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Multiple instances of Livedata observers

I am struggling to find a solution for calling a livedata observer multiple times in one activity and not creating multiple instances of it, this leads to the problem when the database changes I got callbacks from all the instances.
ViewModel
public class RatingsViewModel extends AndroidViewModel {
private RatingsRepository ratingsRepository;
private LiveData<List<Rating>> ratingsList;
public RatingsViewModel(Application application) {
super(application);
ratingsRepository = new RatingsRepository(application);
}
public LiveData<List<Rating>> getRatingsByDate(LocalDate date) {
ratingsList = ratingsRepository.getActivitiesByDate(date);
return ratingsList;
}
Activity
private void getRatingsByDate(LocalDate date) {
ratingsViewModel.getRatingsByDate(date).observe(this, activities -> {
// list populating stuff
});
}
I tried calling hasObserver() but it returns false so I cannot remove the observers.

You should be able to do something like following (in Kotlin but should be easily translatable in to Java if needed)
val dateLiveData: MutableLiveData<Date> = MutableLiveData()
val ratingsList = MediatorLiveData<List<Rating>>().apply {
this.addSource(dateLiveData) {
this.value = ratingsRepository.getActivitiesByDate(dateLiveData.value)
}
}
fun setDate(date: Date) {
dateLiveData.value = date
}
You'd call observe from onCreate() for example in your activity/fragment and then call setDate() when that value changes.

Android ViewModel: Should I "borrow" the observe() method from LiveData like in the official example?

When working with ViewModels the View observes the ViewModel. It has to register as an observer. In the official tutorial of Google this registration is delegated to the observe() method of a LiveData object.
public class MyViewModel extends ViewModel {
private MutableLiveData<List<User>> users;
public LiveData<List<User>> getUsers() {
if (users == null) {
users = new MutableLiveData<List<Users>>();
loadUsers();
}
return users;
}
private void loadUsers() {
// Do an asynchronous operation to fetch users.
}
}
public class MyActivity extends AppCompatActivity {
public void onCreate(Bundle savedInstanceState) {
// Create a ViewModel the first time the system calls an activity's onCreate() method.
// Re-created activities receive the same MyViewModel instance created by the first activity.
MyViewModel model = ViewModelProviders.of(this).get(MyViewModel.class);
model.getUsers().observe(this, users -> {
// update UI
});
}
}
The method getUsers() returns the LiveData object itself. It's observe() method is used to register the observer. The View does not observe the the ViewModel but a part of it's implementation.
Now is this best practice, when working with ViewModels not to observe themselves but parts of their implementation in form of LiveData objects? Or is this an introduction of low quality?

Based on the answer of Chris I give my own answer. I think the tutorial is not best practice for the simple reason, that an object should not expose it's internal implementation. Base on the argumentation of Chris I was looking for an option to get encapsulation without losing the named features. The result is the method observerUsers() which delegates to a LiveData object internally.
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MainActivityViewModel model = ViewModelProviders.of(this).get(MainActivityViewModel.class);
model.observeUsers(this,
new Observer<List<User>>() {
#Override
public void onChanged(#Nullable List<User> users) {
updateUI();
}
}
);
}
void updateUI() {
}
static class MainActivityViewModel extends ViewModel {
private MutableLiveData<List<User>> users;
public void observeUsers(#NonNull LifecycleOwner owner,
#NonNull Observer<List<User>> observer) {
getUsers().observe(owner, observer);
}
private LiveData<List<User>> getUsers() {
if (users == null) {
users = new MutableLiveData<>();
loadUsers();
}
return users;
}
private void loadUsers() {
// Do an asynchronous operation to fetch users.
}
}
static class User {
}
}
Still List<User> exposes internal implementation. It could be improved to a class Users.
I put everything into one file and use inner static class. This is not meant as best practice. It was just to be able to quickly edit everything within one file. Especially the model User belongs into it's very own file, while I often put the ViewModel into the View class it belongs to like this.
My second point of critic matches the case that the ViewModel itself observes an underlying model. In this case the observer method onChange() is very general and requires a very general update method like updateUI(). You may want to observe more specific events of the model to do specific updates.

I'd say yes it's best practice for the ViewModel to expose its data through some form of Observable, whether that be LiveData or something like an RX Observable.
This breaks from other architectures such as MVP where the presenter would typically have a reference to the View which gets called when something changes. The guidelines are quite specific about what a ViewModel should reference.
A ViewModel must never reference a view, Lifecycle, or any class that may hold a reference to the activity context.
By exposing your data as an Observable through the ViewModel this means views can come and go, and once subscribed will receive the latest data and any subsequent updates. Again the guidelines have some detail.
If the activity is re-created, it receives the same MyViewModel instance that was created by the first activity. When the owner activity is finished, the framework calls the ViewModel objects's onCleared() method so that it can clean up resources
https://developer.android.com/topic/libraries/architecture/viewmodel.html

AndroidViewModel - Making duplicate calls doesn't return data in observe function

My question is related to ViewModel second time returns null wherein I am not getting callback inobserve function if I make a repeated call to server. Following is the code I am using -
#Singleton
public class NetworkInformationViewModel extends AndroidViewModel {
private LiveData<Resource<NetworkInformation>> networkInfoObservable;
private final APIClient apiClient;
#Inject
NetworkInformationViewModel(#NonNull APIClient apiClient, #NonNull Application application) {
super(application);
this.apiClient = apiClient;
getNetworkInformation();
}
public LiveData<Resource<NetworkInformation>> getNetworkInfoObservable() {
return networkInfoObservable;
}
// making API calls and adding it to Observable
public void getNetworkInformation() {
networkInfoObservable = apiClient.getNetworkInformation();
}
}
In Activity, the ViewModel is defined as followed -
final NetworkInformationViewModel networkInformationViewModel =
ViewModelProviders.of(this, viewModelFactory).get(NetworkInformationViewModel.class);
observeViewModel(networkInformationViewModel);
The observeViewModel function is used to add observable on ViewModel.
public void observeViewModel(final NetworkInformationViewModel networkInformationViewModel) {
networkInformationViewModel.getNetworkInfoObservable()
.observe(this, networkInformationResource -> {
if (networkInformationResource != null) {
if (networkInformationResource.status == APIClientStatus.Status.SUCCESS) {
Timber.d("Got network information data");
} else {
final Throwable throwable = networkInformationResource.throwable;
if (throwable instanceof SocketTimeoutException) {
final NetworkInformation networkInformation = networkInformationResource.data;
String error = null;
if (networkInformation != null) {
error = TextUtils.isEmpty(networkInformation.error) ? networkInformation.reply : networkInformation.error;
}
Timber.e("Timeout error occurred %s %s", networkInformationResource.message, error);
} else {
Timber.e("Error occurred %s", networkInformationResource.message);
}
if (count != 4) {
networkInformationViewModel.getNetworkInformation();
count++;
// Uncommenting following line enables callback to be received every time
//observeViewModel(networkInformationViewModel);
}
}
}
});
}
Uncommenting the following line in above function allows the callback to come everytime, but there has to be a proper way of doing this.
//observeViewModel(networkInformationViewModel);
Please note:-
I don't need RxJava implementation for implementing this.

Right now in getNetworkInformation() you are:
Creating a new LiveData
Updating the the LiveData using setValue
Instead, you should have a single LiveData for APIClient created as a member variable, then in getNetworkInformation() just update that member LiveData.
More generally, your APIClient is a data source. For data sources, you can have them contain member LiveData objects that update when the data changes. You can provide getters to those LiveData objects to make them accessible in ViewModels, and ultimately listen to them in your Activities/Fragments. This is similar how you might take another data source, such as Room, and listen to a LiveData returned by Room.
So the code in this case would look like:
#Singleton
public class APIClient {
private final MutableLiveData<Resource<NetworkInformation>> mNetworkData = new MutableLiveData<>(); // Note this needs to be MutableLiveData so that you can call setValue
// This is basically the same code as the original getNetworkInformation, instead this returns nothing and just updates the LiveData
public void fetchNetworkInformation() {
apiInterface.getNetworkInformation().enqueue(new Callback<NetworkInformation>() {
#Override
public void onResponse(
#NonNull Call<NetworkInformation> call, #NonNull Response<NetworkInformation> response
) {
if (response.body() != null && response.isSuccessful()) {
mNetworkData.setValue(new Resource<>(APIClientStatus.Status.SUCCESS, response.body(), null));
} else {
mNetworkData.setValue(new Resource<>(APIClientStatus.Status.ERROR, null, response.message()));
}
}
#Override
public void onFailure(#NonNull Call<NetworkInformation> call, #NonNull Throwable throwable) {
mNetworkData.setValue(
new Resource<>(APIClientStatus.Status.ERROR, null, throwable.getMessage(), throwable));
}
});
}
// Use a getter method so that you can return immutable LiveData since nothing outside of this class will change the value in mNetworkData
public LiveData<Resource<NetworkInformation>> getNetworkData(){
return mNetworkData;
}
}
Then in your ViewModel...
// I don't think this should be a Singleton; ViewModelProviders will keep more than one from being instantiate for the same Activity/Fragment lifecycle
public class SplashScreenViewModel extends AndroidViewModel {
private LiveData<Resource<NetworkInformation>> networkInformationLiveData;
#Inject
SplashScreenViewModel(#NonNull APIClient apiClient, #NonNull Application application) {
super(application);
this.apiClient = apiClient;
// Initializing the observable with empty data
networkInfoObservable = apiClient.getNetworkData()
}
public LiveData<Resource<NetworkInformation>> getNetworkInfoObservable() {
return networkInformationLiveData;
}
}
Your activity can be the same as you originally coded it; it will just get and observe the LiveData from the ViewModel.
So what is Transformations.switchMap for?
switchMap isn't necessary here because you don't need to change the underlying LiveData instance in APIClient. This is because there's really only one piece of changing data. Let's say instead your APIClient needed 4 different LiveData for some reason, and you wanted to change which LiveData you observed:
public class APIClient {
private MutableLiveData<Resource<NetworkInformation>> mNetData1, mNetData2, mNetData3, mNetData4;
...
}
Then let's say that your fetchNetworkInformation would refer to different LiveData to observe depending on the situation. It might look like this:
public LiveData<Resource<NetworkInformation>> getNetworkInformation(int keyRepresentingWhichLiveDataToObserve) {
LiveData<Resource<NetworkInformation>> currentLiveData = null;
switch (keyRepresentingWhichLiveDataToObserve) {
case 1:
currentLiveData = mNetData1;
break;
case 2:
currentLiveData = mNetData2;
break;
//.. so on
}
// Code that actually changes the LiveData value if needed here
return currentLiveData;
}
In this case the actual LiveData coming from getNetworkInformation is changes, and you're also using some sort of parameter to determine which LiveData you want. In this case, you'd use a switchMap, because you want to make sure that the observe statement you called in your Activity/Fragment observes the LiveData returned from your APIClient, even if you change the underlying LiveData instance. And you don't want to call observe again.
Now this is a bit of an abstract example, but it's basically what your calls to a Room Dao do -- if you have a Dao method that queries your RoomDatabase based on an id and returns a LiveData, it will return a different LiveData instance based on the id.

I didn't met the same issue, but i came across a similar thing where the number of observers were increasing each time i was saving the data in db. The way i debugged was how many instances or different instances of observers were getting invoked and i came to know that when you are fetching the live data from view model it needs to be checked for non null or you can say only 1 instance is being returned -
private LiveData<T> data;
public LiveData<T> getLiveData(){
if(data ==null){
data = //api call or fetch from db
}
return data;
}
Before i was simply returning the data object and then after checking the source i came to the conclusion that livedata automatically updates your object and everytime without the null check new instance was getting created and new observers were getting attached. Someone can correct me if my understanding regarding livedata is wrong.

I have already updated the linked question's answer. Re-posting here since I have placed a bounty on the question and hopefully someone will verify that this is the proper way to handle the issue.
Following is the updated working solution -
#Singleton
public class SplashScreenViewModel extends AndroidViewModel {
private final APIClient apiClient;
// This is the observable which listens for the changes
// Using 'Void' since the get method doesn't need any parameters. If you need to pass any String, or class
// you can add that here
private MutableLiveData<Void> networkInfoObservable;
// This LiveData contains the information required to populate the UI
private LiveData<Resource<NetworkInformation>> networkInformationLiveData;
#Inject
SplashScreenViewModel(#NonNull APIClient apiClient, #NonNull Application application) {
super(application);
this.apiClient = apiClient;
// Initializing the observable with empty data
networkInfoObservable = new MutableLiveData<Void>();
// Using the Transformation switchMap to listen when the data changes happen, whenever data
// changes happen, we update the LiveData object which we are observing in the MainActivity.
networkInformationLiveData = Transformations.switchMap(networkInfoObservable, input -> apiClient.getNetworkInformation());
}
/**
* Function to get LiveData Observable for NetworkInformation class
* #return LiveData<Resource<NetworkInformation>>
*/
public LiveData<Resource<NetworkInformation>> getNetworkInfoObservable() {
return networkInformationLiveData;
}
/**
* Whenever we want to reload the networkInformationLiveData, we update the mutable LiveData's value
* which in turn calls the `Transformations.switchMap()` function and updates the data and we get
* call back
*/
public void setNetworkInformation() {
networkInfoObservable.setValue(null);
}
}
The Activity's code will be updated as -
final SplashScreenViewModel splashScreenViewModel =
ViewModelProviders.of(this, viewModelFactory).get(SplashScreenViewModel.class);
observeViewModel(splashScreenViewModel);
// This function will ensure that Transformation.switchMap() function is called
splashScreenViewModel.setNetworkInformation();
Watch her droidCon NYC video for more information on LiveData. The official Google repository for LiveData is https://github.com/googlesamples/android-architecture-components/ look for GithubBrowserSample project.
The apiClient.getNetworkInformation() call doesn't need it any parameters to get additional information. Hence, the 'Void' added in MutableLiveData.
public LiveData<Resource<NetworkInformation>> getNetworkInformation() {
final MutableLiveData<Resource<NetworkInformation>> data = new MutableLiveData<>();
apiInterface.getNetworkInformation().enqueue(new Callback<NetworkInformation>() {
#Override
public void onResponse(
#NonNull Call<NetworkInformation> call, #NonNull Response<NetworkInformation> response
) {
if (response.body() != null && response.isSuccessful()) {
data.setValue(new Resource<>(APIClientStatus.Status.SUCCESS, response.body(), null));
} else {
data.setValue(new Resource<>(APIClientStatus.Status.ERROR, null, response.message()));
}
}
#Override
public void onFailure(#NonNull Call<NetworkInformation> call, #NonNull Throwable throwable) {
data.setValue(
new Resource<>(APIClientStatus.Status.ERROR, null, throwable.getMessage(), throwable));
}
});
return data;
}

Making changes to LiveData to "redo" work in ViewModel

So, I have just started experimenting with LiveData - I am busy with a new project, where I am using ViewModel as well as LiveData - with some of the RESTFul services I use to fetch data, they take no parameters and return some data.
A typical setup of the MVVM paradigm with LiveData looks much like this:
public class MyActivity extends AppCompatActivity {
public void onCreate(Bundle savedInstanceState) {
MyViewModel model = ViewModelProviders.of(this).get(MyViewModel.class);
model.getUsers().observe(this, users -> {
// update UI
});
}
}
Now when we leave this activity, and go to a new activity, by using an Intent or some other means, and not pressing the back button (So, finalize is not called) - and then come back to MyActivity - we of course don't fetch the users again, as we should still have that data.
However, what if we did want to fetch them again?
The only way to do this properly, from what I have looked at, seems to call "setValue" on the getUsers() LiveData object
Something like this:
public class MyActivity extends AppCompatActivity {
public void onResume() {
viewModel.setActive(true);
}
}
And the ViewModel would look like this:
private final MutableLiveData<Boolean> activeLiveData = new MutableLiveData<>();
ViewModel(ViewModelRepo repo){
this.repo = repo;
results = Transformations.switchMap(activeLiveData, active ->{
if(active){
return repo.getUsers();
}else {
return AbsentLiveData.create(); //"Null live data"
}
});
}
LiveData<Users>> getUsers() {
return results;
}
//This could be called "update" with no params
void setActive(boolean active) {
activeLiveData.setValue(active);
}
The one reason I have decided to do it like this is because Google does not want us doing this:
class MyViewModel extends ViewModel {
private final PostalCodeRepository repository;
public MyViewModel(PostalCodeRepository repository) {
this.repository = repository;
}
private LiveData<String> getPostalCode(String address) {
// DON'T DO THIS
return repository.getPostCode(address);
}
}
For this reason:
If this is the implementation, the UI would need to unregister from
the previous LiveData and re-register to the new instance each time
they call getPostalCode(). Moreover, if the UI is re-created, it
triggers another call to repository.getPostCode() instead of using the
previous call’s result.
Is there a better way to get the ViewModel to "redo" its repo.getUsers() call? Perhaps I could just make a method that says "Update()" instead of "active" but still - its doing the same thing differently.

Well here you're doing the fetching in the creator of the ViewModel, which locks things in place. Usually they'd advise to fetch the data in the getter, if the data is not there already.
So a good option would be to use the regular pattern first :
private MutableLiveData<Users> users = null;
ViewModel(ViewModelRepo repo){
this.repo = repo;
}
LiveData<Users> getUsers() {
if (users = null) {
fetchUsers();
}
return users;
}
public void fetchUsers() {
users.postValue(repo.getUsers());
}
And then from your Activity/Fragment, whenever you feel necessary to "refresh the users", you'd simply call viewModel.fetchUsers();

Room - LiveData observer does not trigger when database is updated

I am trying to find out in the code below, why is it that Room's LiveData observable does not give me new shifts once I populate the database with new data.
This is put on my activity's onCreate method:
shiftsViewModel = ViewModelProviders.of(this).get(ShiftsViewModel.class);
shiftsViewModel
.getShifts()
.observe(this, this::populateAdapter);
This is the populateAdapter method:
private void populateAdapter(#NonNull final List<Shift> shifts){
recyclerView.setAdapter(new SimpleItemRecyclerViewAdapter(shifts));
}
I also have the following code that populates the database (I use RxJava to do the work on an IO thread since Room needs its code to be called outside the main thread):
#Override
public Observable<List<Shift>> persistShifts(#NonNull final List<Shift> shifts){
return Observable.fromCallable(() -> {
appDatabase.getShiftDao().insertAll(shifts);
return shifts;
})
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread());
}
The problem I have occurs when I call persistShifts after I start observing my shiftsViewModel. I would expect that my observer (LiveData) would be triggered with all the newly added shifts. It turns out the observer is triggered, but an empty list of shifts is returned instead. The only way to make it "work" is if I leave the activity (therefore destroying the current ViewModel) and enter again. This time the viewModel's LiveData gives me all the shifts previously persisted, as expected.
Here is the rest of the code:
#Entity
public class Shift{
#PrimaryKey
private long id;
private String start;
private String end;
private String startLatitude;
private String startLongitude;
private String endLatitude;
private String endLongitude;
private String image;
...
DAO:
#Dao
public interface ShiftDAO {
#Query("SELECT * FROM shift")
LiveData<List<Shift>> getAll();
#Query("SELECT * FROM shift WHERE id = :id")
LiveData<Shift> getShiftById(long id);
#Insert(onConflict = OnConflictStrategy.REPLACE)
void insertAll(List<Shift> shifts);
}
ViewModel:
public class ShiftsViewModel extends AndroidViewModel{
private final ISQLDatabase sqlDatabase;
private MutableLiveData<Shift> currentShift;
private LiveData<List<Shift>> shifts;
private boolean firstTimeCreated;
public ShiftsViewModel(final Application application){
super(application);
this.sqlDatabase = ((ThisApplication) application).getSQLDatabase();
this.firstTimeCreated = true;
}
public MutableLiveData<Shift> getCurrentlySelectedShift(){
if(currentShift == null){
currentShift = new MutableLiveData<>();
}
return currentShift;
}
public LiveData<List<Shift>> getShifts() {
if(shifts == null){
shifts = sqlDatabase.queryAllShifts();
}
return shifts;
}
public void setCurrentlySelectedShift(final Shift shift){
currentShift = getCurrentlySelectedShift();
currentShift.setValue(shift);
}
public boolean isFirstTimeCreated(){
return firstTimeCreated;
}
public void alreadyUsed(){
firstTimeCreated = false;
}
}
Why am I not getting the list of shifts I persist in the observe() callback straightaway?

I had a similar problem using Dagger 2 that was caused by having different instances of the Dao, one for updating/inserting data, and a different instance providing the LiveData for observing. Once I configured Dagger to manage a singleton instance of the Dao, then I could insert data in the background (in my case in a Service) while observing LiveData in my Activity - and the onChange() callback would be called.
It comes down to the instance of the Dao must be the same instance that is inserting/updating data and providing LiveData for observation.

In my case, it was because I was using a MediatorLiveData to convert the entities returned from the database and forgot to call setValue() with the converted result, so the mediator was only relying requests to the database but never notifying results.
override fun getItems() = MediatorLiveData<List<Item>>().apply {
addSource(itemDao().getItems()) {
// I was previously only converting the items, without calling 'value ='
value = it.map(ItemWithTags::toDto)
}
}