Data binding while using fragment manager

Question

I recently started to learn data binding in Android, I understand the basics but when I'm trying to do some more complex data binding such as data binding of view pager with viewpager adapter (uses fragment manager) I don't completely understand how to use the fragment manager of the view (which is a fragment in my case, I use childfragment manager). I found a few incomplete examples and solutions but nothing seemed to work.

Here is the code that I have so far:

• HomeFragment

  class HomeFragment : Fragment() {
  
  lateinit var binding : FragmentHomeBinding
  lateinit var viewModel : HomeViewModel
  
  lateinit var viewPager : ViewPager
  lateinit var tabs : TabLayout
  
  override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
      binding = DataBindingUtil.inflate(inflater, R.layout.fragment_home, container, false)
      val view = binding.root
      viewModel = ViewModelProviders.of(this).get(HomeViewModel::class.java)
      setupViews(view)
  
      return view
  }
  
  fun setupViews(view : View){
      tabs = view.findViewById(R.id.tabs)
      viewPager = view.findViewById(R.id.pager)
  
      val childFragmentManager = childFragmentManager
  
      viewPager.addOnPageChangeListener(TabLayout.TabLayoutOnPageChangeListener(tabs))
  
  
      tabs.addOnTabSelectedListener(object : TabLayout.OnTabSelectedListener {
          override fun onTabSelected(tab: TabLayout.Tab) {
              viewPager.currentItem = tab.position
          }
  
          override fun onTabUnselected(tab: TabLayout.Tab) {
  
          }
  
          override fun onTabReselected(tab: TabLayout.Tab) {
  
          }
      })
  }
  

  }

• home_fragment.xml

  <data>
  
      <variable
          name="fm"
          type="androidx.fragment.app.FragmentManager" />
  
      <variable
          name="viewModel"
          type=".HomeViewModel" />
  
      <variable
          name="adapter"
          type=".HomeViewPagerAdapter" />
  
  </data>
  
  <androidx.constraintlayout.widget.ConstraintLayout
      android:layout_width="match_parent"
      android:layout_height="match_parent">
  
  
      <com.google.android.material.tabs.TabLayout
          android:id="@+id/tabs"
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          app:tabMaxWidth="0dp"
          app:setupWithViewPager="@{pager}"
          app:layout_constraintStart_toStartOf="parent"
          app:layout_constraintTop_toTopOf="parent" >
  
          <com.google.android.material.tabs.TabItem
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:text="@string/upcoming_tab"/>
          <com.google.android.material.tabs.TabItem
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:text="@string/synced_items_tab"/>
  
      </com.google.android.material.tabs.TabLayout>
  
  
      <androidx.viewpager.widget.ViewPager
          android:id="@+id/pager"
          android:layout_width="match_parent"
          android:layout_height="0dp"
          app:adapter="@{adapter(fm)}"
          app:layout_constraintEnd_toEndOf="parent"
          app:layout_constraintStart_toStartOf="parent"
          app:layout_constraintTop_toBottomOf="@+id/tabs" />
  
  
  </androidx.constraintlayout.widget.ConstraintLayout>
  

• HomeViewModel

  class HomeViewModel: ViewModel(){

  @BindingAdapter("adapter")
  fun setAdapter(pager: ViewPager, fm: FragmentManager) {
      pager.adapter = HomeViewPagerAdapter(pager.context,fm)
  }
  

  }

Answer 1

I'll try to provide another way to archive what you're looking while giving some brief explanation. I hope that in the end you can take some benefit from it.

In your <data> leave only the ViewModel and the Adapter

<data>

    <variable
        name="viewModel"
        type=".HomeViewModel" />

    <variable
        name="adapter"
        type=".HomeViewPagerAdapter" />

</data>

Those <variable .../> values need to be set. You can do that using your binding variable, like:

# inside onCreate
binding.viewModel = viewModel
binding.adapter = HomeViewPager(context, fm)

The ViewModel shouldn't be responsible for creating the adapter and even less know of Android components like context or fragment manager, try to keep its dependencies from Fragment/Activity's specifics things at minimum. If this is the only thing that it's currently doing you can safely get rid of it.

Now, for the @BindingAdapter I suggest you to put it inside and object and annotate it with @JvmStatic. You can write it in a separate class and put all yours @BindingAdapters inside of it:

object BindingAdapters {

    @JvmStatic
    @BindingAdapter("adapter")
    fun ViewPager.bindAdapter(adapter: HomeViewPagerAdapter) {
         this.adapter = adapter
    }

}

With that your ViewPager's adapter will be automatically set.

But, I don't think it was worth it. You already have a reference to the ViewPager in your fragment, you could just set the adapter there.