How can I call notification only when Firebase database has changed?

Question

As title what I want to do is only when database had change than run notification for it but a problem is when I run app the notification appear even the data is not change here is my code:

 public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View v= inflater.inflate(R.layout.main,container,false);

        result= (TextView) v.findViewById(R.id.textView3);


        notificationmanager = NotificationManagerCompat.from(ct.getcontext());
        if (user != null) {

            ref.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    if (dataSnapshot.exists()) {
                        String post = dataSnapshot.getValue(String.class);
                        result.setText(post);

                        Notification notification = new NotificationCompat.Builder(ct.getcontext(), CHANNEL_1_ID)
                                .setSmallIcon(R.drawable.ic_launcher_foreground)
                                .setContentTitle(post)
                                .setContentText("test")
                                .setPriority(NotificationCompat.PRIORITY_HIGH)
                                .setCategory(NotificationCompat.CATEGORY_EVENT)
                                .setDefaults(Notification.DEFAULT_ALL)
                                .build();
                        notificationmanager.notify(2,notification);



                    }

                }

                @Override
                public void onCancelled(DatabaseError error) {

                }
            });

        }
        else
        {
            Toast.makeText(getActivity(),"not yet login",Toast.LENGTH_LONG).show();
        }


        return v;
    }

First call to firebase also works like `onDataChange` for it. you can change your call to firebase to `addChildEventListener` and you will get more callbacks like `onChildChanged`. note that you need to change your reference to. — Vadim Eksler, Mar 31 '19 at 11:35
This listener will give you one time only datasnapshot of the value in the reference you used. Regardless of whether a value had changed — ronginat, Mar 31 '19 at 11:37

score 1 · Answer 1 · answered Mar 31 '19 at 12:05

1

Save the value in your local preferences, then on next change, verify if the value if it is really changed by comparing with previously saved value.

answered Mar 31 '19 at 12:05

Umer Waqas - Top rated dev

3,072
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score 0 · Answer 2 · edited Jun 20 '20 at 09:12

0

You need to use addValueEventListener(), from the docs:

public ValueEventListener addValueEventListener (ValueEventListener listener)

Add a listener for changes in the data at this location. Each time time the data changes, your listener will be called with an immutable snapshot of the data.

Using addValueEventListener, everytime there is a change in the database it will be triggered.

edited Jun 20 '20 at 09:12

Community

1
1

answered Mar 31 '19 at 11:54

Peter Haddad

78,874
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A listener will always fire with the current value too, so OP will need to filter that out, for example by tracking it in local preferences as [vicky answered](https://stackoverflow.com/a/55440731). – Frank van Puffelen Mar 31 '19 at 14:22

score 0 · Answer 3 · answered Mar 31 '19 at 12:27

From @mikasaloli's explanation, I suspect the problem lies more in in a mix of the way firebase RTDB( realtime database) / Firestore functions and the wrong listener.

The first problem is, firebase RTDB/firestore by default keeps a local cache copy of the references( not all of them) you've accessed on your phone. From their docs, they say, and I quote: "Asynchronous listeners: Data stored in a Firebase Realtime Database is retrieved by attaching an asynchronous listener to a database reference. The listener is triggered once for the initial state of the data and again anytime the data changes. An event listener may receive several different types of events. This mode of data retrieval is supported in Java, Node.js and Python Admin SDKs." What you need to do is bypass the local cache when reading data for the first time after app startup. here's how I've been able to do that( with some slight tweaking)

The second problem is as previously mentioned by the others, addListenerForSingleValueEvent will get triggered only once. So, assuming you receive the notification change for the first time, your listener will automagically unregister itself implying you won't receive the subsequent onDataChange events.

Now, conjecturing from the previous paragraphs, the problem with the above code is: it attaches a listener for single value event. which then gets triggered with the local ( cached ) version of the data which forcibly isn't the latest state of the server database ( if the application was offline while some changes took place online). But then, upon attaching this listener, it gets triggered with the local version of the data and unregisters itself afterwards. Implying, the above code will get triggered once at most and might never get triggered during it's lifetime while attached.

What I suggest, is you bypass the local cache and use a addValueEventListener.

How can I call notification only when Firebase database has changed?

3 Answers3