@Dao
interface ConcertDao {
    // The Integer type parameter tells Room to use a PositionalDataSource
    // object, with position-based loading under the hood.
    @Query("SELECT * FROM user ORDER BY concert DESC")
    fun concertsByDate(): DataSource.Factory<Int, Concert>
}

class MyViewModel(concertDao: ConcertDao) : ViewModel() {
    val concertList: LiveData<PagedList<Concert>> = LivePagedListBuilder(
            concertDao.concertsByDate(),
            /* page size */ 20
    ).build()
}

class MyActivity : AppCompatActivity() {
    public override fun onCreate(savedState: Bundle?) {
        super.onCreate(savedState)
        val viewModel = ViewModelProviders.of(this)
                .get(MyViewModel::class.java!!)
        val recyclerView = findViewById(R.id.concert_list)
        val adapter = ConcertAdapter()
        viewModel.concertList.observe(this, { pagedList ->
                adapter.submitList(pagedList) })
        recyclerView.setAdapter(adapter)
    }
}

class ConcertAdapter() :
        PagedListAdapter<Concert, ConcertViewHolder>(DIFF_CALLBACK) {
    fun onBindViewHolder(holder: ConcertViewHolder, position: Int) {
        val concert = getItem(position)
        if (concert != null) {
            holder.bindTo(concert)
        } else {
            // Null defines a placeholder item - PagedListAdapter automatically
            // invalidates this row when the actual object is loaded from the
            // database.
            holder.clear()
        }
    }

    companion object {
        private val DIFF_CALLBACK = object : DiffUtil.ItemCallback<Concert>() {
            // Concert details may have changed if reloaded from the database,
            // but ID is fixed.
            override fun areItemsTheSame(oldConcert: Concert,
                    newConcert: Concert): Boolean =
                    oldConcert.id == newConcert.id

            override fun areContentsTheSame(oldConcert: Concert,
                    newConcert: Concert): Boolean =
                    oldConcert == newConcert
        }
    }
}