Room android LiveData 仅观察调用 2 次

Question

I have a problem using LiveData in Room library android.

I have a Fragment where there is a RecyclerView and to populate use LiveData with Observer

mDB.user().getUsers().observe(getViewLifecycleOwner(), new Observer<List<UserModel>>() {
        @Override
        public void onChanged(List<UserModel> userModels) {
            updateUI(userModels);
        }
});

If I click on ADD button, call a startActivity , open a new Activity with my form, compile all fields and @INSERT a new item on DB

AsyncTask.execute(() -> {
        mDB.user().addUser("NAME");
        finish();
});

When finish the current Activity, show again previous Fragment and automatically should be call the onChanged . The observe is called only for 2 times, than, if I click on ADD button (for 3th time) and come back to the fragment, never call and never refresh my list. I don't know why.

I tried with observeForever without results.

I see that in debug, never called removeObserver , so the observer is active.

These are my dependencies

def room_version = "2.2.5"
def lifecycle_version = "2.3.0-alpha01"

// Room database
implementation "androidx.room:room-runtime:$room_version"
annotationProcessor "androidx.room:room-compiler:$room_version"
// ViewModel
implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
// LiveData
implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-service:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-process:$lifecycle_version"

Answer 1

You should call observe() from onResume() of UsersFragment , instead of onCreateView() because for adding a new user, you are switching to a new Activity which pauses the current activity and the fragments attached to it instead of destroying the views of the fragments. So, when you come back to the current activity, onCreateView() of UsersFragment never gets called, onResume() is called however.

I would, however, suggest you to implement a proper ViewModel , Repository pattern for communicating with Room database.