FragmentStatePagerAdapter

FavMainAdapter.kt

class FavMainAdapter constructor(
    val context: Context?,
    fragmentManager: FragmentManager
) : BaseFragmentPagerAdapter(fragmentManager) {

    override fun createItem(position: Int): BaseFragment {
        return when (position) {
            0 -> MyCollectionsFragment()
            1 -> MyPlaylistsFragment()
            else -> throw IndexOutOfBoundsException("IndexOutOfBounds $position")
        }
    }

    override fun getCount(): Int {
        return 2
    }

    override fun getPageTitle(position: Int): CharSequence? {
        val item = FavMainPagerItem.values()[position]
        return context?.getString(item.titleId)
    }
}

FavMainFragment.kt

class FavMainFragment
pagerAdapter override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        setupViewPager()
        tabLayout.setViewPager(viewPager)
    }

    private fun setupViewPager() {
        pagerAdapter = FavMainAdapter(activity, childFragmentManager)
        viewPager.isStopScrollWhenTouch = false
        viewPager.adapter = pagerAdapter
        viewPager.currentItem = FavMainPagerItem.COLLECTIONS.ordinal
    }

FragmentStatePagerAdapter.kt

abstract class BaseFragmentPagerAdapter(
    open val fragmentManager: FragmentManager
) : FragmentStatePagerAdapter(fragmentManager, BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {

    /**
     * State key for the fragment super state.
     */
    private val ADS_FRAGMENT_STATE_SUPER_STATE = "superState"

    /**
     * State key for the fragment pages.
     */
    private val ADS_FRAGMENT_STATE_PAGES_COUNT = "pages_count"

    /**
     * A list of fragments displayed by this adapter.
     */
    private val pages = SparseArray<BaseFragment>()

    fun addFragment(fragment: BaseFragment, position: Int = -1) {
        val index = if (position == -1) pages.size() else position
        pages.put(index, fragment)
    }

    override fun getItem(position: Int): Fragment {
        val fragment = createItem(position)
        addFragment(fragment, position)
        return fragment
    }

    override fun getItemPosition(item: Any): Int {
        // Causes adapter to reload all Fragments when
        // notifyDataSetChanged is called
        return PagerAdapter.POSITION_NONE
    }

    abstract fun createItem(position: Int): BaseFragment

    override fun destroyItem(container: ViewGroup, position: Int, item: Any) {
        if (pages.indexOfKey(position) >= 0) {
            pages.remove(position)
        }
        super.destroyItem(container, position, item)
    }

    override fun saveState(): Parcelable? {
        val parcelable = super.saveState()
        val bundle = Bundle()
        bundle.putParcelable(ADS_FRAGMENT_STATE_SUPER_STATE, parcelable)
        bundle.putInt(ADS_FRAGMENT_STATE_PAGES_COUNT, pages.size())
        if (count > 0) {
            for (i in 0 until count) {
                val position = pages.keyAt(i)
                bundle.putInt(i.cacheIndex(), position)
                val fragment = pages.get(position)
                fragmentManager.putFragment(bundle, position.cacheKey(), fragment)
            }
        }
        return bundle
    }

    override fun restoreState(state: Parcelable?, loader: ClassLoader?) {
        try {
            val bundle = state as? Bundle ?: return
            val pageCount = bundle.getInt(ADS_FRAGMENT_STATE_PAGES_COUNT)
            if (pageCount > 0) {
                for (i in 0 until pageCount) {
                    val position = bundle.getInt(i.cacheIndex())
                    val fragment = fragmentManager.getFragment(bundle, position.cacheKey()) as BaseFragment
                    addFragment(fragment, position)
                }
            }
            val parcelable = bundle.getParcelable<Parcelable?>(ADS_FRAGMENT_STATE_SUPER_STATE)

            super.restoreState(parcelable, loader)
        } catch (e: Exception) {
            Log.e("TAG", "Error Restore State of Fragment : " + e.message, e)
        }
    }

    fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        for (i in 0 until count) {
            val fragment = pages.get(i) ?: continue
            fragment.onActivityResult(requestCode, resultCode, data)
        }
    }

    fun onHandleBackPress(): Boolean {
        var handled = false
        for (i in 0 until count) {
            val fragment = pages.get(i) ?: continue
            if (fragment.onHandleBackPress()) handled = true
        }
        return handled
    }

    fun setRestoredFromBackstack(restoredFromBackstack: Boolean) {
        for (i in 0 until count) {
            val fragment = pages.get(i) ?: continue
            fragment.isRestoredFromBackstack = restoredFromBackstack
        }
    }

    fun setRetainInstance(retain: Boolean) {
        for (i in 0 until count) {
            val fragment = pages.get(i) ?: continue
            fragment.retainInstance = retain
        }
    }
}

/**
 * Create an index string for caching fragment pages.
 *
 * @param index The index of the item in the adapter.
 *
 * @return The key string for caching fragment pages.
 */
private fun Int.cacheIndex(): String {
    return "pageIndex:$this"
}

/**
 * Create a key string for caching fragment pages.
 *
 * @param position The position of the item in the adapter.
 *
 * @return The key string for caching fragment pages.
 */
private fun Int.cacheKey(): String {
    return "page:$this"
}