ViewModel is duplicating items for recyclerview

Question

I am using Firebase firestore pagination and ViewModel class in my project, I have setup an onScroll listener for recyclerview, and fetching data on scroll but when i Navigate to another fragment and back to the main fragment, the whole items are duplicated, How can i fix this issue??

Here is my code

NewsViewModel.kt

class NewsViewModel : ViewModel() {

private val repo = FirebaseRepo(this)

val mutableLiveData = MutableLiveData<List<News>>()

fun getNewsList(tm:Timestamp): LiveData<List<News>> {
    repo.getNewsData(tm)
    return mutableLiveData
}
}

Repository.kt

class FirebaseRepo(private val viewModel: NewsViewModel) {

private val db = FirebaseFirestore.getInstance().collection("news")

fun getNewsData(tm: Timestamp) {
    val newsList = ArrayList<News>()
    if(viewModel.mutableLiveData.value != null) {
        newsList.addAll(viewModel.mutableLiveData.value!!)
    }
    db
        .orderBy("timestamp", Query.Direction.DESCENDING)
        .whereLessThan("timestamp",tm)
        .limit(6)
        .get()
        .addOnSuccessListener {
            Log.i("CodeCamp", it.toString())
            for (doc in it) {
                val imgUrl = doc.getString("imageUrl")
                val heading = doc.getString("headline")
                val timestamp = doc.getTimestamp("timestamp")
                val tagline = doc.getString("tagline")
                val type = doc.getString("type")
                newsList.add(News(doc.id, imgUrl!!, heading!!, tagline!!, type!!, timestamp!!))
            }
            viewModel.mutableLiveData.value = newsList
        }
}
}

MainActivity.kt

viewModel = ViewModelProvider(this).get(NewsViewModel::class.java)
    val layoutManager = LinearLayoutManager(view.context)
    recyclerView.layoutManager = layoutManager
    recyclerView.adapter = newsAdapter
    recyclerView.addItemDecoration(DividerItemDecoration(view.context, RecyclerView.VERTICAL))

    //observe to the viewModel
    viewModel.getNewsList(Timestamp.now()).observe(viewLifecycleOwner, Observer {
        newsAdapter.submitList(it)
    })

    recyclerView.addOnScrollListener(object : RecyclerView.OnScrollListener() {
        override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
            super.onScrolled(recyclerView, dx, dy)
            val visibleItemCount = layoutManager.childCount
            val totalItemCount = layoutManager.itemCount
            val firstVisibleItemPosition = layoutManager.findFirstVisibleItemPosition()
            if ((visibleItemCount + firstVisibleItemPosition) >= totalItemCount
                && firstVisibleItemPosition >= 0
                && totalItemCount >= PAGE_SIZE && !isLoading
            ) {
                isLoading != isLoading
                val list = viewModel.mutableLiveData.value!!
                viewModel.getNewsList(list[list.size - 1].timestamp).value
                Handler().postDelayed({
                    isLoading != isLoading
                },2000)
            }
        }
    })

My Adapter

class NewsAdapter : ListAdapter<News, NewsAdapter.ViewHolder> (NEWS_COMPARATOR) {

companion object {
    private val NEWS_COMPARATOR =  object : DiffUtil.ItemCallback<News>() {
        override fun areItemsTheSame(old: News, new: News): Boolean = old.id == new.id
        override fun areContentsTheSame(old: News, new: News): Boolean = old == new
    }
}

class ViewHolder(private val view: View) : RecyclerView.ViewHolder(view) {

    fun bindView(news: News) {
        Glide.with(view).load(news.imageUrl).into(itemView.img)
        itemView.news_title.text = news.heading
        itemView.news_src.text = news.tagline
        itemView.news_type.text = news.type
        itemView.news_time.text = DateTime.getTimeAgo(news.timestamp.seconds)
        itemView.setOnClickListener {
            it.findNavController().navigate(R.id.action_homeFragment_to_newsFragment, bundleOf("id" to news.id))
        }
    }
}

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
    val view = LayoutInflater.from(parent.context).inflate(R.layout.item_news,parent,false)
    return ViewHolder(view)
}
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
    val news = getItem(position)
    holder.bindView(news)
}

}

Answer 1

The problem is in your destructuring:

Everytime Observer called your ViewModel is re-fetching data from firebase and holding it in variable called mutableLivedata as you defined.

You need to observe mutableLiveData for your recyclerView and call getNewsItem() inside init function as shown below:

ViewModel.kt

val mutableLiveData = MutableLiveData<List<News>>()

fun getNewsList(tm:Timestamp) {
    repo.getNewsData(tm)
}

init {
    getNewsList(Timestamp.now())
}

MainActivity.kt

viewModel.mutableLiveData.observe(viewLifecycleOwner, Observer {
        newsAdapter.submitList(it)
    })

Happy Coding..