Intro to RxScala


Focus on usage

  1. What is an asynchronous stream?
  2. Where are asynchronous streams in a given app?
  3. Enter Rx (Reactive Extensions)
  4. Rx Observable
  5. Rx Observer
  6. Schwatcher w/ callbacks
  7. Schwatcher w/ Rx
  8. Conclusions
  9. Resources

What is an

Asynchronous stream?

What is a collection (immutable)?

A collection is a number of objects defined at 1 instance in time.

  • Examples:
    • Stream.fill(9001)("lo")<- not asynchronous
    • Set(1, 2, 3, 4)
    • List("a", "b", "c")
    • Seq.empty

Asynchronous stream?

An asynchronous stream is a source of objects, but the objects are produced at different points in time.

Asynchronous stream marble diagram showing filtering

Where are asynchronous streams in a given app?

  • Technically, anywhere you want to handle something asynchronously.

  • This means that you can use it to represent a single asynchronous result, such as a Future.

  • For practical reasons though, similar to how nobody wraps singular objects in List or Seq, we normally talk about asynchronous streams when there is a possibility of more than 1 result object.

Where are asynchronous streams in a given app ?


 $("#textbox").addEventListener("keypress", function(event) {
        // do stuff
  • When we use callbacks like this, the objects (in this case, HTML events) are all ephemeral; they are "lost" after the function is run (unless if we store them manually).

  • Things also quickly become unmaintainable if we want to support different behaviour depending on the event (e.g. only run the function if the key was "3")

Enter the Rx


Rx (Reactive Extensions) Framework

  • Started off as a .NET library (!) as the asynchronous dual of LINQ (a collection manipulation library). There are now implementations of Rx for various languages.
  • Netflix engineering created a port of Rx to Java - RxJava, and naturally there is a Scala wrapper for RxJava that makes it syntactically nicer to use called RxScala.

Rx Observable

In Rx, the term Observable is used to describe an asynchronous stream.

So from now on, we will use this (shorter) word.

Observable creation

There are several ways to make an Observable:

// From a collection
val obsInts: Observable[Int] = Observable.items(1, 2, 3)

// Emission of consecutive values at intervals
val obsLongsAtIntervals: Obervable[Long] = Observable.interval(100 millis)
  • The RxScala library evolves very quickly so these may already be outdated by the time you see this. Please check the official examples for up-to-date versions.

Callbacks → Observables

Note: There are other ways of doing this

// a Subject is an observable that allows you to put things in it.
val publishSubject = PublishSubject.create[String]
val someSwingTextField = // your Swing TextField
someSwingTextField.subscribe( { case ValueChanged(x) => publishSubject.onNext(x.text) } )
  • We used a PublishSubject, which implements the Subject interface AND the Observable interface
  • Subjects allow you to push things into them
  • There are different kinds of Subject, all of which behave differently. Read about them here

Composing Obervables

Thanks to RxScala, Observable objects have a composition API that is very similar to the standard Scala library.

val periodic: Observable[Long] = Observable.interval(100 millis) // Start with this


val onlyOdds = periodic.filter(_ % 2 == 1)



val times10 = * 10)


Async transformers

There are also other transformers that deal specifically with the async nature of Observables.







... and many others

For more details, read the guide

Rx Observer

Essentially callback functions as a type.

trait Observer[-T]  {
  def onNext(value : T) : Unit
  def onError(error : hrowable) : Unit
  def onCompleted() : Unit

Rx Observer

val observer = Observer[Int](
  onNext = println(_),
  onError = e => println(e),
  onCompleted = () => println("all done")
val subscriptionObserver = intObservable.subscribe(observer)
val subscriptionFunLit = intObservable.subscribe(onNext = println(_))
  • You pass an Observer to an Observable's subscribe method (or a function literal)
  • The return type is Subscription, which is an object you call unsubscribe on to tell the Observable that it should no longer invoke the Observer for that Subscription.


  • File watching library for Scala using Java7+ WatchService API.
  • 2 APIs: Callbacks & RxScala:
    • Let's compare the two when it comes to monitoring files in a directory.


val fileMonitorActor  = // FileMonitor Actor

val printPath = { p: Path => println(s"Something was modified in $p" }

// Callback for 1 file
fileMonitorActor ! RegisterCallback(
  path = Paths get "/Users/lloyd/Desktop/test" ,
  printPath )

// Callback for another file ...
fileMonitorActor ! RegisterCallback(
  path = Paths get "/Users/lloyd/Desktop/test2" ,
  printPath )


val monitor = RxMonitor()
val observable = monitor.observable

val testObservable = observer.filter(_.path.toString.split('/').last == "test")
val test2Observable = observer.filter(_.path.toString.split('/').last == "test2")

val observer = Observer[EventAtPath](onNext = { event => println(s"Something was modified in: ${event.path}")})

monitor.registerPath(ENTRY_MODIFY, Paths get "/Users/lloyd/Desktop")



  • Asynchronous streams are a useful abstraction for dealing with asynchronous data in a composable way.
  • Rx is an asynchronous stream spec/framework that offers a nice API.