keeping viewpager2 item position when navigatiing to other fragment

Question

I'm using viewpager2 with navigation component. When I click on one item in viewpager, it goes to another fragment, but when I go back to previous fragment with viewpager, the position of viewpager resets to the first one. How can I keep the viewpager position when navigating to other fragment?

score 1 · Answer 1 · answered May 12 '20 at 00:26

The best way to accomplish what you want is to have a viewmodel that will save your viewpager position.

In your fragment:


private lateinit var viewModel : TestViewModel

override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {

viewModel = ViewModelProvider(this).get(TestViewModel::class.java)

#### all other regular fragment stuff still aplies

}

fun setViewPager(){

        ### regular VP2 code here (adapter, etc)

viewModel.viewPagerSelected.observe(viewLifecycleOwner, Observer { page ->
                viewPager.currentItem = page
            })

viewPager.registerOnPageChangeCallback(object : ViewPager2.OnPageChangeCallback() {

            override fun onPageSelected(page: Int) {
                viewModel.updatePageSelected(page)
            }
}

ViewModel

class TestViewModel() : ViewModel() {

    private val _viewPagerSelected = MutableLiveData<Int>()
    val viewPagerSelected: LiveData<Int> = _viewPagerSelected

    fun updatePageSelected(newSelection: Int) {
        _viewPagerSelected.postValue(newSelection)
    }

}

I hope it helps!

is the **viewPager.currentItem = page** placed after the adapter/data is set? — Henrique Vasconcellos, May 12 '20 at 18:16
After. It works with `viewpager.setCurrentItem(page, false)` but viewpager lags a lot. — Mehranjp73, May 12 '20 at 22:37
have you tried set the flag to true? This flag refers to "smooth scroll" it shouldnt matter in your case, it should set the item anyways. — Henrique Vasconcellos, May 12 '20 at 22:44
Are you using the latest viewpager2 library? I have this repo working just fine, take a look https://github.com/chenriquevz/PokeDex/blob/master/app/src/main/java/com/chenriquevz/pokedex/ui/pokemon/PokemonFragment.kt https://github.com/chenriquevz/PokeDex/blob/master/app/src/main/java/com/chenriquevz/pokedex/ui/pokemon/PokemonViewModel.kt — Henrique Vasconcellos, May 12 '20 at 22:58

CTD · Answer 2 · 2021-06-10T09:11:55.170

Initialize FragmentStateAdapter with const ItemCount, I guest the position you want to keep is greater than ItemCount.

class Adapter(val data: List<String>, fragment: Fragment) : FragmentStateAdapter(fragment) {

    override fun getItemCount(): Int = data.size

    override fun createFragment(position: Int): Fragment {
        return  SubFragment.newInstance(data.get(position))
    }
}

viewModel.subFragment.observe(viewLifecycleOwner){
    Adapter(it,this@fragment)
}

keeping viewpager2 item position when navigatiing to other fragment

2 Answers2