Scala-rx-puppeteer is an utility library for Scala.Rx (an experimental change propagation library for Scala) that aims to nullify the risk of introducing cyclic dependencies.

These cyclic dependencies might be introduced when triggering the recalculation of an Rx while performing side effects on an observer (Obs) change.

Scala-rx-puppeteer offers some alternative syntax to create those dependencies and throws an exception if a dependency is added that would lead to a cyclic dependency.

In my Scala applications I got used to using Scala.Rx extensively. For better or worse, this lead to a lot of code looking somewhat like this:

val x = Var(1)
val y = Var(2)

val onXChange = x.triggerLater {
  // do something with the data of x
}

val onYChange = y.triggerLater {
  // do something with the data of y 
  // also: in order to correctly work with y, we indirectly need data x
}

// if the value of x changes; also notify the observers of y
x.triggerLater {
   y.recalc()
}

Specifically, I noticed that I had use recalc() inside triggerLater code blocks quite often. My use cases were mostly that I needed to trigger a redraw in my Scala.js application every time x changes. Also data y's representation indirectly depends on x, so everytime x changes, the observers of y need to be triggered as well so that all data depending on y is redrawn as well.

The Problem: Handling these dependencies can easily get out-of-hand because of cyclic relationships and duplicates. Finding these errors quickly was becoming a problem as the application grew larger and more complex.

Basic example: Using scala-rx-puppeteer, the example given above can be rewritten like this:

import com.github.fbaierl.rxpuppeteer.RxPuppet.rxToPuppet

val x = Var(1)
val y = Var(2)

val onXChange = x.triggerLater {
  // ..
}

val onYChange = y.triggerLater {
  // ..
}

x ~~~> y 

Writing x ~~~> y in this case equals x.triggerLater { y.recalc() } but throws an AssertionError if this dependency introduces a cyclic relationship or has already been added once.

Advanced example: Code can be executed before and after the change propagation. Also, the change propagation can be bound to a condition.

val x = Var(1)
val y = Var(2)
val condition = false

x.trigger {
  /* code to run before change propagation */}
  if(condition){
    y.recalc()
  }
  /* code to run after after propagation */
}

Can be rewritten using rx-puppeteer like this:

import com.github.fbaierl.rxpuppeteer.RxPuppet.rxToPuppet

val x = Var(1)
val y = Var(2)
def condition: Boolean = false

(x runBefore  {/* code to run before change propagation */}
   runAfter   {/* code to run after change propagation */}
   activateIf condition
  ) ~~> y 

~~> results in trigger being called on the Rx, ~~~> results in triggerLater being called on the Rx.

Scala-rx-puppeteer is available on Maven Central. In order to get started, simply add the following to your build.sbt:

libraryDependencies += "com.github.fbaierl" %% "scala-rx-puppeteer" % "0.1.3"

For Scala.js add:

libraryDependencies += "com.github.fbaierl" %%% "scala-rx-puppeteer" % "0.1.3"

Copyright 2018 Florian Baierl

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