Reset/clear viewmodel or livedata

Question

I am following the one-single-activity app pattern advised by Google, so if I want to share data between Fragments I have to share a ViewModel whose owner must be the parent Activity. So, the problem becomes because I want to share data between only two Fragments, independently from the others.

Imagine I have MainFragment, CreateItemFragment and ScanDetailFragment. So, from first one I navigate to CreateItemFragment in which whenever I press a button I navigate to ScanDetailFragment in order to scan a barcode and, in consequence, through a LiveData object inside the ViewModel I can get the scanned value back into the CreateItemFragment once ScandDetailFragment finishes. The problem becomes when I decide to cancel the creation of the item: I go back to the `MainFragment' and because the ViewModel's owner was the Activity's lifecycle, once I go again into CreateItemFragment, the previously scanned value is still there.

Any idea to reset that ViewModel?

Answer 1

You can use callbacks in such cases to share data between fragments. or if you use DB/Sharedpreference/Content provider then you do not have to worry about sharing data each page will fetch its own data from the store(DB/SharedPreference/Contentprovider).

you can also try https://medium.com/@lucasnrb/advanced-viewmodels-part-iii-share-a-viewmodel-between-fragments-59c014a3646 if this guide helps

Answer 2

You can clear LiveData value every time when you go into CreateItemFragment from MainFragment.

Or you can just clear it from the CreateItemFragment in onBackPressed() method.

Answer 3

When you cancel the creation of item,set livedata value to null.then within observer code if(updatedvalue!=null) write your code using updated live data value.in this way you can avoid last updated value.

Answer 4

At the moment (on 2022), the method viewmodel.getViewModelStore.clear(); or onCleared(); is deprecated.

So, if you want to clear data holded by ViewModel or clear value of LiveData, you just need use 1 line code like this:

mainViewModel.getLiveData().getValue().clear();

• getLiveData() is my method inside MainViewModel class to return liveData variable
• getValue() is defaut method provided by LiveData (MutableLiveData: setValue(), postValue())

If you need to clear data when user press on Back button in Fragment, you can do like the code below & put it inside the onViewCreated method - the method of LifecycleFragment.

private void handleOnBackPressed() {
        requireActivity().getOnBackPressedDispatcher().addCallback(new OnBackPressedCallback(true) {
            @Override
            public void handleOnBackPressed() {
                Objects.requireNonNull(mainViewModel.getLiveData().getValue()).clear();
                requireActivity().finish();
            }
        });
    }

My project on Git if you want to refer code (it still updated): https://github.com/Nghien-Nghien/PokeAPI-Java/blob/master/app/src/main/java/com/example/pokemonapi/fragment/MainFragment.java