Room, LiveData example

RoomExample.java

// this file is just a pseudo code of the whole idea

// we have 3 entities - Booking, User and Address(Origin and Destination)

// example userDao
class UserDao {
 
    public User getUser(int id);
}

// example addressDao
class AddressDao {
 
    public Address getAddress(int id)
}

// example booking model
class Booking {
 
    int userId = 0; // or shipperId?
    int originId = 0;
    int destinationId = 0;
  
    User shipper;
    Address origin;
    Address destination;
}

// create a viewmodel
class ExampleViewModel extends ViewModel {
 
    // create a livedata object
    private LiveData<Booking> liveData = MutableLiveData<Booking>();
    
    // if you want to execute the database query in a background thread, declare an ExecutorService
    // it is optional but highly recommended
    private ExecutorService executor = Executors.newCachedThreadPool();
  
    // create a method to fetch the data
    public void getBooking() {
     
        Booking b = network.getBooking(); // perform network call or something
      
        // do the whole database query
        executor.execute(new Runnable() {
            
            // grab DAOs from anywhere you've init'd them
            // UserDao userDao = Database.getInstance().getUserDao() // or something similar
            // AddressDao addressDao = ....
          
            b.shipper = userDao.getUser(b.userId);
            b.origin = addressDao.getAddress(b.originId);
            b.destination = addressDao.getAddress(b.destinationId);
            
            // notify livedata
            liveData.postValue(b);
        });
    }
    
    // create getter for livedata
    public LiveData<Booking> getLiveData() {
      return liveData;
    }  
}

// in your activity file
class ExampleActivity extends AppCompatActivity {
    
    private ExampleViewModel viewModel;
    
    @override
    public void onCreate() {
     
        ....
        ....
        
        viewModel = ViewModelProviders.of(this).create(ExampleViewModel::class);
        viewModel.getLiveData().observe(this, new Observer() {
            
            public void onChanged(Booking b) {
                
                 // you can now use booking here
                final Booking fetchedBooking = b;
            }
        });
    }
}