Kotlin Sequences: Deep Dive

Android developers spend countless hours dealing with lists of data, and with the vast amount of collection operations available to us (think ‘map’, ‘filter’ or ‘sort’) it’s become easier than ever to manipulate that data into the format we want. But all that processing can come at a cost, as sometimes we sacrifice efficiency in the name of readable and concise code. It would be great if there was a way we could do both - be a good citizen when dealing with data, while also still using all the handy operations that we’ve grown to love...

Enter Kotlin Sequences! They offer a different take on iterating through data than a Collection, and in many cases allow you to work with your data more efficiently. Join us as we go through the ins and outs of Kotlin Sequences - what they are, when to use them, and what advantages they offer over traditional collections.

Transcript

1. Kotlin Sequences Deep Dive Phil Shadlyn | @physphil November 16, 2020 #dcAmericas
2. What is a Sequence?
3. What are Kotlin Sequences? “A sequence is an iterable type that can be operated upon without creating unnecessary intermediate collections, by running all applicable operations on each element before moving onto the next.” https://typealias.com/guides/kotlin-sequences-illustrated-guide/
4. Great... What does that mean?
5. An Example • TV Show reference app • Display 10 favourites and their rating • Consume API of 30,000 TV Shows
6. data class NetworkTvShow( val id: String val title: String, val year: String?, val ratings: List<Int>?, val favourite: Boolean, val characters: List<Character>? ) data class TvShow( val title: String, val year: Int, val rating: Int, val favourite: Boolean ) Server model App model
7. Order of operations: • Get list of shows from API • Filter by favourite • Convert network model to domain model • Take first 10 elements • Display in list
8. val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList)
9. val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList)
10. val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList) New collection
11. val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList) New collection
12. val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList) New collection
13. val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList)
14. val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) 30,000 shows 31,941 operations 30,000 operations 1,931 operations 10 operations 3 new collections
15. val favouritesList = tvShowApi.getShowsList() .asSequence() .filter { it.favourite } .map { it.toTvShow() } .take(10) .toList() 30,000 shows 39 operations 19 operations 10 operations 10 operations
16. Whoa… what just happened?
17. What are Kotlin Sequences? “A sequence is an iterable type that can be operated upon without creating unnecessary intermediate collections, by running all applicable operations on each element before moving onto the next.” https://typealias.com/guides/kotlin-sequences-illustrated-guide/
18. The Wire Favourite Seinfeld Favourite Breaking Bad Game of Thrones Favourite Mad Men filter { it.favourite } The Wire Seinfeld Game of Thrones map { it.toTvShow() } The Wire Seinfeld Game of Thrones take(10) The Wire Seinfeld Game of Thrones … … … …
19. Collection Processing • Operations executed eagerly • Operate on every element • Intermediate collections
20. The Wire Favourite Seinfeld Favourite Breaking Bad Game of Thrones Favourite Mad Men filter { it.favourite } The Wire Seinfeld Game of Thrones map { it.toTvShow() } The Wire Seinfeld Game of Thrones take(10) The Wire Seinfeld Game of Thrones … …
21. Sequence Processing • Operations executed lazily • One element at a time • Chain executed with terminal operator • No intermediate collections • Can short circuit
22. Intermediate Operators • Return a Sequence • Build out operation chain • filter, map, take, etc...
23. val favouritesList = tvShowApi.getShowsList() .asSequence() .filter { it.favourite } .map { it.toTvShow() } .take(10) .toList()
24. Terminal Operators • Return a result • Trigger execution of operation chain • toList(), count(), maxOf(), etc...
25. val favouritesList = tvShowApi.getShowsList() .asSequence() .filter { it.favourite } .map { it.toTvShow() } .take(10) .toList()
26. Sequence Summary • Operations executed lazily • Execute chain on each item • Chain executed with terminal operator • No intermediate collections • Can short circuit
27. How can I create a Sequence?
28. Creating Sequences /** * Creates a [Sequence] instance that wraps the original * collection returning its elements when being iterated. */ public fun <T> Iterable<T>.asSequence(): Sequence<T> val showSequence = tvShowApi.getShowsList().asSequence()
29. Creating Sequences /** * Creates a sequence that returns the specified values. */ public fun <T> sequenceOf(vararg elements: T): Sequence<T> val showSequence = sequenceOf(TheWire, Seinfeld, GameOfThrones)
30. Creating Sequences /** * Returns a sequence defined by the starting value [seed] * and the function [nextFunction] */ public fun <T : Any> generateSequence( seed: T?, nextFunction: (T) -> T? ): Sequence<T> val incrementalSequence = generateSequence(1) { it + 1 }
31. Should I use a Collection or Sequence?
32. Use a Collection if: • Few items • Few operators • Stateful operations • Missing Sequence operators
33. Use a Sequence if: • Many items • Many operations • Stateless operations • Short circuits
34. Want to Learn More? • Kotlin Sequences docs https://kotlinlang.org/docs/reference/sequences.html • Kotlin Sequences: An Illustrated Guide https://typealias.com/guides/kotlin-sequences-illustrated-guide/ • Kotlin Sequence Tutorial https://winterbe.com/posts/2018/07/23/kotlin-sequence-tutorial/ • Kotlin Vocabulary: Collections and Sequences https://www.youtube.com/watch?v=77hfjIYwouw
35. Thank You! Phil Shadlyn | @physphil November 16, 2020 #dcAmericas