To implement that you firstly need to create a class which has the status of the response ,
Loading which is before the fetching of the data and there you can set progress bar to visible then on success you would set the data to your adapter and right after your hide your progress bar and in the on failure one , you show the toast message error

• This is the generic class

class AuthResource<T>(
    var authStatus : AuthStatus? = null,
    var data : T,
    var msg : String? = null

)

fun <T> success(@Nullable data: T): AuthResource<T> {
    return AuthResource(
        AuthStatus.Success,
        data,
        null
    )
}

fun <T> Error(@NonNull msg: String?, @Nullable data: T) : AuthResource<T>? {
    return AuthResource(
        AuthStatus.ERROR,
        data,
        msg
    )
}

fun <T> loading(@Nullable data: T): AuthResource<T>? {
    return AuthResource(
        AuthStatus.LOADING,
        data,
        null
    )
}


enum class AuthStatus {
    Success, ERROR, LOADING
}

• This is my view model where i implement the authResource with the api response

class MainViewModel @Inject constructor( private var webAuth: WebAuth,
    private var favFoodDao: FavFoodDao,
    private var application: Application) : ViewModel() {

    /// you have to create MediatorLiveData with authresource which contains your modelclass 

    private var mediatorLiveData = MediatorLiveData<AuthResource<WrapLatestMeals>>()
    
    ///Here you return a livedata object 
    fun ObserverCountries(): LiveData<AuthResource<WrapCountries>> {
        var liveData = LiveDataReactiveStreams.fromPublisher(
            webAuth.getCountries()
                 ///onerrorreturn , rxjava operator which returns error in case 
                 ///of response failure 
                .onErrorReturn(object : Function<Throwable, WrapCountries> {
                    override fun apply(t: Throwable): WrapCountries {
                        var country = WrapCountries()
                        return country
                    }
                })
                .map(object : Function<WrapCountries, 
                     AuthResource<WrapCountries>> {
                    override fun apply(t: WrapCountries): 
                        AuthResource<WrapCountries> {
                        if(t.meals.isNullOrEmpty())
                        {
                            return Error(
                                "Error",
                                t
                            )!!
                        }
                        return success(t)
                    }
                })
                .subscribeOn(Schedulers.io())
        )

       
        //add that data to mediatorLivedata 
        mediatorLiveDataCountries.addSource(liveData, Observer {
            mediatorLiveDataCountries.postValue(it)
            mediatorLiveDataCountries.removeSource(liveData)
        })
        return mediatorLiveDataCountries

    }

• This is how you handle the status in your MainActivity

 mainViewModel = ViewModelProvider(this,provider)[MainViewModel::class.java]
        mainViewModel.ObserverCountries().observe(viewLifecycleOwner, Observer {
             when(it.authStatus) {
                 AuthStatus.LOADING -> /// here you show progressbar in response pre-fetch
                 {
                     
                     countriesFragmentBinding.countryprogress.show()
                 }
                 AuthStatus.Success -> { // here you update your ui

                     countriesAdapter = CountriesAdapter(it.data.meals!!, 
                     requireContext())
                     countriesFragmentBinding.recyclercountries.adapter = countriesAdapter
                     countriesAdapter!!.deleteCategory(23)
                     countriesFragmentBinding.countryprogress.hide()
                 }
                 AuthStatus.ERROR -> // here you hide your progressbar and show your toast
                 {
                   
                     countriesFragmentBinding.countryprogress.hide()
                     ToastyError(requireContext(),getString(R.string.errorretreivingdata))
                 }

             }
        })

        return countriesFragmentBinding.root
    }

}

I am using MVVM pattern in my app I have separate repository class for network operations. In repository class I am getting response from the server. How can I show Toast message send from the server in my main activity.

Below is my code:

Repository.java

public class MyRepository {

MutableLiveData<List<Facts>> mutableLiveData = new MutableLiveData<>();
    
Application application;

public MyRepository(Application application) {
    this.application = application;
}

public MutableLiveData<List<Facts>> getMutableLiveData(){

    Retrofit retrofit = RetrofitClient.getInstance();
    ApiService apiService = retrofit.create(ApiService.class);

    apiService.getFacts().subscribeOn(Schedulers.io())
                         .observeOn(AndroidSchedulers.mainThread())
                         .subscribe(new Observer<List<Facts>>() {
                             @Override
                             public void onSubscribe(Disposable d) {

                             }

                             @Override
                             public void onNext(List<Facts> facts) {

                                 if(facts.size() > 0 && facts != null){

                                     mutableLiveData.setValue(facts);
                                 }
                             }

                             @Override
                             public void onError(Throwable e) {

                                 TastyToast.makeText(application,e.getMessage(),TastyToast.LENGTH_SHORT,
                                         TastyToast.ERROR).show();
                             }

                             @Override
                             public void onComplete() {

                             }
                         });

       return mutableLiveData;
  }
 }

FactsViewModel.java

public class FactsViewModel extends AndroidViewModel {

MyRepository repo;

public FactsViewModel(@NonNull Application application) {
    super(application);

    repo = new MyRepository(application);
  }

public LiveData<List<Facts>> getAllFacts(){

    return repo.getMutableLiveData();
  }
}

MainActivity.java

 private void myFacts(){

    FactsViewModel viewModel = new ViewModelProvider(this).get(FactsViewModel.class);  

    viewModel.getAllFacts().observe(this, new Observer<List<Facts>>() {
       @Override
       public void onChanged(List<Facts> facts) {

           adapter = new FactsAdapter(facts,getActivity());
           recycle.setAdapter(adapter);
       }
   });

}

How can I show error toast messages in MainActivity?