dependencies {
  def paging_version = "1.0.0"

  implementation "android.arch.paging:runtime:$paging_version"

  // alternatively - without Android dependencies for testing
  testImplementation "android.arch.paging:common:$paging_version"

  // optional - RxJava support, currently in release candidate
  implementation "android.arch.paging:rxjava2:1.0.0-rc1"
}

class ConcertViewModel {
  fun search(query: String): ConcertSearchResult {
    val boundaryCallback =
        ConcertBoundaryCallback(query, myService, myCache)
    // Error-handling not shown in this snippet.
    val networkErrors = boundaryCallback.networkErrors
  }
}

class ConcertBoundaryCallback(
    private val query: String,
    private val service: MyService,
    private val cache: MyLocalCache
) : PagedList.BoundaryCallback<Concert>() {
  override fun onZeroItemsLoaded() {
    requestAndSaveData(query)
  }

  override fun onItemAtEndLoaded(itemAtEnd: Concert) {
    requestAndSaveData(query)
  }
}

@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
    }
  }
}

 class MyViewModel(concertDao: ConcertDao) : ViewModel() {
   val concertList: Flowable<PagedList<Concert>> = RxPagedListBuilder(
       concertDao.concertsByDate(),
       /* page size */ 50
   ).buildFlowable(BackpressureStrategy.LATEST)
 }

class MyActivity : AppCompatActivity() {
  private lateinit var adapter: ConcertAdapter<Concert>
  private lateinit var viewModel: MyViewModel

  private val disposable = CompositeDisposable()

  public override fun onCreate(savedState: Bundle?) {
    super.onCreate(savedState)
    val recyclerView = findViewById(R.id.concert_list)
    viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java!!)
    adapter = ConcertAdapter()
    recyclerView.setAdapter(adapter)
  }

  override fun onStart() {
    super.onStart()
    disposable.add(viewModel.concertList.subscribe({
        flowableList -> adapter.submitList(flowableList)
    }))
  }

  override fun onStop() {
    super.onStop()
    disposable.clear()
  }
}

private val adapter = ConcertPagedListAdapter()
private lateinit var viewModel: ConcertViewModel

override fun onCreate(savedInstanceState: Bundle?) {
  viewModel = ViewModelProviders.of(this)
      .get(ConcertViewModel::class.java)
  viewModel.concerts.observe(this, adapter::submitList)
}

class ConcertPagedListAdapter() : PagedListAdapter<Concert, ConcertViewHolder>(
    object : DiffUtil.ItemCallback<Concert>() {
  // The ID property identifies when items are the same.
  override fun areItemsTheSame(oldItem: Concert, newItem: Concert)
      = oldItem.id = newItem.id

  // Use the "==" operator (or Object.equals() in Java-based code) to know
  // when an item's content changes. Implement equals(), or write custom
  // data comparison logic here.
  override fun areContentsTheSame(oldItem: Concert, newItem: Concert) =
      oldItem.name == newItem.name && oldItem.date == newItem.date
  }
) {
  override fun onBindViewHolder(holder: ConcertViewHolder, position: Int) {
    val concert: Concert? = getItem(position)

    // Note that "concert" is a placeholder if it's null
    holder.bind(concert)
  }
}

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

// The Integer type argument corresponds to a PositionalDataSource object.
val myConcertDataSource : DataSource.Factory<Integer, Concert> =
    concertDao.concertsByDate()

val myPagedList = LivePagedListBuilder(myConcertDataSource, /* page size */ 20)
    .build()

val myPagingConfig = PagedList.Config.Builder()
    .setPageSize(50)
    .setPrefetchDistance(150)
    .setEnablePlaceholders(true)
    .build()

// The Integer type argument corresponds to a PositionalDataSource object.
val myConcertDataSource : DataSource.Factory<Integer, Concert> =
    concertDao.concertsByDate()

val myPagedList = LivePagedListBuilder(myConcertDataSource, myPagingConfig)
    .setFetchExecutor(myExecutor)
    .build()

class ConcertTimeDataSource(private val concertStartTime: Date) :
    ItemKeyedDataSource<Date, Concert>() {
  override fun getKey(item: Concert) = item.startTime

  override fun loadInitial(
      params: LoadInitialParams<Date>,
      callback: LoadInitialCallback<Concert>) {
    val items = fetchItems(concertStartTime, params.requestedLoadSize)
    callback.onResult(items)
  }

  override fun loadAfter(
      params: LoadParams<Date>,
      callback: LoadCallback<Concert>) {
    val items = fetchItemsAfter(
      date = params.key,
      limit = params.requestedLoadSize)
    callback.onResult(items)
  }
}

class ConcertTimeDataSourceFactory(private val concertStartTime: Date) :
    DataSource.Factory<Date, Concert>() {
  val sourceLiveData = MutableLiveData<ConcertTimeDataSource>()
  override fun create(): DataSource<Date, Concert> {
    val source = ConcertTimeDataSource(concertStartTime)
    sourceLiveData.postValue(source)
    return source
  }
}

class ConcertActivity : AppCompatActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    ...

    concertViewModel.refreshState.observe(this, Observer {
      swipeRefreshLayout.isRefreshing =
          it == NetworkState.LOADING
    })
    swipeRefreshLayout.setOnRefreshListener {
      concertViewModel.invalidateDataSource()
    }
  }
}

class ConcertViewModel : ViewModel() {
  val concertDescriptions : LiveData<PagedList<String>>
    init {
      val factory = database.allConcertsFactory()
          .map { concert ->
              concert.name + " - " + concert.date
          }
      concerts = LivePagedListBuilder(factory, 30).build()
    }
  }
}