I have the following database structure in Firebase:
In my app when I launch an activity. I want to get all the level0Node's nodes only once when the activity is started. This I achieve like this:
rootNode.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Iterable<DataSnapshot> level0Nodes = dataSnapshot.getChildren();
}
#Override
public void onCancelled(DatabaseError databaseError) {}
});
But the problem I am facing is that if any updates occur on the level1Node's, onDataChanged is invoked again which I do not want. How can I achieve this?
You're looking for a SingleValueListener.
Replace your ValueEventListener with one, the code would look like this:
rootNode.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Iterable<DataSnapshot> level0Nodes = dataSnapshot.getChildren();
}
#Override
public void onCancelled(DatabaseError databaseError) {}
});
For more info refer to the Official Docs
you have two methods for getting total data in one go from a particular reference.
rootNode.addValueEventListener(new ValueEventListener(){}
// this you are using and according to documentation this is triggered when ever the data is change at this node- rootNode. if you want to use it only once your can remove your listner by calling
rootNode.removeEventListener() method for removing further updates.
use rootNode.addListenerForSingleValueEvent(new ValueEventListener() {}
according to documentation this is only triggered once so when ever there is a change in your child this listner will not trigger and your problem will be solve.
plus i just want to suggest you to read full documentation on firebase as your question was very basic.
Related
I know that when we retrieve data from Firebase ,it will be asynchronous, so ussally i will put all the code inside addChildEventListener, like example i want to sort userList below. But i am confused, if the List is really big, like million Users, so it means the method sortUser(user) will be called million times ? Can anyone explain this to me, I'm new to firebase
myRef.child("User").addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
User user= dataSnapshot.getValue(User.class);
userList.add(user);
sortUser(userList);
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
You currently use a ChildEventListener, which means your onChildAdded gets called for each child node immediately and then later whenever a new child is added. This indeed can be a lot of invocations.
If you use a ValueEventListener, its onDataChange will only be called once for the initial data (no matter how many child nodes there are), and then once for each change.
By adding a ValueEventListener to your current set up, you can keep things simple: add the child nodes to the lit like you're already doing, but only sort in onDataChange.
myRef.child("User").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
sortUser(userList);
}
#Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
};
Firebase will only synchronize the data for the User node once, even when you have two listeners on it.
You should probably retrieve the data sorted server side by using order-by methods and then listen to that one.
var userRef = firebase.database().ref('posts').orderByChild('Users');
If I guess correctly, you would not need separate sorting client side.
You can also filter data. Do refer the docs
Wanted to know is there a particular reason or benefit to use the Query dataSnapshot, rather than the regular DataBase Reference dataSnapshot. Please see the example- where I am trying to find information about a player via playerName. My Json Hierarchy goes like this:-
Player >> PlayerId(firebase generated Id) >>{playerName, playerAge, playerScore}
I can do this both ways
OPTION 1... with Query
gamesDBref = FirebaseDatabase.getInstance().getReference("Players");
Query query = gamesDBref.orderByChild("playerName").equalTo("FlashBoy");
query.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Log.e("R&D", "toString()" + dataSnapshot.toString());
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
OPTION 2... with Database Reference
gamesDBref = FirebaseDatabase.getInstance().getReference("Players");
gamesDBref.orderByChild("playerName").equalTo("FlashBoy").addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Log.e("R&D", "toString()" + dataSnapshot.toString());
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
Both my results are the same, But is one of them an inefficient way?
Thanks it advance for the help/knowledge.
The two pieces of code accomplish exactly the same in exactly the same way. With just the current snippets, they will even create the exact same number of objects.
If you need use the same Query in multiple places, then your first approach allows you to re-use the same Query object. That would reduce memory usage.
But to be honest, it's a micro-optimization that is unlikely to help in any substantial way.
is there any ways to read data from Firebase once the Activity is loaded. At this moment I am using the regular valueEventListener, but in order for it to work, there has to be some sort of a change in the database
mDatabaseReference.child("Users").child(mUser.getUid()).
child("Posts").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
arrayOfQuestionForms.clear();
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
QuestionForm tempQuestionForm = postSnapshot.getValue(QuestionForm.class);
arrayOfQuestionForms.add(tempQuestionForm);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
What I am looking for is some way to read data from Firebase without using listeners. I have looked at other similar posts but I don't think there is a clear answer for this yet.
There is no way for reading data from a Firebase database without using listeners. Everything is about listeners when it comes to Firebase. It's true that when setting a value, we just need to use a method named setValue() directly on the reference. Unfortunately, there is no method within Firebase, let' say getValue(), which acts in the same way as setValue().
To solve this, i recommend you using addListenerForSingleValueEvent.
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.
in order for it to work, there has to be some sort of a change in the database
This is not true and a common source of confusion for developers.
With your current code, Firebase will immediately start reading the data from the server. Once it gets that data, it invokes your onDataChange().
From the documentation:
This method is triggered once when the listener is attached and again every time the data, including children, changes.
for such purpose I used different kind of listener - ChildEventListener. It has different #Override methods. The method onChildAdded returns every child-nodes of the node when called first time (i.e. on activity start).
Put attention - maybe you will need to slightly change the reference to DB (trim back one hierarchy level), to point to the parent node. If you expanded snapshot of your DB structure, I can look.
Here is updated code (sorry is made any typo - I couldn't test it as have no your DB :)
mDatabaseReference.child("Users").child(mUser.getUid()).child("Posts").addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
arrayOfQuestionForms.clear();
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
QuestionForm tempQuestionForm = postSnapshot.getValue(QuestionForm.class);
arrayOfQuestionForms.add(tempQuestionForm);
}
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
I have this data in Firebase Database:
Negocios:
-KjfCu56lFZCybYldBZy
lugarReference: "Restaurante"
numeroMesas:39
password:"pass123"
user:"user123"
user_id:"usr_1"
I have two applications connected to one project in Firebase, one of them upload data of restaurants reservation, and the other one displays reservations in a RecyclerView.
In the first application, once I do a reservation, it must minus one to numeroMesas value, the other application should display any changes to this key.
In the reservation application, I have this piece of code that I did base in this question:
Update specific keys using Firebase for Android
It supposes to modify the value of numeroMesas, but it does not:
mReference.child("Negocios").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot postSnapshot : dataSnapshot.getChildren()){
Lugar lugar = postSnapshot.getValue(Lugar.class);
if(nombre_lugar.equals(lugar.getUser_id())){
DatabaseReference numMesasReference = mReference.child(dataSnapshot.getKey()).child("numeroMesas");
numMesasReference.setValue(lugar.getNumeroMesas() - 1);
}
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
What it does is the next thing:
Negocios:
-KjfCu56lFZCybYldBZy
lugarReference: "Restaurante"
numeroMesas:39
password:"pass123"
user:"user123"
user_id:"usr_1"
-Negocios
-KjfCu56lFZCybYldBZy
numeroMesas:39
How can be fixed?
I need to minus one once I do a reservation...
Greetings!
Using postSnapshot.getRef() you obtain a reference to the source location for this snapshot. So what's next is to add a child to that reference, that reference will be "numeroMesas" that is the key you want to modify.
As easy as that, the code will be like this.
mReference.child("Negocios").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot postSnapshot : dataSnapshot.getChildren()){
Lugar lugar = postSnapshot.getValue(Lugar.class);
if(nombre_lugar.equals(lugar.getUser_id())){
DatabaseReference numMesasReference = postSnapshot.getRef().child("numeroMesas");
numMesasReference.setValue(lugar.getNumeroMesas() - 1);
}
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
P.S: If you want to minus one just once by reservation, addValueEventListener() won't work, it will minus multiple time as a loop. Use addListenerForSingleValueEvent() instead addValueEventListener().
As the name says, this listener fits for single changes in the data at any location.
I guess the problem is with dataSnapshot.getKey() in this line
DatabaseReference numMesasReference = mReference.child(dataSnapshot.getKey()).child("numeroMesas");
Here you getting the key for the whole list of Negocios, not for the specific child replace it with `
postSnapshot.getKey()
The problem is that your DatabaseReference that you are using is wrong. You are missing a child. So in order to solve this problem, please use the following code:
mReference.child("Negocios").child(negociosId).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
int numeroMesas = (int) dataSnapshot.child("numeroMesas").getValue();
if(nombre_lugar.equals(lugar.getUser_id())){
mReference.child("Negocios").child(negociosId).child("numeroMesas").setValue(numeroMesas - 1);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {}
});
In which negociosId is the unique id generated by the push() method.
Hope it helps.
I'm new to firebase and android. But If the above solutions doesn't work please do the following. I think the problem is that you didn't convert the value of numeroMesas to string. Sorry if there's anything wrong.
Hope it helps.
mReference.child("Negocios").child(negociosId).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
if(nombre_lugar.equals(lugar.getUser_id())){
mReference.child("Negocios").child(negociosId).child("numeroMesas").setValue((Integer.parseInt(dataSnapshot.child("numeroMesas").getValue().toString())) - 1);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {}
});
I keep getting the old values (which no longer is in the database) from my Firebase database. Here is how the database looks right now:
I am getting the info under friendlist. It used to be only one child there with key-value set soosk: true, but now it looks like in the photo. When using addListenerForSingleValueEvent() to my databaseRef, the friendlist retrieved only has soosk: true in it. Here is my code:
mFirebaseDatabaseReference = FirebaseDatabase
.getInstance()
.getReference(
"users/"+FirebaseAuth.getInstance().getCurrentUser().getUid()
);
mFirebaseDatabaseReference.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
User user = dataSnapshot.getValue(User.class);
Log.d(TAG, user.getFriendlist().toString());
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
The method you need to use to have all the values updated is addValueEventListener(). Because you need to listen for more than one value this is the only way to achieve it and to use addListenerForSingleValueEvent().
The most important thing is to remove the listener in your onDestroy method like this:
yourReference.removeEventListener(valueEventListener);