Force Firebase to use cached data

Question

I would like to force Firebase to give me cached data and not go get the latest from the server. Is this possible?

The idea was to enable persistence, call keepSynced(true) on a particular location, add a listener to be notified when the data is available, then call keepSynced(false) to ensure that future listeners get the cached data.

However, in a test app this doesn't seem to work. Calling keepSynced(false) doesn't prevent a listener from getting the latest from the server. (Code below)

What is the correct way to work with a temporarily-unchanging set of Firebase data?

public class MainActivity extends AppCompatActivity {
  private static String TAG = "tag";
  Button keepSyncedTrue;
  Button keepSyncedFalse;
  Button getData;
  TextView data;
  private DatabaseReference myRef;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    FirebaseDatabase.getInstance().setPersistenceEnabled(true);
    myRef = getDatabaseReference();
    setContentView(R.layout.activity_main);
    keepSyncedTrue = (Button)findViewById(R.id.keepSyncTrue);
    keepSyncedFalse = (Button)findViewById(R.id.keepSyncFalse);
    getData = (Button)findViewById(R.id.getData);
    data = (TextView)findViewById(R.id.data);

    keepSyncedTrue.setOnClickListener(new View.OnClickListener() {
      @Override public void onClick(View view) {
        myRef.keepSynced(true);
      }
    });

    keepSyncedFalse.setOnClickListener(new View.OnClickListener() {
      @Override public void onClick(View view) {
        myRef.keepSynced(false);
      }
    });

    getData.setOnClickListener(new View.OnClickListener() {
      @Override public void onClick(View view) {
        // Read from the database
        myRef.addListenerForSingleValueEvent(new ValueEventListener() {
          @Override
          public void onDataChange(DataSnapshot dataSnapshot) {
            // This method is called once with the initial value and again
            // whenever data at this location is updated.
            String value = dataSnapshot.getValue(String.class);
            Log.d(TAG, "Value is: " + value);
            data.setText(value);
          }

          @Override
          public void onCancelled(DatabaseError error) {
            // Failed to read value
            Log.w(TAG, "Failed to read value.", error.toException());
          }
        });
      }
    });

  }

  private DatabaseReference getDatabaseReference() {
    return FirebaseDatabase.getInstance().getReference("message");
  }
}

and

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout android:id="@+id/activity_main"
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:orientation="vertical"
    android:paddingTop="@dimen/activity_vertical_margin">


    <Button
        android:id="@+id/keepSyncTrue"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="keepSynced(true)" />

    <Button
        android:id="@+id/keepSyncFalse"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="keepSynced(false)" />

    <Button
        android:id="@+id/getData"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="get data" />

    <TextView
        android:id="@+id/data"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</LinearLayout>

Answer 1

It depends by your final goal.

If you want to avoid any kind of synchronization you can use

DatabaseReference.goOffine();

As you can see in the documentation:

Manually disconnect the Firebase Database client from the server and disable automatic reconnection.

Note: Invoking this method will impact all Firebase Database connections.

Otherwise:

FirebaseDatabase.goOffline();

As described in doc:

Shuts down our connection to the Firebase Database backend until goOnline() is called

Morover if you want to keep alive your synchronization and work with local cached data you can use:

FirebaseDatabase.getInstance().setPersistenceEnabled(true);

As reported in doc:

By enabling persistence, any data that the Firebase Realtime Database client would sync while online persists to disk and is available offline, even when the user or operating system restarts the app. This means your app works as it would online by using the local data stored in the cache. Listener callbacks will continue to fire for local updates.

In this case you app work with cached data but the client continues to sync with server data.
More info here.

Also in this case to keep specific locations in sync use:

yourRef.keepSynced(true);

Answer 2

You can use Firebase Offline Capabilities by enabling Disk Persistence in your Application. This is how it's done:

public class MyApp extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        if (!FirebaseApp.getApps(this).isEmpty()) {
            FirebaseDatabase.getInstance().setPersistenceEnabled(true);
        }
    }
}

After you enable disk persistence, Firebase listener will look up to cache data first to be given in DataSnapshot. So, to prevent it fetching data from server, you should use Firebase addListenerForSingleValueEvent everytime you want to read data, because it will only read once and remove itself after that, which means it will read from cache data.

I think you don't need to use keepSynced(boolean value) for this case.