Within my app I often have the need to read data once. I originally started by using the addListenerForSingleValueEvent() method for this, however I ran into problems using this method as it does not work as I wanted when offline capabilities are enabled (see here the issue: Firebase Offline Capabilities and addListenerForSingleValueEvent)
In the question above it is mentioned that a workaround is to use the addValueEventListener() method, however I do not fully understand how to do this (particularly how to remove the ValueEventListener as soon I am finished grabbing the data I need).
Take this method which I created in a standalone class to query the Users node on Firebase where I store the users FCM Token. It seems to have an issue of not returning the latest token from the server everytime.
public class SendFCMMessage {
String userToken;
String currentUser;
String userName;
ValueEventListener userListener;
public void sendMessage(final String contactNumber) {
final DatabaseReference ref = FirebaseDatabase.getInstance().getReferenceFromUrl(link).child("Users").child(contactNumber);
userListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
User user = dataSnapshot.getValue(User.class);
userToken = user.getToken();
// Send FCM Message after getting user token and then remove event listener
ref.removeEventListener(userListener);
}
#Override
public void onCancelled(DatabaseError databaseError) {
Log.d("TAG", "Something terrible went wrong: " + databaseError);
}
};
ref.addValueEventListener(userListener);
}
}
If I remove the line
ref.removeEventListener(userListener);
Then this code works fine, however I would like to know how I could remove the ValueEventListener as soon as I receive the data I need?
Thanks,
R
ValueEventListener vel; //Declared Global
Listen your DatabaseReference like this;
vel = yourDatabaseReference.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChanged(DataSnapshot dataSnapShot) {
//Do your stuff here. I suggest you create another method for this if you don't want a problem with inner class.
//For example, workDataSnapshot(DataSnapshot dataSnapShot) <-- Work here
yourDatabaseReference.removeEventListener(vel);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
Hope it helps you.
Related
I would to compare a value with a child of my Firebase Realtime Database but I don't know how to do. The structure of my database is:
This is the code that I wrote:
email = loadPreferences();
mAuth = FirebaseAuth.getInstance();
FirebaseDatabase database = FirebaseDatabase.getInstance();
final DatabaseReference myRef = database.getReference().child("users");
myRef.orderByChild("email").equalTo(email).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot snapshot) {
if(snapshot.getValue() != null) {
//loop through the keys
for(DataSnapshot datasnap : snapshot.getChildren()) {
if(!email.equalsIgnoreCase("")) {
myRef.child("users").child("email").child("address").setValue(getAddress());
}
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError error) {
}
});
I tried with debugger but when it arrived at "orderByChild()" it skip all and jumps to end. Anyone can help me? Thanks in advance
Data is loaded from Firebase asynchronously, because it needs to be loaded from the cloud. Instead of blocking the app for your users, Firebase instead loads the data in the background and lets your main code continue, which is what you see happening when you step through the code.
Instead of stepping through the code, place a breakpoint on the first line inside onDataChange, and allow the code to run. Then when the data is available, the debugger will hit your breakpoint and you can continue debugging.
You should also implement onCancelled, as you're now ignoring possible problems. At its minimum, this method should be:
public void onCancelled(#NonNull DatabaseError databaseError) {
throw databaseError.toException();
}
I have some trouble trying to check if user information is stored already in the FireBase database.
Basically I'm trying to do something stupid like this:
"select user_name from user where user_id="+userID+"
And if the nickname exists it should make the boolean var isFirstTime = false and if it doesn't it should stay true. And after that it should show register box or not.
This is my db:
Firebase
And this is my code in onCreate method:
databaseReference = FirebaseDatabase.getInstance().getReference();
DatabaseReference dbRefFirstTimeCheck = databaseReference.child("User").child(user.getUid()).child("Nickname");
isFirstTime = true;
dbRefFirstTimeCheck.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
if(dataSnapshot.getValue() != null) {
isFirstTime=false;
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
if(isFirstTime) {
showNewUserBox();
}
else {
}
No matter what I do, the methor showNewUserBox() is being called. How do I get the data i need and check if it's there?
As others have commented, data is loaded from Firebase asynchronously. By the time you check isFirstTime, the data hasn't been loaded yet, onDataChange hasn't been run yet, so ifFirstTime will have its default value (false for a boolean).
All code that requires data from the database should be inside onDataChange (or invoked from within there). The simplest fix for your code is:
databaseReference = FirebaseDatabase.getInstance().getReference();
DatabaseReference dbRefFirstTimeCheck = databaseReference.child("User").child(user.getUid()).child("Nickname");
dbRefFirstTimeCheck.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
if(dataSnapshot.exists()) {
showNewUserBox();
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException(); // don't ignore errors
}
});
Also see some of the many questions about asynchronous loading from Firebase, such as getContactsFromFirebase() method return an empty list (or this quite old classic: Setting Singleton property value in Firebase Listener).
I'm creating an Android app for the first time, I've got a simple Realtime Firebase Database with a couple of records in it. I have the following code;
public void onStart() {
super.onStart();
// Read from the database
databaseMatches.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot matchSnapshot : dataSnapshot.getChildren()) {
matches match = matchSnapshot.getValue(matches.class);
matchesList.add(match);
}
matchList adapter = new matchList (getActivity(), matchesList);
listViewMatch.setAdapter(adapter);
}
#Override
public void onCancelled(DatabaseError error) {
// Failed to read value
Log.w(TAG, "Failed to read value.", error.toException());
}
});
If I put a breakpoint on the databaseMatches.addValueEventListener(new ValueEventListener() { it shows me that the database connection has been set and is returning the correct object (In my view).
The challenge I have is the part after, the break points for public void onDataChange nor onCancelled ever get hit. I'm lost here and not sure what might be the next step as it appears to be connecting, but I am not able to retrieve records.
I'm doing this in a fragment instead of a activity. Any help is appreciated.
Detecting Connection State
it is useful for your app to know when it is online or offline. Firebase Realtime Database provides a special location at /.info/connected which is updated every time the Firebase Realtime Database client's connection state changes. Here is an example: If you are not sure.
https://firebase.google.com/docs/database/android/offline-capabilities#section-connection-state
DatabaseReference connectedRef =
FirebaseDatabase.getInstance().getReference(".info/connected");
connectedRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
boolean connected = snapshot.getValue(Boolean.class);
if (connected) {
System.out.println("connected");
} else {
System.out.println("not connected");
}
}
#Override
public void onCancelled(DatabaseError error) {
System.err.println("Listener was cancelled");
}
Firebase also loads and synchronizes data asynchronously
see Setting Singleton property value in Firebase Listener
Thanks.
There must have been some strange caching issue as the following morning when I ran the exact same code, no problem. And I've not had a problem since.
I have a firebase database from which I save and retrieve data from, to and from. I know how datasnapshot works inside an addValueEventListener. The problem is that this is only called or triggered when the firebase database detects change in its data. I only want to access data and read it to be able to store it in an arraylist or the same thing.
I have a code like this:
public void foo(){
DatabaseReference x= FirebaseDatabase.getInstance().getReference().child("x");
reservations.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
String userID = client.getId();
for(DataSnapshot snap : dataSnapshot.getChildren()){
if(snap.child("someId").equals(someId)) number++;
if(snap.child("userID").getValue().equals(client.getId())){
isAlreadyReserved = true; // if user has already reserved the item
alreadyReserved();
break;
}
Log.e("isAlreadyReserved: ", isAlreadyReserved+"");
numberOfReservations++;
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
if(isAlreadyReserved) {
alreadyReserved();
}
else if(number == numberOfCopies){
// material is no longer available
OtherActivity.showMaterialUnavailable();
}
else{
Reservation reservation = new Reservation();
reservation.setBookId(this.bookId);
reservation.setResID((numberOfReservations+1)+"");
reservation.setUserID(client.getId());
String key = reservations.push().getKey();
reservations.child(key).setValue(reservation);
Log.e("Reservations: ", "reserve successful");
AlertDialog.Builder builder = new AlertDialog.Builder(this.context);
builder.setTitle(R.string.reservationSuccess_title)
.setMessage(R.string.reservationSuccess_body);
AlertDialog dialog = builder.create();
dialog.show();
}
}
You can see that inside onDataChange I only count materials and set some flags, which I can supposedly do outside the ValueEventListener.
But I notice that this is faulty because onDataChange is called only when writing to the Firebase database occurs. Which should not be the case.
What can I do to loop through the values inside the DatabaseReference x without calling onDataChange, or without using DataSnapshot?
You cannot loop inside a DatabaseReference without using a listener. When we are talking about Firebase, we are talking only about listeners. So in order to get those values, you need to use a listener and than get the data out from the dataSnapshot.
What i think your problem is in your case, is that onDataChange method is called in an asynchronously way. This means that everything you are doing outsite this method is actually executed before onDataChange method has been called. So in order to understand what is actually going on, please see this post and this post. Reading this posts, will teach you how to query data in Firebase and how to retrieve data from Firebase asynchronously.
Hope it helps.
In order to get the values of DatabaseReference x, you should use addListenerForSingleValueEvent
x.addListenerForSingleValueEvent(new ValueEventListener()
{
#Override
public void onDataChange(DataSnapshot dataSnapshot)
{
//do something
}
#Override
public void onCancelled(DatabaseError databaseError)
{
//do something
}
});
as mentioned in the firebase documentation:
public void addListenerForSingleValueEvent (ValueEventListener
listener)
Add a listener for a single change in the
data at this location. This listener will be triggered once with the
value of the data at the location.
I am using Firebase addValueEventListener to fetch the data from firebase database, below is my code.
DatabaseReference chatMessagesDb = FirebaseDatabase.getInstance().getReference("ChatRooms");
DatabaseReference usersDb = FirebaseDatabase.getInstance().getReference("users");
chatMessagesDb.child(chatRoomId).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot messages : dataSnapshot.getChildren()) {
Message message = messages.getValue(Message.class); // Message is a data Model for chatMessages.
userDb.child(message.getCreatorId()).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
I am applying addValueEventListener on chatMessagesDb and providing a key chatRoomId, so it will fetch all the messages from database of the chatRoom with id equals to chatRoomId. Now, for each message I want to fetch the creator of the Message, so I am adding a addValueEventListener on users database to fetch the details of the creator of a Message.
It should work like, for 1st loop of Message it should call the addValueEventListener on users db for the creator of that message, but it doesn't work like this. First, it loop through all the messages, then it starts calling addValueEventListener on users db.
How can I solve it? Please let me know if anyone have idea about this, this would be a great help.
Thanks a lot in advanced.
I think it's little late to answer, but I think other guys who are learning can be helpful. In case of message from db, use ChildValueEventListener. It returns 4 methods to implement, so any change in the db of a particular node will be listened and return that value.