Sharing data between fragments using new architecture component ViewModel

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On Last Google IO, Google released a preview of some new arch components, one of which, ViewModel.

In the docs google shows one of the possible uses for this component:

It is very common that two or more fragments in an activity need to communicate with each other. This is never trivial as both fragments need to define some interface description, and the owner activity must bind the two together. Moreover, both fragments must handle the case where the other fragment is not yet created or not visible.

This common pain point can be addressed by using ViewModel objects. Imagine a common case of master-detail fragments, where we have a fragment in which the user selects an item from a list and another fragment that displays the contents of the selected item.

These fragments can share a ViewModel using their activity scope to handle this communication.

And shows a implementation example:

public class SharedViewModel extends ViewModel {
    private final SavedStateHandle state;

    public SharedViewModel(SavedStateHandle state) {
        this.state = state;
    }

    private final MutableLiveData<Item> selected = state.getLiveData("selected");

    public void select(Item item) {
        selected.setValue(item);
    }

    public LiveData<Item> getSelected() {
        return selected;
    }
}

public class MasterFragment extends Fragment {
    private SharedViewModel model;

    @Override
    protected void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        model = new ViewModelProvider(getActivity()).get(SharedViewModel.class);
        itemSelector.setOnClickListener(item -> {
            model.select(item);
        });
    }
}

public class DetailFragment extends Fragment {
    @Override
    protected void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        SharedViewModel model = new ViewModelProvider(getActivity()).get(SharedViewModel.class);
        model.getSelected().observe(this, { item ->
           // update UI
        });
    }
}

I was quite excited about the possibility of not needing those interfaces used for fragments to communicate through the activity.

But Google's example does not show exactly how would I call the detail fragment from master.

I'd still have to use an interface that will be implemented by the activity, which will call fragmentManager.replace(...), or there is another way to do that using the new architecture?

master-detail android-architecture-components android-fragments android-activity android
8 Answers

As written in the official Google tutorial now you may obtain a shared view model with by activityViewModels()

// Use the 'by activityViewModels()' Kotlin property delegate
// from the fragment-ktx artifact
private val model: SharedViewModel by activityViewModels()
user_67566830

I have found a similar solution as others according to google codelabs example. I have two fragments where one of them wait for an object change in the other and continues its process with updated object.

for this approach you will need a ViewModel class as below:

import android.arch.lifecycle.MutableLiveData;
import android.arch.lifecycle.ViewModel;
import yourPackage.YourObjectModel;

public class SharedViewModel extends ViewModel {

   public MutableLiveData<YourObjectModel> item = new MutableLiveData<>();

   public YourObjectModel getItem() {
      return item.getValue();
   }

   public void setItem(YourObjectModel item) {
      this.item.setValue(item);
   }

}

and the listener fragment should look like this: 

public class ListenerFragment extends Fragment{
   private SharedViewModel model;
  @Override
  public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);

    model.item.observe(getActivity(), new Observer<YourObjectModel>(){

        @Override
        public void onChanged(@Nullable YourObjectModel updatedObject) {
            Log.i(TAG, "onChanged: recieved freshObject");
            if (updatedObject != null) {
                // Do what you want with your updated object here. 
            }
        }
    });
}
}

finally, the updater fragment can be like this: 

public class UpdaterFragment extends DialogFragment{
    private SharedViewModel model;
    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
   }
   // Call this method where it is necessary
   private void updateViewModel(YourObjectModel yourItem){
      model.setItem(yourItem);
   }
}

It is good to mention that the updater fragment can be any form of fragments(not DialogFragment only) and for using these architecture components you should have these lines of codes in your app build.gradle file. source

dependencies {
  def lifecycle_version = "1.1.1"
  implementation "android.arch.lifecycle:extensions:$lifecycle_version"
}
user_103095540

For those using Kotlin out there try the following approach:

  • Add the androidx ViewModel and LiveData libraries to your gradle file

  • Call your viewmodel inside the fragment like this:

      class MainFragment : Fragment() {

      private lateinit var viewModel: ViewModel

      override fun onActivityCreated(savedInstanceState: Bundle?) {
          super.onActivityCreated(savedInstanceState)

          // kotlin does not have a getActivity() built in method instead we use activity, which is null-safe
          activity?.let {
              viemModel = ViewModelProvider(it).get(SharedViewModel::class.java)
          }
      }
  }

The above method is a good practice since it will avoid crashes due to null pointer exceptions

Edit: As btraas complemented: activity is compiled into getActivity() which is marked as @Nullable in the android SDK. activity and getActivity() are both accessible and equivalent.

user_135756290
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