Handler().postDelayed({
    startActivity(Intent(this, MainActivity::class.java))
    finish(); overridePendingTransition(anim.fade_in, anim.fade_out)  // use fade animation
}, DELAY_MILLIS)  // jump to MainActivity after delay

// Determine current layout: https://developer.android.com/training/multiscreen/adaptui#TaskDetermineCurLayout
val fragmentContainer: View? = findViewById(R.id.fragmentContainer)
isDualPane = fragmentContainer?.visibility == View.VISIBLE

if (isDualPane) {
    // However, if we're being restored from a previous state,
    // then we don't need to do anything and should return or else
    // we could end up with overlapping fragments.
    if (savedInstanceState != null) return
    val defaultFragment = PlayerFragment()  // create a new Fragment to be placed in the activity layout
    val bundle = Bundle(); bundle.putParcelable(Item.EXTRA_KEY, items[0])  // pack first item up with a bundle
    defaultFragment.arguments = bundle  // pass first item to default fragment
    // Add the fragment to the 'fragmentContainer' FrameLayout
    supportFragmentManager.beginTransaction().add(R.id.fragmentContainer, defaultFragment).commit()
}

override fun onItemClick(item: Item) {
    if (isDualPane) {
        val newFragment = PlayerFragment()
        val bundle = Bundle(); bundle.putParcelable(Item.EXTRA_KEY, item)  // pack the item up with a bundle
        newFragment.arguments = bundle  // pass first item to first fragment
        supportFragmentManager.beginTransaction().replace(R.id.fragmentContainer, newFragment).commit()
    }  // layout is at sw600dp variation, replace fragment on list item clicked.
    else {
        startActivity(Intent(this, DetailActivity::class.java).putExtra(Item.EXTRA_KEY, item))
    }  // layout is at normal variation, start detail activity on list item clicked.
}  // implement RecyclerAdapter.OnItemClickListener

// Provide a reference to the views for each item
class ItemViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
    fun bind(item: Item, listener: OnItemClickListener) = with(itemView) {
        title.text = item.title
        duration.text = item.duration
        setOnClickListener { listener.onItemClick(item) }
    }  // bind an item object to its view
}

val playerFragment = PlayerFragment()  // create a player fragment
val item = intent.getParcelableExtra<Item>(Item.EXTRA_KEY)  // get item object from MainActivity
supportActionBar?.title = item.title  // set actionbar title with item's
val bundle = Bundle(); bundle.putParcelable(Item.EXTRA_KEY, item)  // pack item object up with a bundle
playerFragment.arguments = bundle  // pass item object to player fragment through bundle
supportFragmentManager.beginTransaction().add(R.id.detailContainer, playerFragment).commit()  // show fragment

class PlayerFragment : Fragment() {
    private lateinit var player: SimpleExoPlayer
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        if (container == null) return null  // create and return view only in dual pane mode
        val playerView = PlayerView(activity)
        // https://exoplayer.dev/hello-world.html
        player = ExoPlayerFactory.newSimpleInstance(activity)  // initialize player instance
        playerView.player = player  // bind the player to the view
        val item = arguments?.getParcelable<Item>(Item.EXTRA_KEY)  // get item object from list activity
        player.apply {
            playWhenReady = true  // whether auto-play video when ready
            prepare(
                ProgressiveMediaSource.Factory(DefaultHttpDataSourceFactory(getString(R.string.app_name)))
                    .createMediaSource(Uri.parse(item?.url))
            )  // prepare the player with `ProgressiveMediaSource`, which is for regular media files like "*.mp4".
        }
        return playerView
    }
    override fun onDestroy() {
        super.onDestroy()
        player.release()  // release the player when fragment is destroyed, or it will still play in background.
    }
}

@Parcelize  // for intent extra, see more in RecyclerAdapter.ItemViewHolder.onClick() and DetailActivity.onCreate()
@Serializable  // for reading list from json resource file, see more in MainActivity.onCreate()
data class Item(val title: String, val duration: String, val url: String) : Parcelable {
    companion object {
        const val EXTRA_KEY = "${BuildConfig.APPLICATION_ID}.Item"
    }  // intent extra key, use app's package name as prefix to be unique
}