override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  ...

  selectionTracker = SelectionTracker.Builder(
      "selection-demo",
      recyclerView,
      StableIdKeyProvider(recyclerView),
      itemDetailsLookup,
      StorageStrategy.createLongStorage())
    .withSelectionPredicate(selectionPredicate)
    .build()

  selectionTracker.addObserver(object : SelectionTracker.SelectionObserver<Long>() {
    override fun onSelectionChanged() {
      title = if (selectionTracker.hasSelection()) {
        "Selection ${selectionTracker.selection.size()} / $MAXIMUM_SELECTION"
      } else {
        RecyclerViewSelectionActivity::class.java.simpleName
      }
    }
  })
}