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Implement a Companion App

This lesson teaches you to

  1. Create a companion app module
  2. Display database entries with Firebase UI

Try it out

A smart doorbell might or might not have a local display. A companion app for mobile devices allows the user to remotely and automatically see new doorbell ring events and images as they are inserted by the embedded device.

In this lesson, you will build an Android mobile app containing a Firebase Realtime Database that is synchronized with the doorbell database.

Create a companion app module

Create a second module in your project alongside the embedded doorbell app. To create a new project module:

  1. Follow the instructions to Create a New Module in Android Studio.
  2. Create a new Phone & Tablet Module.

Add Firebase to your module

To enable Firebase Realtime Database for your companion app module:

  1. Install the Firebase Android SDK into your app module.
  2. Download and install the google-services.json file as described in the instructions.

  3. Add the Firebase Realtime Database and Firebase UI dependencies to your app-level build.gradle file:

    dependencies {
        compile ''
        compile ''
        compile 'com.firebaseui:firebase-ui-database:0.5.3'

Display doorbell events with Firebase UI

To simplify the interaction with the Firebase data model, create a DoorbellEntry model class that contains the same properties described in the JSON schema.

public class DoorbellEntry {

    Long timestamp;
    String image;
    Map<String, Float> annotations;

    public DoorbellEntry() {

    public DoorbellEntry(Long timestamp, String image, Map<String, Float> annotations) {
        this.timestamp = timestamp;
        this.image = image;
        this.annotations = annotations;

    public Long getTimestamp() {
        return timestamp;

    public String getImage() {
        return image;

    public Map<String, Float> getAnnotations() {
        return annotations;

The FirebaseRecyclerAdapter from the FirebaseUI library simplifies binding Firebase data to a Recyclerview for display. Subclasses of the adapter must override populateViewHolder() and provide the logic to bind the data from the model into the provided ViewHolder.

The following DoorbellEntryAdapter binds data from a DoorbellEntry instance into a DoorbellEntryViewHolder:

public class DoorbellEntryAdapter extends FirebaseRecyclerAdapter<DoorbellEntry, DoorbellEntryAdapter.DoorbellEntryViewHolder> {

     * ViewHolder for each doorbell entry
    static class DoorbellEntryViewHolder extends RecyclerView.ViewHolder {

        public final ImageView image;
        public final TextView time;
        public final TextView metadata;

        public DoorbellEntryViewHolder(View itemView) {

            this.image = (ImageView) itemView.findViewById(;
            this.time = (TextView) itemView.findViewById(;
            this.metadata = (TextView) itemView.findViewById(;

    private Context mApplicationContext;

    public DoorbellEntryAdapter(Context context, DatabaseReference ref) {
        super(DoorbellEntry.class, R.layout.doorbell_entry, DoorbellEntryViewHolder.class, ref);

        mApplicationContext = context.getApplicationContext();

    protected void populateViewHolder(DoorbellEntryViewHolder viewHolder, DoorbellEntry model, int position) {
        // Display the timestamp
        CharSequence prettyTime = DateUtils.getRelativeDateTimeString(mApplicationContext,
                model.getTimestamp(), DateUtils.SECOND_IN_MILLIS, DateUtils.WEEK_IN_MILLIS, 0);

        // Display the image
        if (model.getImage() != null) {
            // Decode image data encoded by the Cloud Vision library
            byte[] imageBytes = Base64.decode(model.getImage(), Base64.NO_WRAP | Base64.URL_SAFE);
            Bitmap bitmap = BitmapFactory.decodeByteArray(imageBytes, 0, imageBytes.length);
            if (bitmap != null) {
            } else {
                Drawable placeholder =
                        ContextCompat.getDrawable(mApplicationContext, R.drawable.ic_placeholder);

        // Display the metadata
        if (model.getAnnotations() != null) {
            ArrayList<String> keywords = new ArrayList<>(model.getAnnotations().keySet());

            int limit = Math.min(keywords.size(), 3);
            viewHolder.metadata.setText(TextUtils.join("\n", keywords.subList(0, limit)));
        } else {
            viewHolder.metadata.setText("no annotations yet");


Create an instance of the DoorbellEntryAdapter and attach it to a RecyclerView in your activity. The FirebaseUI adapters automatically register event listeners with the database. To avoid memory leaks, initialize the adapter in onStart() and call its cleanup() method in onStop().

To ensure that each new doorbell event is visible to the user, scroll the list to the latest item on each data set change. You can listen for changes events using an AdapterDataObserver attached to your adapter.

public class MainActivity extends AppCompatActivity {

    private DatabaseReference mDatabaseRef;

    private RecyclerView mRecyclerView;
    private DoorbellEntryAdapter mAdapter;

    protected void onCreate(Bundle savedInstanceState) {

        mRecyclerView = (RecyclerView) findViewById(;
        // Show most recent items at the top
        LinearLayoutManager layoutManager =
                new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, true);

        // Reference for doorbell events from embedded device
        mDatabaseRef = FirebaseDatabase.getInstance().getReference().child("logs");

    protected void onStart() {

        mAdapter = new DoorbellEntryAdapter(this, mDatabaseRef);

        // Make sure new events are visible
        mAdapter.registerAdapterDataObserver(new RecyclerView.AdapterDataObserver() {
            public void onItemRangeChanged(int positionStart, int itemCount) {

    protected void onStop() {

        // Tear down Firebase listeners in adapter
        if (mAdapter != null) {
            mAdapter = null;

Congratulations! You have built a cloud-enabled doorbell for the connected home using Android Things!

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