Android：将 LiveData 设置为 MutableLiveData

Question

Firstly i might not actually be understanding what mutable live data is correctly.

i get data from room using live data and then show it to the UI, it is a question with answers, so at the end of the question i would like to update the database with correct answers, the time it took etc etc.

I cannot work out how to use Mutable live data as there is next to no useful information on it or i'm incredibly stupid!

So firstly, can i actually update the database with mutable live data?

if so how? (i don't like asking this but i am a really stumped)

Dao

@Query("SELECT * FROM question_table WHERE :id = uoe_id")
LiveData<Question> getQuestionLiveData(int id);

Repo

public LiveData<Question> getQuestionLiveData(int id) {
  return questionDao.getQuestionLiveData(id);
}

ViewModel

public LiveData<Question> getQuestionLiveData(int id) {
  return questionRepository.getQuestionLiveData(id);
}

And then observing it in the View

viewModel.getQuestionLiveData(packageId).observe(getViewLifecycleOwner(), new Observer<com.questionTest.practice.Model.Question>() {
            @Override
            public void onChanged(com.questionTest.practice.Model.QuestionQuestion question) {

   Do stuff here////

                }
            }
        });

The next part is where i'm not sure. i added this in the view model

MutableLiveData mutableLiveData = new MutableLiveData();

and then tried to assign the this to the question

mutableLiveData = (MutableLivedata) getQuestionLiveData(id);

so i could use update the values but this throws an Casting error.

Im either missing something or i cannot do this so any help will be welcomed thanks

Answer 1

In this case, There is no need for MutableLiveData . MutableLiveData is LiveData which publicly exposes setValue() and postValue() method. So if you are not setting LiveData values inside ViewModel class, there is no need for MutableLiveData . Here the Dao class generate LiveData for you and you should observe that LiveData .