Sharing data between fragments using new architecture component ViewModel

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()

Updated on 6/12/2017,

Android Official provide a simple, precise example to example how the ViewModel works on Master-Detail template, you should take a look on it first.Share data between fragments

As @CommonWare, @Quang Nguyen methioned, it is not the purpose for Yigit to make the call from master to detail but be better to use the Middle man pattern. But if you want to make some fragment transaction, it should be done in the activity. At that moment, the ViewModel class should be as static class in Activity and may contain some Ugly Callback to call back the activity to make the fragment transaction.

I have tried to implement this and make a simple project about this. You can take a look it. Most of the code is referenced from Google IO 2017, also the structure. https://github.com/charlesng/SampleAppArch

I do not use Master Detail Fragment to implement the component, but the old one ( communication between fragment in ViewPager.) The logic should be the same.

But I found something is important using these components

1. What you want to send and receive in the Middle man, they should be sent and received in View Model only
2. The modification seems not too much in the fragment class. Since it only change the implementation from "Interface callback" to "Listening and responding ViewModel"
3. View Model initialize seems important and likely to be called in the activity.
4. Using the MutableLiveData to make the source synchronized in activity only.

1.Pager Activity

public class PagerActivity extends AppCompatActivity {
    /**
     * The pager widget, which handles animation and allows swiping horizontally to access previous
     * and next wizard steps.
     */
    private ViewPager mPager;
    private PagerAgentViewModel pagerAgentViewModel;
    /**
     * The pager adapter, which provides the pages to the view pager widget.
     */
    private PagerAdapter mPagerAdapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_pager);
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
        mPager = (ViewPager) findViewById(R.id.pager);
        mPagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager());
        mPager.setAdapter(mPagerAdapter);
        pagerAgentViewModel = new ViewModelProvider(this).get(PagerAgentViewModel.class);
        pagerAgentViewModel.init();
    }

    /**
     * A simple pager adapter that represents 5 ScreenSlidePageFragment objects, in
     * sequence.
     */
    private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
       ...Pager Implementation
    }

}

2.PagerAgentViewModel (It deserved a better name rather than this)

public class PagerAgentViewModel extends ViewModel {
    private final SavedStateHandle state;
    private final MutableLiveData<String> messageContainerA;
    private final MutableLiveData<String> messageContainerB;

    public PagerAgentViewModel(SavedStateHandle state) {
        this.state = state;

        messageContainerA = state.getLiveData("Default Message");
        messageContainerB = state.getLiveData("Default Message");
    }

    public void sendMessageToB(String msg)
    {
        messageContainerB.setValue(msg);
    }
    public void sendMessageToA(String msg)
    {
        messageContainerA.setValue(msg);

    }
    public LiveData<String> getMessageContainerA() {
        return messageContainerA;
    }

    public LiveData<String> getMessageContainerB() {
        return messageContainerB;
    }
}

3.BlankFragmentA

public class BlankFragmentA extends Fragment {

    private PagerAgentViewModel viewModel;

    public BlankFragmentA() {
        // Required empty public constructor
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        viewModel = new ViewModelProvider(getActivity()).get(PagerAgentViewModel.class);


        textView = (TextView) view.findViewById(R.id.fragment_textA);
        // set the onclick listener
        Button button = (Button) view.findViewById(R.id.btnA);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                viewModel.sendMessageToB("Hello B");
            }
        });

        //setup the listener for the fragment A
        viewModel.getMessageContainerA().observe(getViewLifecycleOwner(), new Observer<String>() {
            @Override
            public void onChanged(@Nullable String msg) {
                textView.setText(msg);
            }
        });

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_blank_a, container, false);
        return view;
    }

}

4.BlankFragmentB

public class BlankFragmentB extends Fragment {
 
    public BlankFragmentB() {
        // Required empty public constructor
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        viewModel = new ViewModelProvider(getActivity()).get(PagerAgentViewModel.class);

        textView = (TextView) view.findViewById(R.id.fragment_textB);
        //set the on click listener
        Button button = (Button) view.findViewById(R.id.btnB);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                viewModel.sendMessageToA("Hello A");
            }
        });

        //setup the listener for the fragment B
        viewModel.getMessageContainerB().observe(getViewLifecycleOwner(), new Observer<String>() {
            @Override
            public void onChanged(@Nullable String msg) {
                textView.setText(msg);

            }
        });
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_blank_b, container, false);
        return view;
    }

}