This is my code for reading Firebase data:
final String[] booknum = {"0"};
databaseReference.child("All BID").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
booknum[0] =Long.toString(snapshot.getChildrenCount());
Toast.makeText(getApplicationContext(), booknum[0],Toast.LENGTH_LONG).show();
}
#Override public void onCancelled(DatabaseError databaseError) {
Log.w(TAG, "loadPost:onCancelled", databaseError.toException());
}
});
Toast.makeText(getApplicationContext(), booknum[0],Toast.LENGTH_LONG).show();
When I am executing this, the first toast(inside ValueEventListener) prints the right answer (eg '8'). But the toast outside always prints 0 no matter what.
Please Help!
The ValueEventListener is asynchronous. The lines of code inside of the listener, including the assignment of booknum[0], are not guaranteed to execute before the lines of code written outside of the listener. If you depend on the new value of booknum[0] for some operation, consider moving that operation inside of onDataChange() to ensure it uses the new value.
Related
In the code below i get the number of children but i want to use it outside the onDataChange method.
mRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot)
{
DateStorage dateStorage = null;
for (DataSnapshot result : dataSnapshot.getChildren())
{
Log.e(result.getKey(),result.getChildrenCount() + "");
in[0] = result.getChildrenCount();
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
Can anyone help me?
Data is loaded from Firebase asynchronously. Your main code continues to run while the data is loading, and then when the data is available the onDataChange method is called. What that means is easiest to see if you add a few log statements:
Log.d("TAG", "Before attaching listener");
mRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot)
{
Log.d("TAG", "Got data");
}
#Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
Log.d("TAG", "After attaching listener");
When you run this code, it logs:
Before attaching listener
After attaching listener
Got data
This is probably not the order you expected, but is completely normal when calling asynchronous APIs. And it explains why you'll get the wrong value if you print it outside of the onDataChange().
The problem is not that you can't use the data outside of the onDataChange(), the problem is that you must ensure that onDataChange() has run before you use the data.
The simplest way to do that is to put all code that requires data from the database inside the onDataChange method. But you can also create your own callback interface, and pass that into the method where you load the data. For an example of both of these approaches, see my answer here: getContactsFromFirebase() method return an empty list
I want to count elements in a firebase database, I have seen different topics and tried this code:
final Query dataQuery = myRef.equalTo(MainActivity.user.getUid()).getRef();
dataQuery.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Log.e("ERROR",""+dataSnapshot.child(MainActivity.user.getUid()).getChildrenCount());
}
#Override
public void onCancelled(DatabaseError databaseError) {
Log.e(TAG, "onCancelled", databaseError.toException());
}
});
Now, the value in log error is correct but, if I try to assign it in field, or static field is always 0 out of this method;
How can I use this value in other class?
You need to use a callback and call a method on the callback in your onDataChange. Then once that callback is returned you can continue with the rest of your logic.
You can see an example of that here:
https://github.com/Austin-Android/austin-feeds-me/blob/master/app/src/main/java/com/austindroids/austinfeedsme/data/firebase/FirebaseEventsDataSource.java#L40
fireBase.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Event event = snapshot.getValue(Event.class);
events.add(event);
}
callback.onEventsLoaded(events);
}
#Override
public void onCancelled(DatabaseError firebaseError) {
}
});
You cannot simply take that value and use it outside onDataChange() method, because it will always be null. This is happening because this method has an asynchronous behaviour, which means that is called even before you are getting the data out from the database. A quick fix would be to use that value only inside onDataChange() method, or to dive into the asynchronous world and see the last part of my answer from this post.
I have been trying to set a value change listener like this:
final DatabaseReference chat_ref =
FirebaseDatabase.getInstance().getReference(Constants.ARG_CHAT_ROOMS +
"/" + room_type_1);
chat_ref.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(final DataSnapshot dataSnapshot) {
showAlert(dataSnapshot);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
The problem is that, the parenthesis after the showAlert method keeps looping over, creating several alerts when I only want one.
What could be causing this to loop and how can I address it?
Your help will be most appreciated!
I am not sure where I am getting it wrong, because even this loops as well:
chat_ref.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
showAlert(dataSnapshot);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
You have those several alerts because of the way in which ValueEventListener works. But remember ValueEventListener is not a loop is an interface. Please see more details here. And as you see in the offical doc, it is triggered every time the data at a particular location changes.
To solve this, just get the call of the showAlert() out from the onDataChange method.
Hope it helps.
If you add ValueEventListener with addValueEventListener it will fire every time value of you reference changes so showAlert can be displayed several times.
Use addListenerForSingleValueEvent: your listener will be called only once.
https://firebase.google.com/docs/database/android/read-and-write#read_data_once
I'm totally new to Firebase and need to know how to check if my writing task was successful because if I don't, the MainActivity starts and messes up my Register progress.
This checks if Username is already taken and registers the user if it isn't:
Query usernamequery = myRef.orderByChild("Username").equalTo(Username);
usernamequery.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
if (snapshot.exists()) {
// TODO: handle the case where the data already exists
editText.setError("Username taken!");
return;
}
else {
// TODO: handle the case where the data does not yet exist
myRef.child("Users").child(Username).child("Userid").setValue(user.getUid());
myRef.child("Users").child(Username).child("Username").setValue(Username);
startActivity(maps);
finish();
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
Toast.makeText(Username.this, "Error", Toast.LENGTH_LONG).show();
}
});
But I want the Intent to Main Activity (maps) only be fired when
myRef.child("Users").child(Username).child("Userid").setValue(user.getUid());
and the other one is finished with its task and is successful.
What can I do?
To know when a write operation has completed on the server, add a completion listener:
myRef.child("Users").child(Username).child("Userid").setValue(user.getUid(), new DatabaseReference.CompletionListener() {
void onComplete(DatabaseError error, DatabaseReference ref) {
System.err.println("Value was set. Error = "+error);
// Or: throw error.toException();
}
});
If there was an error, details will be in the error operation. If the write operation was completed without problems, the error will be null.
If you want to write to multiple locations with a single operation, you'll want to look at the update() method
The correct overload for setValue() is documented here.
I have been looking for a way to get one child object's data in Android Firebase.
I have found things like Firebase retrieve child Android. All the solutions are suggesting using a "ChildEventListener", however I need to get this data at this moment, not when it is moved, deleted, updated, etcetera.
My data is kept in https://.firebaseio.com/users//creation as a string. I figure there must be some simple way to access that without needing to do too much, because if I copy the exact URL to my browser, I can see the: 'creation: "2015/05/31 21:33:55"' right there in my "Firebase Forge Dashboard".
How can I access this without a listener?
Firebase listeners fire for both the initial data and any changes.
If you're looking to synchronize the data in a collection, use ChildEventListener. If you're looking to synchronize a single object, use ValueEventListener. Note that in both cases you're not "getting" the data. You're synchronizing it, which means that the callback may be invoked multiple times: for the initial data and whenever the data gets updated.
This is covered in Firebase's quickstart guide for Android. The relevant code and quote:
FirebaseRef.child("message").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
System.out.println(snapshot.getValue()); //prints "Do you have data? You'll love Firebase."
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
In the example above, the value event will fire once for the initial state of the data, and then again every time the value of that data changes.
Please spend a few moments to go through that quick start. It shouldn't take more than 15 minutes and it will save you from a lot of head scratching and questions. The Firebase Android Guide is probably a good next destination, for this question specifically: https://firebase.google.com/docs/database/android/read-and-write
You don't directly read a value. You can set it with .setValue(), but there is no .getValue() on the reference object.
You have to use a listener. If you just want to read the value once, you use ref.addListenerForSingleValueEvent().
Example:
Firebase ref = new Firebase("YOUR-URL-HERE/PATH/TO/YOUR/STUFF");
ref.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
String value = (String) dataSnapshot.getValue();
// do your stuff here with value
}
#Override
public void onCancelled(FirebaseError firebaseError) {
}
});
Source: https://www.firebase.com/docs/android/guide/retrieving-data.html#section-reading-once
just fetch specific node data and its working perfect for me
mFirebaseInstance.getReference("yourNodeName").getRef().addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
Log.e(TAG, "======="+postSnapshot.child("email").getValue());
Log.e(TAG, "======="+postSnapshot.child("name").getValue());
}
}
#Override
public void onCancelled(DatabaseError error) {
// Failed to read value
Log.e(TAG, "Failed to read app title value.", error.toException());
}
});
I store my data this way:
accountsTable ->
key1 -> account1
key2 -> account2
in order to get object data:
accountsDb = mDatabase.child("accountsTable");
accountsDb.child("some key").addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
try{
Account account = snapshot.getChildren().iterator().next()
.getValue(Account.class);
} catch (Throwable e) {
MyLogger.error(this, "onCreate eror", e);
}
}
#Override public void onCancelled(DatabaseError error) { }
});