_off.isChecked = viewModel.getSwRoundTimerInit()**
subscribeToLiveData() //To read liveData
switch_on_off.setOnCheckedChangeListener { _, isChecked ->
viewModel.setOnOff(isChecked)
}
}//End of onActivityCreated
//other code...
/////////////////////////////////////////////////////////////////////////
//On the fragment ViewModel
class myViewModel(private val **mState: SavedStateHandle**) : ViewModel() {
//SavedStateHandle Keys to save and restore states in the App
private val swStateKey = "SW_STATE_KEY"
private var otherSwitch:Boolean //other internal states.
//Init for the other internal states
init {
otherSwitch = false
}
fun getSwRoundTimerInit():Boolean{
val state = mState[swStateKey] ?: "false"
return state.toBoolean()
}
fun setOnOff(swValue:Boolean){
mState.set(swStateKey, swValue.toString())
}
}
This does not work. It always loads the default (off) value, as if the savedState is null all the time.
change
//fragment scope
viewModel = ViewModelProvider(thisSavedStateVMFactory(this)).get(myViewModel::class.java)
to
//activity scope
viewModel = activity?.let { ViewModelProviders.of(it,SavedStateVMFactory(this)).get(myViewModel::class.java) }
https://developer.android.com/topic/libraries/architecture/viewmodel#sharing
I have an activity, TabBarActivity that hosts a fragment, EquipmentRecyclerViewFragment. The fragment receives the LiveData callback but the Activity does not (as proofed with breakpoints in debugging mode). What's weird is the Activity callback does trigger if I call the ViewModel's initData method. Below are the pertinent sections of the mentioned components:
TabBarActivity
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
initVM()
setContentView(R.layout.activity_nav)
val equipmentRecyclerViewFragment = EquipmentRecyclerViewFragment()
supportFragmentManager
.beginTransaction()
.replace(R.id.frameLayout, equipmentRecyclerViewFragment, equipmentRecyclerViewFragment.TAG)
.commit()
navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener)
}
var eVM : EquipmentViewModel? = null
private fun initVM() {
eVM = ViewModelProviders.of(this).get(EquipmentViewModel::class.java)
eVM?.let { lifecycle.addObserver(it) } //Add ViewModel as an observer of this fragment's lifecycle
eVM?.equipment?.observe(this, loadingObserver)// eVM?.initData() //TODO: Not calling this causes Activity to never receive the observed ∆
}
val loadingObserver = Observer<List<Gun>> { equipment ->
...}
EquipmentRecyclerViewFragment
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
columnCount = 2
initVM()
}
//MARK: ViewModel Methods
var eVM : EquipmentViewModel? = null
private fun initVM() {
eVM = ViewModelProviders.of(this).get(EquipmentViewModel::class.java)
eVM?.let { lifecycle.addObserver(it) } //Add ViewModel as an observer of this fragment's lifecycle
eVM?.equipment?.observe(this, equipmentObserver)
eVM?.initData()
}
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
val view = inflater.inflate(R.layout.fragment_equipment_list, container, false)
if (view is RecyclerView) { // Set the adapter
val context = view.getContext()
view.layoutManager = GridLayoutManager(context, columnCount)
view.adapter = adapter
}
return view
}
EquipmentViewModel
class EquipmentViewModel(application: Application) : AndroidViewModel(application), LifecycleObserver {
var equipment = MutableLiveData<List<Gun>>()
var isLoading = MutableLiveData<Boolean>()
fun initData() {
isLoading.setValue(true)
thread { Thread.sleep(5000) //Simulates async network call
var gunList = ArrayList<Gun>()
for (i in 0..100){
gunList.add(Gun("Gun "+i.toString()))
}
equipment.postValue(gunList)
isLoading.postValue(false)
}
}
The ultimate aim is to have the activity just observe the isLoading MutableLiveData boolean, but since that wasn't working I changed the activity to observe just the equipment LiveData to minimize the number of variables at play.
To get same reference of ViewModel of your Activity you need to pass the same Activity instance, you should use ViewModelProviders.of(getActivity). When you pass this as argument, you receive instance of ViewModel that associates with your Fragment.
There are two overloaded methods:
ViewModelProvider.of(Fragment fragment)
ViewModelProvider.of(FragmentActivity activity)
For more info Share data between fragments
I put this code inside the onActivityCreated fragment, don't underestimate getActivity ;)
if (activity != null) {
globalViewModel = ViewModelProvider(activity!!).get(GlobalViewModel::class.java)
}
globalViewModel.onStop.observe(viewLifecycleOwner, Observer { status ->
Log.d("Parent Viewmodel", status.toString())
})
This code helps me to listening Parent ViewModel changes in fragment.
Just for those who are confused between definitions of SharedViewModel vs Making two fragments use one View Model:
SharedViewModel is used to share 'DATA' (Imagine two new instances being created and data from view model is being send to two fragments) where it is not used for observables since observables look for 'SAME' instance to take action. This means you need to have one viewmodel instance being created for two fragments.
IMO: Google should somehow mention this in their documentation since I myself thought that under the hood they are same instance where it is basically not and it actually now makes sense.
EDIT : Solution in Kotlin: 11/25/2021
In Your activity -> val viewModel : YourViewModel by viewModels()
In Fragment 1 - >
val fragmentViewModel =
ViewModelProvider(requireActivity() as YourActivity)[YourViewModel::class.java]
In Fragment 2 - >
val fragmentViewModel =
ViewModelProvider(requireActivity() as YourActivity)[YourViewModel::class.java]
This Way 2 fragments share one instance of Activity viewmodel and both fragments can use listeners to observe changes between themselves.
When you create fragment instead of getting viewModel object by viewModels() get it from activityViewModels()
import androidx.fragment.app.activityViewModels
class WeatherFragment : Fragment(R.layout.fragment_weather) {
private lateinit var binding: FragmentWeatherBinding
private val viewModel: WeatherViewModel by activityViewModels() // Do not use viewModels()
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
binding = FragmentWeatherBinding.inflate(inflater, container, false)
binding.viewModel = viewModel
// Observing for testing & Logging
viewModel.cityName.observe(viewLifecycleOwner, Observer {
Log.d(TAG, "onCreateView() | City name changed $it")
})
return binding.root
}
}
Kotlin Answer
Remove these two points in your function if you are using:
= viewModelScope.launch { }
suspend