android - Problem with LiveData observer changing rather than staying the same

Question

I have a problems with a RecyclerView linked to LiveData. The RecyclerView content goes back up all the way to the top every time there is an update.

Apparently the problem lies in the way I created the adapter, as the Observer changes every time. Or so I understood. That observer is stuck inside a lambda, which is one of the many things I am still very remotely familiar with.

This is where the problem lies, apparently:

languagesViewModel.getLanguagesForUser().observe ( viewLifecycleOwner, { languages ->
        LANGUAGES_SCREEN_VIEWS.languagesRecyclerView.adapter = LanguagesRecyclerViewAdapter ( languages ) } )

I tried to get bits out of it, but it didn't work...

My first attempt was this:

var languageAdapter = LANGUAGES_SCREEN_VIEWS.languagesRecyclerView.adapter

languagesViewModel.getLanguagesForUser().observe( requireActivity(), { languages ->
            languageAdapter = LanguagesRecyclerViewAdapter(languages) } )

There was a warning that the variable is assigned but never accessed, and the list of languages is empty.

My second attempt was this:

    val languages = mutableListOf < LanguagesEntity > ()
    var languagesAdapter = LanguagesRecyclerViewAdapter(languages)

    languagesViewModel.getLanguagesForUser().observe( requireActivity(), { languages ->
        LANGUAGES_SCREEN_VIEWS.languagesRecyclerView.adapter = languagesAdapter } )

I hope it's clear that unfortunately I haven't got a clue of what I should really be doing. Not for lack of trying because I stabbed in the dark for 3 days before asking for help, and apparently that is the part of the code which is the problem. I do try to answer my own unanswered questions if I find a solution too. This time it just won't happen.

Thank you very much in advance.

question from:https://stackoverflow.com/questions/66056506/problem-with-livedata-observer-changing-rather-than-staying-the-same

Answer 1

you should have an internal function in adapter for setting value and notify the adapter. And also, DONT set your adapter in LiveData observer.

You should consider this architecture.

Adapter

// STEP 1 - make a function to notify the variable
internal fun setLanguage(lang: List<LanguagesEntity>) {
       languages = lang
       notifyDataSetChanged()
 }

// STEP 2 - setup recyclerview before everything
val languages = mutableListOf < LanguagesEntity > ()
var languagesAdapter = LanguagesRecyclerViewAdapter(languages)
LANGUAGES_SCREEN_VIEWS.languagesRecyclerView.adapter = languagesAdapter 


// STEP 3 - set new value to adapter
languagesViewModel.getLanguagesForUser().observe( requireActivity(), { languages ->
    
    languagesAdapter.setLanguage(language)

} )

Hope it is clear to you.