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Trouble connecting RxPagingSource to a Volley call using Java

I am trying to Load and display paged data from Volley and having a little trouble connecting the data source to my backend service. Most examples are in Kotlin but I have been following the Android documentation and a LoopWiki example. Since the latter is using Retrofit and I am using Volley I have been trying to bounce between the two to come up with something.
This piece of code from the Android documentation shows the call but uses a hypothetical backend. I would like to plug my Volley call into this:
#NotNull
#Override
public Single<LoadResult<Integer, User>> loadSingle(
#NotNull LoadParams<Integer> params) {
// Start refresh at page 1 if undefined.
Integer nextPageNumber = params.getKey();
if (nextPageNumber == null) {
nextPageNumber = 1;
}
return mBackend.searchUsers(mQuery, nextPageNumber)
.subscribeOn(Schedulers.io())
.map(this::toLoadResult)
.onErrorReturn(LoadResult.Error::new);
}
When I have been using volley I have been passing in a listener and calling it
public void getData(Application application, ResultsListener listener) {
...
if (listener!=null)
listener.onReturn(arrayList);
}
public interface ResultsListener {
void onReturn(List<MyObject> objectList);
}
But this approach doesn't seem like a good fit here. I am somewhat new and have gotten in the habit of using listeners but I expect there is a better approach.
What do I need to do to get .subscribeOn, .map, and .OnErrorReturn on a search method in a class?
I have the paging functioning in my API.

Changes are not observable by viewmodel

I have created an app which is relied on my local server which fetch profile image and information about user..Code works fine without any problem but when I change my data in the local server (for example profile picture )the updated profile is not reflecting in the application until activity is restarted but this should not be happened because live data should reflect the change immediately as soon as changes occurred in the database.
below is the code of live data class
private MutableLiveData<Profile> profileMutableLiveData;
public void init(String token){
if (profileMutableLiveData!=null){
return;
}
repository=Repository.getInstance();
profileMutableLiveData=repository.getProfile(token);
}
public LiveData<Profile> getProfile(){
return profileMutableLiveData;
}
here is my Repository code
public class Repository {
private static Repository instance;
public static Repository getInstance(){
if (instance==null){
instance=new Repository();
}
return instance;
}
public MutableLiveData<Profile> getProfile(String token){
MutableLiveData<Profile> data=new MutableLiveData<>();
RetrofitApi retrofitApi=RetrofitInstance.getInstance();
Call<Profile> call=retrofitApi.getProfile(token);
call.enqueue(new Callback<Profile>() {
#Override
public void onResponse(Call<Profile> call, Response<Profile> response) {
Profile profile=response.body();
if (response.isSuccessful()){
data.setValue(profile);
}
}
#Override
public void onFailure(Call<Profile> call, Throwable t) {
}
});
return data;
}
}
Code in main activity to observe changes....
actually I am showing profile image in navigation drawer ... like telegram app
viewModelClass = new ViewModelProvider(this).get(ViewModelClass.class);
viewModelClass.init(token);
viewModelClass.getProfile().observe(this, new Observer<Profile>() {
#Override
public void onChanged(Profile profile) {
Picasso.get().load("http://192.168.43.216:8000" + profile.getProfile_photo()).into(profileImage);
fName = profile.getFirst_name();
lName = profile.getLast_name();
image = profile.getProfile_photo();
nameView.setText("Hello " + profile.getFirst_name());
}
});
}
The code is working fine but I want the data must be updated as soon as changes made in my server...
but data is updated when I restart the activity or opening app again after closing the activity...

May be the problem - is that you begin to observe in your activity one instance of MutableLiveData, and then you replace it with another one.
In your ViewModel:
profileMutableLiveData=repository.getProfile(token);
you override it instead of setting new value with "postValue"
In your Repository:
MutableLiveData<Profile> data=new MutableLiveData<>();
you make another instance of LiveData
You can try to change your return value from a Repository to a "Profile" and set it as a new value of MutableLiveData in your ViewModel with "postValue"
UPDATED
I've read your question more carefully. I think my answer above wouldn't give you what you expect (in case you expect Retrofit should update LiveData instantly like ROOM does)
So my thoughts:
You expect too much using LiveData+Retrofit. Just using them doesn't mean you'll get on-line updates of your data on your server. To achieve that you have to change mechanism of your interaction with your server, not just fix few lines in code you've shown.
There is mechanism LiveData+ROOM that works with local DB (Sqlite) in a way, that you expect from LiveData+Retrofit. But there is no magic there. Room is using mechanic, that built-in in Sqlite for notifying (triggering) when there are some changes in DB tables occur. But Retrofit doesn't implement similar mechanism with Rest Api and actually it's not its responsibility.
To achieve what you want you can look at several possibilities:
To use some Cloud Service API, that contains that built-in mechanism for notifying your device when data changes (Firebase, for example)
To implement some kind of periodic synchronisation of your app data with server. After this synchronisation you'll have all data on device and depending on where you put your data you could observe changes with LiveData+Room or FileObserver.
To simplify your case and refresh your data from the server at activity explicitly after click on Button "Refresh" on your activity. In that case you can implement steps that I wrote at first version of my answer.

Previous task(s) waiting on Firebase Realtime Database need to complete first before new one starts

I'm using the Task API in my app to retrieve data from Firebase Database, which is usually from different nodes. I have a helper class for Firebase Database like so:
public class FirebaseDbHelper {
public Task<DataSnapshot> getData() {
TaskCompletionSource<DataSnapshot> source = new TaskCompletionSource<>();
DatabaseReference dbRef = FirebaseDatabase.getInstance().getReference(FIRST_NODE).child(SUB_NODE);
dbRef.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
source.setResult(dataSnapshot);
}
#Override
public void onCancelled(DatabaseError databaseError) {
source.setException(databaseError.toException());
}
});
return source.getTask();
}
}
As you can see, getData() returns a Task object, which I use on my interactor class (I'm using the MVP architecture for my app) like so:
public class TestDbInteractor {
private FirebaseDbHelper mDbHelper;
private Listener mListener;
public TestDbInteractor(#NonNull Listener listener) {
mDbHelper = new FirebaseDbHelper();
mListener = listener;
}
void getData() {
mDbHelper.getData().addOnCompleteListener(task -> {
if (task.isSuccessful()) {
mListener.onGetDataSuccess(new MyObject(task.getResult()));
} else {
mListener.onGetDataFailed(task.getException());
}
});
}
public interface Listener {
void onGetDataSuccess(MyObject object);
void onGetDataFailed(Exception exception);
}
}
This works as expected. However, we noticed a behavior that when retrieving a lot of data, even if the activity that started the task is already finish()ed, the task still proceeds and attempts to complete. This I believe, is something that could be considered as a memory leak, since a process is still going even though it's supposed to be stopped/destroyed already.
What's worse is that when I try to get a different data (using a different Task in a different activity to a different node in Firebase), we noticed that it waits for the previous task to complete first before proceeding with this new one.
To give more context, we're developing a chat app similar to Telegram, where users could have multiple rooms and the behavior we saw is happening when a user enters a room. This is the flow:
User enters room, I request data for the room details.
Upon getting the room details, I display it, then request for the messages. I only retrieve the most recent 10. During this time, I just show a progress bar on the activity.
In order for the message details to be complete, I get data from different nodes on Firebase, this is where I use Tasks mainly.
After getting the messages, I pass it on to the View, to display the messages, then I attach a listener for new messages. Everything works as expected.
The behavior I mentioned at the beginning is noticeable when the user does something like this:
User enters a room with messages, room details are retrieved instantly, messages are still loading.
User leaves the room (presses the back button), this gets the user back to the room list, and enters a different one.
At this point, the retrieval of the room details takes such a long time - which we thought was odd, since the data isn't really that big to begin with.
After a few more testing, we concluded that the long retrieval time was caused by the current task (get room details) is still waiting for the previous task (get messages) started in a different activity, to finish first before starting.
I attempted to implement my answer here, trying to use a CancellableTask, but I am at a loss on how to use it with my current implementation, where I use a TaskCompletionSource, where you could only set a result or an exception.
I was thinking this could work if I move the task completion source to the interactor class level instead of the helper -- I haven't tried it yet. I think it's possible, but would take a lot of time to refactor the classes I already have.
So I figure why not try Doug's answer, using activity-scoped listeners. So I tested it like below.
In my activity, I added a getActivity() method, which can be called in the presenter:
public class TestPresenter
implements TestDbInteractor.Listener {
private View mView;
private TestDbInteractor mDbInteractor;
#Override
void bindView(View view) {
mView = view;
mDbInteractor = new TestDbInteractor(this);
}
#Override
void requestMessages() {
mDbInteractor.getData(mView.getActivity());
}
// Listener stuff below
}
and updated my getData() like so:
void getData(#NonNull Activity activity) {
mDbHelper.getData().addOnCompleteListener(activity, task -> {
if (task.isSuccessful()) {
mListener.onGetDataSuccess(new MyObject(task.getResult()));
} else {
mListener.onGetDataFailed(task.getException());
}
});
}
Unfortunately, this doesn't seem to work though, exiting the activity still waits for the tasks to complete, before the new task initiated in a different activity starts.

If you kick off a query to Realtime Database, it will always run to completion, whether or not there are any listeners attached to the Task that was returned. There is no way to cancel that work, neither by removing the last listener manually, nor by using activity-scoped listeners that are removed automatically. Queries in motion stay in motion. Also, all traffic to and from RTDB is pipelined over a single socket, which implies that the results of subsequent queries after one that's incomplete will have to wait for the everything ahead of it in the queue to complete first. This is likely the root cause for your observation - you have an incomplete query that other queries are waiting on, regardless of your use of the Task API.
Fortunately, if you have persistence enabled, the second query should be served by the cache of the first query, and not require another round trip to the server.
If you need to make sure that you retain the results of the first query across configuration changes that destroy the activity, then you should use something like LiveData from the Android architecture components to manage this, so that you can pick up the query where it left off after a configuration change. If you do this, don't use activity-scoped listeners.
I've written a three-part blog post about using architecture components with Firebase, which may also be of interest.

Hey You can use childEventListener. use dataSnapshot.getChildrenCount().
dbFriend=FirebaseDatabase.getInstance().getReference("Friend");
dbFriend=dbFriend.child(mPreferences.getString("username","")).child("already");
dbFriend.addChildEventListener(new ChildEventListener() {
int already=0;
#Override
public void onChildAdded(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
Username u=dataSnapshot.getValue(Username.class);
already=alread+1;
if(already >= dataSnapshot.getChildrenCount()){
//get to know when data fetching got completed
}
}
#Override
public void onChildChanged(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
}
#Override
public void onChildRemoved(#NonNull DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});

Android MVP - RxJava and retrofit - best approach

I'm figuring out how to develop an Android app, using MVP, RxJava2 and retrofit.
In my presenter, here is the code:
public void loadData() {
compositeDisposable.dataModelRepository.getDataList().subscribeOn(Schedulers.io())
.observeOn(mainScheduler).subscribe(new Consumer<List<Data>>() {
#Override
public void accept(List<Data> dataList) throws Exception {
if (!dataList.isEmpty())
view.displayData(dataList);
else
view.displayEmpty();
}
}, new Consumer<Throwable>() {
#Override
public void accept(Throwable throwable) throws Exception {
System.out.println(throwable.toString());
view.displayError("boooom");
}
});
}
Retrofit interface has been defined in the following way:
#GET("/fooURL")
Single<List<Data>> getDataList();
And the repository is just
public Single<List<Data>> getDataList() {
return retrofitApi.getDataList();
}
And it is working fine. Question is as follows: my intention is to fetch network data only when data is not available locally, in db.
Having this in mind, is it correct that schedulers are managed in the presenter? Or should they be managed in the Repository?
My guess is that presenter is the correct place, as it creates a thread so repository can do its stuff sequentially (fetch db, if nothing, then fetch network/cache; return data wherever it has been fetched), and when data is provided, notify the view inside the accept method of the Consumer.
Is it correct? Or should it be done in a different way?
Another point is: how can i test using Mockito the repository? The dataModelRepository.getDataList() method i mean? Not sure how to do any Assert for Single objects...
Thanks in advance!

I suggest you to offload all business logic that is related to fetching, retrieving data to a central repository.
One way to achieve somewhat similar to what you have described is to use a concat operator.
Observable<List<Data>> getData() {
return Observable
.concat(localRepository.getData(), remoteRepository.getData())
.first();
}
This will try to get data from your local repository first and if it has no data it will make a network request.
I assume your local and remote repositories will be observed on a new thread, but if you need to perform any action on the UI, simply subscribe on a main thread in your presenter.

Handling errors with RxJava and MVVM pattern in Android

I'm developing application wherein I want to use MVVM pattern. Currently, all events from xml are handled by the activity which pass them next to ViewModel. E.g. user clicks login button, the event is handled by activity; now the activity call view model's method, inside this method I'm calling RxFirebase (Rx wrapper on Firebase) method which returns Observable, subscribe to them and return it; in view I'm again subscribe to this observable for doing UI update. This situation is presented below.
My question is if this approach is correct? In my opinion, the better solution is to handle the error in ViewModel, but how then I can update UI? One of the solutions is to create interface, e.g. ShowMessageListener, next pass it to ViewModel and use to show message, but I prefer harness RxJava to this.
View method:
public void onLoginClick(View view) {
mBinding.clProgress.setVisibility(View.VISIBLE);
mViewModel.onLoginClick().subscribe(authResult -> {
mBinding.clProgress.setVisibility(View.GONE);
startAnotherActivity();
}, throwable -> {
mBinding.clProgress.setVisibility(View.GONE);
if (throwable instanceof FirebaseApiNotAvailableException) {
Snackbar.make(mBinding.getRoot(), R.string.google_play_services_unavilable, Snackbar.LENGTH_LONG).show();
} else {
Snackbar.make(mBinding.getRoot(), throwable.getMessage(), Snackbar.LENGTH_LONG).show();
}
});
}
ViewModel method:
public Observable<AuthResult> onLoginClick() {
Observable<AuthResult> observable = RxFirebaseAuth.signInWithEmailAndPassword(mAuth, mEmail.get(), mPassword.get());
observable.subscribe(authResult -> {
//save user
}, throwable -> {
//handle error
});
return observable;
}

Your answer is almost correct except that you should really seperate View and (Business)-Logic. This would be the attempt if you use databinding which is highly recommend when using Architecture Components.
That means that everything which updates the UI should be in your View, everything which is not relevant for the view should be in the ViewModel.
That means that you can pass your ViewModel to your Layout, which has a onClick and call the Method in the ViewModel. Example:
<?xml version="1.0" encoding="utf-8"?>
<layout ..>
<data><variable name="viewModel" type="YourVm" /></data>
<Button onClick="#{viewModel::onButtonClick}
</layout>
Now you can handle the onClick inside your ViewModel like
public void onClick(View view) {
Log.d("Click", "My Button was clicked");
}
If you "really" want to observe for errors from your View you could either Create an ObservableBoolean which is set to True onec there's an error and subscribe for changes. You can put it inside the ViewModel like:
public final ObservableBoolean observableError = new ObservableBoolean();
public void onClick(...) { observableError.set(true); }
Now you can observe the Boolean inside your View
yourViewModel.obserableError.observe(this, result -> {
// do your error stuff
});
If you don't use Databinding it's almost the same except that you pass a ClickListener to the Button.
Means that you listen for the OnClick in your View, call the "processing"-method in your ViewModel and update the ObservableBoolean if an error occured. Since your a Listening for changes you can process the SnackBar stuff inside your View.
Snackbar and everything which involves the view should really be seperated from the ViewModel except a navigator. In this case you should create WeakReferences to avoid leaks.
Take care that the ObservableBoolean is NOT part of RxJava. It's part of Architecture Components.
If you want to solve it using RxJava you could create a PublishSubject in your ViewModel like:
Viewmodel.java
public final PublishSubject<String> ps = PublishSubject.create<>()
public void onClick(...) { ps.next("my evil error string"); }
And finally Observe it in your view
myViewModel.ps.subscribe( data -> {...}, error -> { ... } )
Take care that you dispose your RxJava Subscriptions in onCleared() which is in your ViewModel interface.
Edit: I haven't tested the code since i have only Kotlin Projects at the moment but should work in java.
Found an issue in your code that you didnt validate if mBinding is null. This may be null since you subscribe for changes and try to create the SnackBar in the View which may be disposed already. Always use if (mBinding != null) Snackbar.snackysnacky(..)