1. Functional and
Reactive UIs
Henri Muurimaa
SVP of Engineering, Vaadin
henri@vaadin.com @henrimuurimaa

2. Number of metal bands per capita

3. Functional
programming
Reactive
programming
Functional and reactive UI programming

4. A style of programming that
expresses computation as the
evaluation of mathematical
functions

5. Recursion
Lazy evaluation
Lambda expressions
Type theory
Monads
Referential
transparency
Currying
Entscheidungsproblem
Pattern matching
Tuples

6. Something practical?
Side effects?
State?
No
Bad

7. Okay…

9. What instead of how

10. val square = (number: Int) => number * number
Functional programming 101
val tenSquared = square(10)
val nums = List(1,2,3)
val squares = nums map square // List(1, 4, 9)
val evenNums = nums filter { _ % 2 == 0 } // List(2)
val letters = List("a", "b", "c")
val numbersWithLetters = squares zip letters // List((1,a), (4,b), (9,c))

11. Null checks
are a code smell

12. val presentNumber: Option[Int] = Some(10)
Dealing with absent
values on the type level
val absentNumber: Option[Int] = None
val wrappedNull: Option[Int] = Option(null) // None

13. Working with Option
case class User(name: String, id: Int)
trait UserRepository {
def findById(i: Int): Option[User]
}
val user = userRepository.findById(1) // Option[User]
if(user.isDefined) {
println("User name: " + user.get.name)
}
user foreach { u => println("User name: " + u.name) }
val nameOpt = user map { _.name } // Option[String]
val name = user map { _.name } getOrElse "No user found"

14. Player
Enemy
Illuminated
areaPlayer
sight
range
github.com/hezamu/DungeonGame

15. case class Cell(x: Int, y: Int) {
def +(other: Cell) = Cell(x + other.x, y + other.y)
}
class Dungeon {
var floors: Set[Cell] = Set()
var entities: Map[Entity, Cell] = Map()
def visibleIlluminatedCells = ???
}
Dungeon model

16. class Dungeon {
var floors: Set[Cell] = Set()
var entities: Map[Entity, Cell] = Map()
def playerOpt = entities.keys collect { case p: Player => p } headOption
def playerCellOpt = playerOpt map entities
def playerSightRange = playerOpt map { _.sightRange } getOrElse 0
def inPlayerSightRange(cell: Cell) = playerCellOpt map { playerPos =>
MapLogic.distance(playerPos, cell) <= playerSightRange
} getOrElse false
def visibleIlluminatedCells = {
val lightSources = entities map {
case (entity, cell) => (cell, entity.illuminationRadius)
}
val visibles = lightSources flatMap MapLogic.area filter inPlayerSightRange
floors intersect visibles.toSet
}
}

17. Functional summary
Option lets you get rid of NPEs forever
Use small functions with clear
responsibilities
Chaining functional operations is
practically a superpower

18. What is
Reactive
Programming?

19. int a = b + 1; =B1+1
Spot the difference?

20. Vaadin
ComboBox cities = new ComboBox();
Label selectedCity = new Label();
selectedCity.setPropertyDataSource(cities);
Tessell
void onInit() {
employee.name.set(name);
binder.bind(employee.name).to(textOf(view.employeeName()));
}
void someBusinessLogic() {
// only have to set the name
employee.name.set(newName);
}

21. Listening to events
is a code smell

22. val numbers = Observable.from(List(1, 2, 3))
numbers subscribe { n => println(n) }
Observables to the rescue
val squares = numbers map { num => num * num }
val letters = Observable.from(List("a", "b", "c"))
val numbersWithLetters = squares zip letters

23. RxVaadinwww.vaadin.com/blog/-/blogs/reactive-functional-ui-development-with-vaadin

24. val movesObserver = Vector(
up.clickEvents map { e => tryMove(Cell(0, -1)) },
down.clickEvents map { e => tryMove(Cell(0, 1)) },
left.clickEvents map { e => tryMove(Cell(-1, 0)) },
right.clickEvents map { e => tryMove(Cell(1, 0)) }
)
def tryMove(delta: Cell) = board.dungeon.playerPosition map { cell =>
cell + delta
} filter board.dungeon.canMoveTo
Observing player moves
// Emit a Option[Cell] every time player tries to move
val moveObserver = Observable from movesObserver flatten

25. // Map a legal destination cell to the set of visible cells after the move
val visibleCellsObserver = moveObserver collect { case Some(cell) =>
board.dungeon.playerOpt foreach { board.dungeon.put(_, cell) }
board.dungeon.visibleIlluminatedCells
}
// Subscribe the game board instance to the stream of legal moves.
// This will call board.onNext() with a set of visible cells whenever
// player performs a legal move.
visibleCellsObserver subscribe board
Handling legal moves

26. // Subscribe to the illegal move stream to show a notification every
// time player tries an illegal move.
moveObserver filter { _.isEmpty } subscribe { none =>
Notification.show("That direction is blocked", Notification.Type.Tray)
}
Handling illegal moves

27. Summary
These techniques are very powerful and
they can be learned gradually
Both functional and reactive techniques
are great with UIs
Functional puts FUN back in
programming!

28. Thank You!
github.com/hezamu
@henrimuurimaa
henri@vaadin.com