LiveData-sample.java

 public class LiveDataTimerViewModel extends ViewModel {

    private static final int ONE_SECOND = 1000; //1초 스케줄링 (카운팅)

    private MutableLiveData<Long> mElapsedTime = new MutableLiveData<>(); // LiveData

    private long mInitialTime;

    public LiveDataTimerViewModel() {
        mInitialTime = SystemClock.elapsedRealtime();
        Timer timer = new Timer();

        // Update the elapsed time every second.
        timer.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                final long newValue = (SystemClock.elapsedRealtime() - mInitialTime) / 1000;
                // setValue() cannot be called from a background thread so post to main thread.
                mElapsedTime.postValue(newValue);
            }
        }, ONE_SECOND, ONE_SECOND);

    }

    /**
     * LiveData ElapsedTime 를 반환한다.
     */
    public LiveData<Long> getElapsedTime() {
        return mElapsedTime;
    }
}
public class ChronoActivity3 extends AppCompatActivity {

    private LiveDataTimerViewModel mLiveDataTimerViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.chrono_activity_3);

        // ViewModel을 생성(등록?) 한다.
        mLiveDataTimerViewModel = ViewModelProviders.of(this).get(LiveDataTimerViewModel.class);

        subscribe();
    }

    /**
     * LiveData ElapsedTime 을 구독한다.
     */
    private void subscribe() {
        final Observer<Long> elapsedTimeObserver = new Observer<Long>() {
            @Override
            public void onChanged(@Nullable final Long aLong) {
                // 데이터 변경시 UI update
                String newText = ChronoActivity3.this.getResources().getString(
                        R.string.seconds, aLong);
                ((TextView) findViewById(R.id.timer_textview)).setText(newText);
                Log.d("ChronoActivity3", "Updating timer");
            }
        };
        // LiveDat elapsedTime에 observer를 등록한다.
        mLiveDataTimerViewModel.getElapsedTime().observe(this, elapsedTimeObserver);
    }
}