从数据库onDataChange读取的Firebase Android App未调用

Question

我从firebase数据库中获取数据时出现问题。 具体问题是，onDataChange方法（ValueEventlistener）未被调用。

申请继续：

DatabaseConnectionManager()
readFromDatabase()
getEventFormulaGet()

eventFormulaGet为空，因为onDataChange未被调用....

代码：

package bbcag.ch.miivent_luca;

    import com.google.firebase.database.DataSnapshot;
    import com.google.firebase.database.DatabaseError;
    import com.google.firebase.database.DatabaseReference;
    import com.google.firebase.database.FirebaseDatabase;
    import com.google.firebase.database.ValueEventListener;


    public class DatabaseConnectionManager implements ValueEventListener {
    private FirebaseDatabase database;
    private DatabaseReference databaseReference;
    private EventFormula eventFormulaSend;
    private EventFormula eventFormulaGet;

    public DatabaseConnectionManager() {
        createConnectionToDatabase();
    }

    public void createConnectionToDatabase() {

        database = FirebaseDatabase.getInstance();
        databaseReference = database.getReference();

    }

    public void readFromDatabase() {
        databaseReference.child("events").addListenerForSingleValueEvent(this);
    }

    public void deleteFromDatabase() {

    }

    public void setEventFormula(EventFormula eventFormula) {
        this.eventFormulaSend = eventFormula;
    }

    public void sendFormulaToDatabase() {
        databaseReference.child("events").child(eventFormulaSend.getEventTitle()).setValue(eventFormulaSend);
    }

    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        eventFormulaGet = dataSnapshot.getValue(EventFormula.class);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
    }

    public EventFormula getEventFormulaGet() {
        return eventFormulaGet;
    }

}