RowsSupportFragment which also remembers position in last selected row.

SubRowsSupportFragment.kt

@file:Suppress("PackageDirectoryMismatch")
package android.support.v17.leanback.widget

import android.os.Bundle
import android.support.v17.leanback.app.RowsSupportFragment
import android.support.v7.widget.RecyclerView
import android.view.View

/**
 * RowsSupportFragment which also remembers the subposition in the rows.
 * Warning: very nasty code, because of lifecycle things.
 * Created by Daniel Zolnai on 2018-03-08.
 */
class SubRowsSupportFragment : RowsSupportFragment() {

    companion object {
        private const val KEY_SELECTED_SUB_POSITION = "selected_sub_position"
    }

    // The saved instance state is used when the fragment is restored from a saved state.
    // However if coming from the back stack, there is no saved state, but the internal variables
    // of the fragment still remain. In this case we use this property to set the fragment up and running again.
    var subPositionOnBackStackSave: Int? = null
    private set

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        (getRowViewHolder(selectedPosition)
                as? ListRowPresenter.ViewHolder)?.selectedPosition?.let {
            outState.putInt(KEY_SELECTED_SUB_POSITION, it)
        }
    }

    override fun onStop() {
        super.onStop()
        subPositionOnBackStackSave = (getRowViewHolder(selectedPosition)
                as? ListRowPresenter.ViewHolder)?.selectedPosition
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        // If it is 0, we should not wait.
        if (savedInstanceState != null && savedInstanceState.containsKey(KEY_SELECTED_SUB_POSITION) && savedInstanceState.getInt(KEY_SELECTED_SUB_POSITION) > 0 ||
                subPositionOnBackStackSave != null) {

            findRowViewHolderByPosition(selectedPosition)?.let {
                (presenterSelector.getPresenter(it.row) as? RowPresenter)?.setSelectLevel(it, 1f)
            }
            val subPosition = savedInstanceState?.getInt(KEY_SELECTED_SUB_POSITION)
                    ?: subPositionOnBackStackSave!!
            (verticalGridView.layoutManager as GridLayoutManager).addOnChildViewHolderSelectedListener(object : OnChildViewHolderSelectedListener() {
                override fun onChildViewHolderSelected(parent: RecyclerView?, child: RecyclerView.ViewHolder?, position: Int, subposition: Int) {
                    if (position == selectedPosition) {
                        // I've tried setting the position with the setSelectedPosition and the attached task.
                        // The problem with this is, that the task is only ran after the position has been set on the row.
                        // So you will see the position 0 appear for a frame, and immediately the correct position.
                        // So instead I intercept the layout manager to give me the view before it has been made visible.
                        // Here I set the subposition I want.
                        (child as? ItemBridgeAdapter.ViewHolder)?.mHolder?.let {

                            if (it is RowPresenter.ContainerViewHolder) {
                                (it as? RowPresenter.ContainerViewHolder)?.mRowViewHolder?.let {
                                    // Set the position
                                    (it.view as? ListRowView)?.gridView?.selectedPosition = subPosition
                                    // Set as selected
                                    it.mSelectLevel = 1f
                                    (it as? ListRowPresenter.ViewHolder)?.mListRowPresenter?.onSelectLevelChanged(it)
                                }
                            } else if (it is ListRowPresenter.ViewHolder) {
                                (it as? ListRowPresenter.ViewHolder)?.let {
                                    // Set the position
                                    (it.view as? ListRowView)?.gridView?.selectedPosition = subPosition
                                    // Set as selected
                                    it.mSelectLevel = 1f
                                    (it as? ListRowPresenter.ViewHolder)?.mListRowPresenter?.onSelectLevelChanged(it)
                                }
                            }
                            (verticalGridView.layoutManager as GridLayoutManager).removeOnChildViewHolderSelectedListener(this)
                        }
                    }
                }
            })
        }
    }

    override fun onDestroyView() {
        // Fixes a memory leak where the grid view whould be registered as an observer at the adapter.
        // Each new gridview would subscribe but not unsubscribe, so the old views would be kept in memory and not be collected.
        // We could null out the adapter immediately, but then it would hide the items while the fragment is animating
        verticalGridView?.let {
            val tempRef = it
            tempRef.postDelayed({
                tempRef.adapter = null
            }, 800) // Enough time to finish animations
        }
        super.onDestroyView()
    }
}