Firebase SDK in Kotlin - onChildChanged not firing on first child in list

Question

I'm developing a server side app using Kotlin for managing my list of web customers in Firebase. I've successfully added the Java Firebase Admin SDK to my Kotlin and have gotten a functional connection with my Realtime Database.

However, I just noticed one problem. I added a ChildEventListener for listening to changes to their data, and it works nice on every customer but the first one in the list. No matter if I order the data or no with "orderBy", the first queried item never receives a ChildChanged callback when actually edited in the database.

var ref=FirebaseDatabase.getInstance().getReference("web-
customers").orderByChild("displayName")
    ref.addChildEventListener(object:ChildEventListener{
        override fun onCancelled(p0: DatabaseError?) {
                if(p0!=null)
            println(p0.message)
        }

        override fun onChildMoved(p0: DataSnapshot?, p1: String?) {

            println("A child moved !!!!!!!!!!!!!!!!!!")
        }

        override fun onChildChanged(p0: DataSnapshot, p1: String) {
                println("A child changed !!!!!!!!!!!!!!!!!!")
                val key=p0.key as String
                println("A customer changed: "+key)
                for(value:HashMap<String,Any> in list){
                    println(value.get(".key") as String+": "+value.get("displayName") as String)
                    if((value.get(".key") as String).equals(key)){
                        val valor=p0.value as HashMap<String,Any>

                        for(llave in valor.keys){
                            val dato=valor.get(llave) as Any

                            value.put(llave,dato)

                        }

                        println("Customer "+valor.get("displayName") as String+" modified!!!!!!")
                        refresh()
                        break
                    }
                }

        }

        override fun onChildAdded(p0: DataSnapshot, p1: String?) {

            println("A child added !!!!!!!!!!!!!!!!!!")
            val value=p0.value as HashMap<String,Any>
            value.put(".key",p0.key as String)
            list.add(value)
        }

        override fun onChildRemoved(p0: DataSnapshot?) {
            println("A child removed !!!!!!!!!!!!!!!!!!")

        }


    })

I thought there could be some problem with the data in my customers, so it crashed without even throwing an exception, but even after adding a single println for checking a ChildChanged callback was fired, it doesn't do a thing with the first customer in the list!

I've done a similar code in the past for Java (without Kotlin) and they always worked great.

I'm using latest Firebase Java SDK version, 5.2.0.

Has anyone experienced a similar error?

Answer 1

Thanks to a comment by @Alex Mamo, I addressed my attention to the previousChildName params, and there was the problem that had me trapped for hours!

Given Kotlin's default's behavior about not allowing Non-null references in variables without a "?" at the end, the onChildChanged method wasn't fired only on my first item because the previousChildName for it was actually null.

So, I changed p1: String to p1: String? and problem solved!

It didn't work like this:

override fun onChildChanged(p0: DataSnapshot, p1: String) { ... }

It works this way!:

override fun onChildChanged(p0: DataSnapshot, p1: String?) { ... }