pishen / akka-ui

AkkaUI - Build your reactive UI using Akka.js



Build your reactive UI using Akka.js

import akka.ui._

implicit val system = ActorSystem("AkkaUI")
implicit val materializer = ActorMaterializer()

val textBox = input(placeholder := "Name").render
val name = span().render

val source: Source[Event, NotUsed] = textBox.source(_.oninput_=)
val sink: Sink[String, NotUsed] = name.sink(_.textContent_=)

source.map(_ => textBox.value).runWith(sink)

val root = div(
  h2("Hello ", name)



libraryDependencies += "net.pishen" %%% "akka-ui" % "0.4.3"

AkkaUI is built on top of Scala.js, Akka.js, and scala-js-dom.

Setup Akka

Since AkkaUI uses Akka Streams underneath, we have to provide an Akka environment for it:

implicit val system = ActorSystem("AkkaUI")
implicit val materializer = ActorMaterializer()

Creating Sources

AkkaUI supports creating a Source from an EventTarget:

import akka.ui._
import akka.stream.scaladsl.Source
import org.scalajs.dom.raw.HTMLButtonElement
import org.scalajs.dom.raw.MouseEvent
import scalatags.JsDom.all._

val btn: HTMLButtonElement = button("Click me!").render
val source: Source[MouseEvent, akka.NotUsed] = btn.source {
  (elem: HTMLButtonElement) => elem.onclick_=

Here we use Scalatags to generate a HTMLButtonElement (which is also an EventTarget) in scala-js-dom. (Scalatags is not required. You can use whatever tool you want to build a DOM element.)

After getting the HTMLButtonElement, we can build a Source from it using .source(). (If you are not familiar with Source, you may check the document of Akka Streams). The .source() function will expect you to return a listener setter to it. Since onclick is a var in HTMLButtonElement, we can use onclick_= to refer the setter function.

Note that you can only call .source() once for each listener. If you call .source(_.onclick_=) more than one time on the same element, the old listener will be overwritten by the new one and the old Source will not function properly. Instead, feel free to reuse (materialize) the same Source multiple times or pass it to other functions.

// Don't do this
btn.source(_.onclick_=).runForeach(_ => println("Stream 1 is clicked!"))
btn.source(_.onclick_=).runForeach(_ => println("Stream 2 is clicked!"))

// Instead, do this
val source = btn.source(_.onclick_=)
source.runForeach(_ => println("Stream 1 is clicked!"))
source.runForeach(_ => println("Stream 2 is clicked!"))

Calling preventDefault()

When creating the Source, one can add a parameter .source(_.onclick_=, preventDefault = true) to call preventDefault() on the source Event.

Creating Sinks

Similar to what you saw in previous section, AkkaUI also supports creating a Sink from Element:

import akka.ui._
import akka.stream.scaladsl.Sink
import org.scalajs.dom.raw.HTMLSpanElement
import scalatags.JsDom.all._

val name: HTMLSpanElement = span().render
val sink: Sink[String, akka.NotUsed] = name.sink {
  (elem: HTMLSpanElement) => elem.textContent_=

Given an Element instance, we can create a Sink on its...

  • Properties
val sink: Sink[String, NotUsed] = span.sink(_.textContent_=)
val sink: Sink[Double, NotUsed] = span.sink(_.scrollTop_=)
val sink: Sink[String, NotUsed] = span.sink(_.style.backgroundColor_=)
  • Children
val sink: Sink[Seq[Element], NotUsed] = div.childrenSink
  • Class
val sink: Sink[Seq[String], NotUsed] = div.classSink

Unlike the situation in previous section, you can create any number of Sink on the same property here. But it's recommended to also reuse the Sink instance here, since each call to .sink() will create an Actor and take up some space in your program.

Here is a simple example which mixes three kinds of Sinks:

import akka.ui._
import org.scalajs.dom.ext._

def todoItem(content: String) = {
  val checkbox = input(`type` := "checkbox").render
  val status = span(cls := "font-weight-bold").render

  val clsSink = status.classSink.contramap[String] { stat =>
    if (stat == "TODO") {
      status.classList.filterNot(_ == "text-success") :+ "text-primary"
    } else {
      status.classList.filterNot(_ == "text-primary") :+ "text-success"
  val txtSink = status.sink(_.textContent_=)

    .scan("TODO")((old, event) => if (old == "TODO") "DONE" else "TODO")

  div(checkbox, " ", status, " ", content).render

val content = input().render
val btn = button("Add").render
val todoList = div().render

  .map(_ => todoList.children :+ todoItem(content.value))

val root = div(content, btn, todoList)

Note that when we import org.scalajs.dom.ext._ and akka.ui._, we will be able to operate Element.children and Element.classList like an immutable Seq, thanks to implicit classes.

If you want to keep some states in your stream, try using the scan() function from Akka Streams like above.

Prevent Memory Leak

Each time you materialize a stream (with run, runForeach, or runWith), there will be several actors created underneath to handle the stream messages. These actors will not be terminated until the stream is completed from the Source or canceled from the Sink. Furthermore, if you materialize a stream using Source.actorRef() or Sink.actorRef(), the Source and Sink actors will keep listening for new message and will never complete. Hence, it's the users' responsibility to terminate the streams by themselves. (By sending a PoisonPill to the Source or Sink actors for example.)

In AkkaUI, when you create a Source or Sink from an Element, we will keep a binding information in the internal hashmap. When the Element is going to be removed from the DOM by childrenSink, all the Source and Sink related to this Element will be completed or canceled, hence prevent the stream from leaking memory. (These streams will not be terminated if you are removing the DOM element by yourself, so make sure you use childrenSink to do the modification.)

Dynamic Stream Handling

In some use cases, we may have to reference back to a Source that's not yet materialized, or merge more Source into an already materialized stream. In these cases, one can use Akka's MergeHub and BroadcastHub to achieve the task. Following is another Todo-list example which add a "Remove" button after each Todo item, and cycle the "Remove" signal back to list's source:

val contentInput = input().render
val addBtn = button("Add").render
val todoList = div().render

val (removeSink, removeSource) = MergeHub.source[String]
  .map("remove" -> _)
  .alsoTo(todoList.dummySink) // prevent memory leak
  .toMat(BroadcastHub.sink[(String, String)])(Keep.both)

def todoItem(content: String) = {
  val checkbox = input(`type` := "checkbox").render
  val contentSpan = span(content).render
  val removeBtn = button("Remove").render

    .scan(false)((done, _) => !done)
        done => if (done) "line-through" else "none"

  // connect removeBtn to removeSink
  removeBtn.source(_.onclick_=).map(_ => content).runWith(removeSink)

  div(checkbox, " ", contentSpan, " ", removeBtn).render

  .map(_ => "add" -> contentInput.value)
  .scan(Map.empty[String, HTMLDivElement]) {
    case (map, ("add", content)) =>
      map + (content -> todoItem(content))
    case (map, ("remove", content)) =>
      map - content

val root = div(contentInput, addBtn, todoList)

Starting from a source that will merge the "add" and "remove" signal, we eventually convert each signal into several Todo items, where each Todo item will contain a Remove button which sends its "remove" signal (onclick) back to the starting source. To achieve this, we use MergeHub and BroadcastHub to get the Sink that can consume signals from the Remove button(s) and the Source which can be connected to todoList's childrenSink.

Notice that when we call run(), an unhandled stream will be materialized, which we have to terminate by ourselves to prevent memory leak. Here we use a trick that add one more Sink to the Source before we materialize it, which is the dummySink on todoList, this Sink will consume and ignore all the signals sending to it, and will cancel the stream when its binding DOM element is removed. When the stream is canceled, the materialized MergeHub and BroadcastHub will be terminated as well, hence preventing the memory leak. We can use this technique to connect the unhandled materialized stream to a DOM element which has the same life-cycle as the stream.