Simple Example for Scala React

Question

Based on the paper Deprecating the Observer Pattern with Scala.React I tried to set up a simple example from the paper, but it throwed an exception Exception in thread "main" java.lang.AssertionError: assertion failed: This method must be run on its domain scala.react.NilDebug@1502c065

A related question is Running a simple Scala.React expression.

How do I set up everything to use the great power of the scala react library?

Besides the library scala-react, I used the following example:

object MyFirstReact extends App {

  object MyDomain extends scala.react.Domain {
    protected val scheduler: Scheduler = new ManualScheduler
    protected val engine: Engine = new Engine
  }
  import MyDomain._

  case class MouseEvent(position: (Int, Int))
  class Path(var positions: Seq[(Int, Int)]) {
    def this(pos: (Int, Int)) = this(Seq(pos))
    def lineTo(pos: (Int, Int)) { positions = positions :+ pos }
    def close { positions = positions :+ positions.head }
  }

  val mouseDown: Events[MouseEvent] = Events.once(MouseEvent((0, 0)))
  val mouseMove: Events[MouseEvent] = Events.once(MouseEvent((1, 1)))
  val mouseUp: Events[MouseEvent] = Events.once(MouseEvent((2, 2)))
  def draw(path: Path) { /* ... */ }

  Reactor.loop { self =>
    // step 1
    val path = new Path((self await mouseDown).position)
    self.loopUntil(mouseUp) { // step 2
      val m = self awaitNext mouseMove
      path.lineTo(m.position)
      draw(path)
    }
    path.close // step 3
    draw(path)
  }

}

Answer 1

You got the exception because Reactor.loop must be wrapped with a schedule { ... } block. But fixing that is not enough (I got an java.lang.StackOverflowError).

Anyhow, I prepared a full working example using a scala-swing in which you can actually draw a line with the very same reactor code that you have. I put a sbt-buildable copy with scala-react included to github: https://github.com/zsoltdonca/scala-react-line-drawing

package zsd

import scala.swing._
import java.awt.event.{MouseMotionAdapter, MouseEvent, MouseAdapter}
import java.awt.{Color, Point}
import scala.react.Domain

object MyDomain extends Domain {
  val scheduler = new SwingScheduler()
  val engine = new Engine
}

import MyDomain._

object ScalaReactLineDrawing extends SimpleSwingApplication with Observing {

  override def main(args: Array[String]) {
    schedule { startup(args) }
    start() // starts the scala-react engine
  }

  override def top: Frame = new MainFrame() {
    contents = new FlowPanel() {
      val mouseDown = EventSource[Point]
      val mouseMove = EventSource[Point]
      val mouseUp = EventSource[Point]

      val mainProgramFlow = Reactor.loop {
        self =>
        // step 1
          val path = new Path(self await mouseDown)
          self.loopUntil(mouseUp) {
            // step 2
            val m = self awaitNext mouseMove
            path.lineTo(m)
            draw(path)
          }
          path.close() // step 3
          draw(path)
      }

      peer.addMouseListener(new MouseAdapter {
        override def mousePressed(e: MouseEvent): Unit = mouseDown << e.getPoint
        override def mouseReleased(e: MouseEvent): Unit = mouseUp << e.getPoint
      })
      peer.addMouseMotionListener(new MouseMotionAdapter {
        override def mouseDragged(e: MouseEvent): Unit = mouseMove << e.getPoint
      })

      class Path(var positions: Seq[Point]) {
        def this(pos: Point) = this(Seq(pos))

        def lineTo(pos: Point) {
          positions = positions :+ pos
        }

        def close() {
          positions = positions :+ positions.head
        }
      }

      var pathDrawn = new Path(new Point(0, 0))
      def draw(path: Path) {
        pathDrawn = path
        repaint()
      }

      override protected def paintComponent(g: swing.Graphics2D): Unit = {
        super.paintComponent(g)

        val xPoints = pathDrawn.positions.map(pos => pos.x).toArray
        val yPoints = pathDrawn.positions.map(pos => pos.y).toArray
        g.setColor(Color.BLACK)
        g.drawPolyline(xPoints, yPoints, pathDrawn.positions.length)
      }
    }
    preferredSize = new Dimension(400, 300)
    pack()
  }
}