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

问题

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 ...

回答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.

回答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;
 }

来源：https://stackoverflow.com/questions/48341561/will-the-onchanged-method-trigger-just-as-we-change-the-data-even-if-we-dont

标签

android

viewmodel

android-livedata