u0652804 / easyexlivedata

a easy case to study live data

EasyExLiveData

a easy case to study live data

step by step

1. Add dependce in build.gradle

    // liveData and ViewModel
    implementation 'android.arch.lifecycle:viewmodel:1.1.1'
    implementation 'android.arch.lifecycle:extensions:1.1.1'
    annotationProcessor 'android.arch.lifecycle:compiler:1.1.1'
   

2. Create your ViewModel e.g. LiveDataTimerViewModel

3. Edit in your View class(Activity or fragment)

     // a. init ViewModel
     // b. create observer and override onChanged event
     // c. use viewModel.getYourLiveData.observe(this, observer)
   

4. When liveData changed by post value or set value, View will triggered onChanged event to do something about update UI

Part of coding

ViewModel :

    public class LiveDataTimerViewModel extends ViewModel {
        private MutableLiveData<Long> mElapsedTime;
        ...
        // this function let View can observer liveData in this ViewModel
        public MutableLiveData<Long> getElapsedTime() {
            if (null == mElapsedTime){
                mElapsedTime = new MutableLiveData<>();
            }
            return mElapsedTime;
        }

View :

    public class MainActivity extends AppCompatActivity {
        private LiveDataTimerViewModel liveDataTimerViewModel;
        ...
        protected void onCreate(Bundle savedInstanceState) {
            // 1. init ViewModel
            liveDataTimerViewModel = ViewModelProviders.of(this).get(LiveDataTimerViewModel.class);
            // 2. create observers
            final Observer<Long> elapsedTimeObserver = new Observer<Long>() {
                @Override
                public void onChanged(@Nullable final Long aLong) {
                    // 5. update UI
                    ((TextView)findViewById(R.id.text1)).setText(aLong.toString());
                }
            };               
            // 3. observe LiveData by get it from ViewModel and bind lifeCycle of this View
            liveDataTimerViewModel.getElapsedTime().observe(this, elapsedTimeObserver);
        }
        // 4. ... do someMethod cause liveData change value

Demo