如何通过cursorAdapter使用新的系统架构ViewModel / LiveData？

Question

While learning the new android Architecture component's ViewModel and LiveData, having a little confusion when observe the LiveData changing from database source change, and how this would work with Cursor adapter.

in https://developer.android.com/reference/android/widget/CursorAdapter.html , it says

int FLAG_REGISTER_CONTENT_OBSERVER
If set the adapter will register a content observer on the cursor 
and will call onContentChanged() when a notification comes in. Be 
careful    when using this flag: you will need to unset the current 
Cursor from the adapter to avoid leaks due to its registered 
observers. This flag is not needed when using a CursorAdapter 
with a CursorLoader.

so the with cursorAdaptor it has a way to get the 'live update' when the database data is updated.

is there a way to use the LiveData (to observe the database data update) with the cursorAdaptor?

trying to show the question of where to use the liveData updating the cursor in snippet below: (with the sample of https://codelabs.developers.google.com/codelabs/android-persistence )

the Book:

@Entity
public class Book {
    public @PrimaryKey String id;
    public String title;
}

The ViewModel:

public class BooksBorrowedByUserViewModel extends AndroidViewModel {

public final LiveData<List<Book>> books;

private AppDatabase mDb;

public BooksBorrowedByUserViewModel(Application application) {
    super(application);
    createDb();
    // Books is a LiveData object so updates are observed.
    books = mDb.bookModel().findBooksBorrowedByName("Mike");   //<=== this ViewModel specific to one type query statement
}

public void createDb() {
    mDb = AppDatabase.getInMemoryDatabase(this.getApplication());

    // Populate it with initial data
    DatabaseInitializer.populateAsync(mDb);
}
}

is this the way to use LiveData observer to force reload cursor?

private CursorAdapter listAdapter;
private BooksBorrowedByUserViewModel mViewModel;

private void subscribeUiBooks() {
    mViewModel.books.observe(this, new Observer<List<Book>>() {
        @Override
        public void onChanged(@NonNull final List<Book> books) {

            showBooksInUi(books, mBooksTextView); //<== the sample’s code

            // if would like to update the cursorAdaptor
            //
            // ??? to requery the database and swap cursor here?
            // Cursor data = queryData(buildSqlStatement());  // build the same sql statement as used in the BooksBorrowedByUserViewModel
            // listAdapter.swapCursor(data)

        }
    });
}

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //having a list using CursorAdaptor
    ListView list = getListView();
    listAdapter = new CursorAdapter(getActivity(), null, 0)
    list.setAdapter(listAdapter);

    // Get a reference to the ViewModel for this screen.
    mViewModel = ViewModelProviders.of(this).get(BooksBorrowedByUserViewModel.class);

    subscribeUiBooks();
}

Answer 1

CursorAdapter is an old stuff, you should use Room + LiveData + RecyclerView.

Your data layer:

public LiveData<List<UserEntity>> getUsers() {
    return userDao.getUsers();
}

Your activity:

viewModel.getUsers().observe(this, new Observer<List<UserEntity>>() {
    @Override
    public void onChanged(@Nullable List<UserEntity> users) {
        if (users != null) {
            adapter.setUsers(users);
        }
    }
});

In adapter:

private List<UserEntity> users = new ArrayList<>();

public void setUsers(List<UserEntity> users) {
    this.users.clear();
    this.users.addAll(users);
    notifyDataSetChanged();
}

So when your activity lunch you should get live data from Room and subscribe to it. After that when you add something to that table, room automatically update observers, so you should just setup new data to the adapter and notify it.

Guide to App Architecture

LiveData

Room