Will the `onChanged` method trigger just as we change the data? Even if we dont change the state of activity?

Question

Will the onChanged method be called if we dont change the state of the activity or restart the activity? ( Even if the data changes ) . If it doesnt happen how shall i make it happen ?

onCreate() {
    viewModel.getHolidaysDatesFromServer(..).observe(.....this,new Observer<GetHolidaysDatesFromServer>) {
        onChanged(GetHolidaysDatesFromServer GetHolidaysDatesFromServer)
    }    
}

and my ViewModel class ...

public class CalendarLifeCycleOwner extends AndroidViewModel {
    Context context;
    MutableLiveData<GetHolidaysDatesFromServer> MutableLiveDataGetHolidays=new MutableLiveData<>();
    public CalendarLifeCycleOwner(@NonNull Application application) {
        super(application);
        this.context=application.getApplicationContext();
    }

    public MutableLiveData<GetHolidaysDatesFromServer> getHolidaysDatesFromServer(String upd_token,int product_id) {
        JustProductId justProductId=new JustProductId();
        justProductId.setProduct_id(product_id);
        UserListInterface userListInterface= UserListService.createService(UserListInterface.class, Utility.ACCEPT_HEADER_V4);
        Call<GetHolidaysDatesFromServer> getHolidaysDatesFromServerCall=userListInterface.GetProductHolidays(upd_token,justProductId);
        getHolidaysDatesFromServerCall.enqueue(new Callback<GetHolidaysDatesFromServer>() {
            @Override
            public void onResponse(Call<GetHolidaysDatesFromServer> call, Response<GetHolidaysDatesFromServer> response) {
                if (response.isSuccessful()) {
                    GetHolidaysDatesFromServer getHolidaysDatesFromServer=response.body();
                    MutableLiveDataGetHolidays.postValue(getHolidaysDatesFromServer);
                }
            }

            @Override
            public void onFailure(Call<GetHolidaysDatesFromServer> call, Throwable t) {
                ErrorObject errorObject=new ErrorObject(t);
                GetHolidaysDatesFromServer getHolidaysDatesFromServer=new GetHolidaysDatesFromServer();
                getHolidaysDatesFromServer.setErrorObject(errorObject);
                MutableLiveDataGetHolidays.setValue(getHolidaysDatesFromServer);

            }
        });
        return MutableLiveDataGetHolidays;
    }
}

But after i just change the data onChanged() is not getting called ...

Precast

If you restart your activity than it will be called or when you change the MutableLiveData – Featured

i have changed the data..but its not getting called unless i restart or rotate or anything ... do i always have to write .observe(....) to make it trigger ..?? i have observed in oCreate once – Precast

You need to write only once on onCreate...please post some code to be more clear – Featured

Are you using ViewModel with LiveData ?? – Featured

I am using ViewModel with MutableLiveData ...1st time in onCreate its getting called if the data is changed ...but after that even if the data changes onChanged is not called – Precast

After i rotate or change the activity state its getting called..it isnt getting called just by changing the data .. – Precast

How you are changing the data ? please post some code – Featured

have added pseudo code...its just simple as that...i am getting the response from retrofit async using enqueue.. – Precast

i am setting the value of mutablelivedata by mutablelivedata.setValue(DataObject) do i need to use .postValue instead ? – Precast

If you are changing the value from a WorkerThread or off the UIThread than you need to use postValue – Featured

same..issue both with both the operator (setValue and postValue) – Precast

Need more code to clarify the issue – Featured

Ok i am posting ViewModel class code – Precast

already updated code... just observed once in onCreate – Precast

Did you check is respone is sucessful ? and use setValue on onResponse method it runs on UIThread only – Featured

My code is fine i just wanna know just as the data changes does the observer gets triggered or do i need change the activity state?? and Ya i used setValue at first... both giving same result... – Precast

You don't need to change the state it will automatically updated the livedata – Featured

then it isnt but it should.. :( do i need to add anything else ? – Precast

Your code is fine..just put the debug on onRespone() callback and check that is it respone.isSuccessfull() is true ?...that's the only thing i have to say – Featured

ya thats true.. if it wasnt true ...if i rotate activity then it would throw error... just that automatically its not updating ..i am having to change the state... ok i am debugging .. – Precast

debugged...the onResponse method is called 1st time then i change data... its not called , then i rotate ,then again its called 2nd time – Precast

Answer 1

Google actually described that (or very familiar to that) kind of issue as an anti-pattern on the dev-summit. The recommended approach is to use switchMap from android.arch.lifecycle.Transformations class. You can see the documentation page for more details.

Raina

Answer 2

Try using this and delete the global MutableLiveData variable

public MutableLiveData<GetHolidaysDatesFromServer> getHolidaysDatesFromServer(String upd_token,int product_id) {

       MutableLiveData<GetHolidaysDatesFromServer> MutableLiveDataGetHolidays=new MutableLiveData<>();

       //Your API call code goes here

       return MutableLiveDataGetHolidays;
 }

Featured

Unfortunately same issue...is there any other method for retrofit callbacks?? like retrofit+viewmodel? – Precast

Just try changing the livedata value without retrofit and remove retrofit callback instead to test setValue after some duration to find the issue where it lies – Featured

then i also have to call the request after specific duration... like for(infinite){ call.enque...} I think this will work.. but continously i have to keep calling API – Precast

I didn't get you @SantanuSur – Featured

and it isnt working too.. :p see now i got the reason why it isnt updating but i dont know how to solve it... its not updating because 1st time as the response is received the onChanged method is called... 2nd time i update the data to server... but i am not calling the getHolidaysDatesFromServer 2nd time in the UI – Precast

did u get my point ?? i have to manually call the getHolidayDatesFromServer after i update... but this doesnt make sense ..why shall i have to call it again after i update... (i am updating to server ) .. – Precast

Checkout my example here github.com/burhanrashid52/Android-Room-Data-Persistent and compare it your code – Featured