Retrieve ServerValue.Timestamp from Firebase in Android app, when data is sent

Question

I would like to know how to use Firebase\'s ServerValue.TIMESTAMP method, when I want to create a timestamp at the Firebase server, and then retrieve it to the local client.

Answer 1

I know this question already been answered, but I wanted to share my version of the solution.

When I use the server's timestamp, i usually need to use it more than once, i.e I have some kind of startTime and endTime which both depend on the server's time, where startTime is NOW and endTime is X seconds / minutes / hours after startTime, so to spare the multiple requests to the server I save the server's time in a root child called serverTime in the database, and I use it to set all dependent values.

Another thing, because of the way Firebase works with the ServerValue.Timestamp, eventually it fires 2 events (added and changed), the first one with local timestamp and the second with the actual server's timestamp. So to overcome the issue of not receiving the correct time I added a simple OnCompleteListener.

A short example code:

import android.support.annotation.NonNull;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.ServerValue;
import com.google.firebase.database.ValueEventListener;

public class ServerTime {

    public interface OnTimeRetrievedListener {

        void onTimeRetrieved(Long timestamp);
    }

    private final DatabaseReference db;

    public ServerTime(DatabaseReference db) {
        this.db = db.child("serverTime");
    }

    /**
     * Gets the server's timestamp in milliseconds.
     * @param listener {@link OnTimeRetrievedListener}
     */
    public void getTime(final OnTimeRetrievedListener listener) {
        if (listener == null) {
            return;
        }

        db.setValue(ServerValue.TIMESTAMP).addOnCompleteListener(new OnCompleteListener() {
            @Override
            public void onComplete(@NonNull Task task) {

                db.addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        listener.onTimeRetrieved(dataSnapshot.getValue(Long.class));
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) { }
                });

            }
        });
    }
}