SplitChildSignal does not re-sync when restarted

[TomasStanek]

Hello everybody,
I've encountered an unexpected behavior of a signal created by the split operator.

First the code:

import com.raquo.laminar.api.L._


object SplitSignalBehavior {

  def render() = {
    // set of visible keys
    val visibleSetVar: Var[Set[Int]] = Var(Set.empty)

    def toggleVisibility(i: Int) = {
      val s = visibleSetVar.now()

      val s2 =
        if (s.contains(i)) s - i
        else s + i

      visibleSetVar.set(s2)
    }


    var j = 0

    // example of a sequence with _1 key and _2 value
    def computeSequence(): Vector[(Int, Int)] = {
      (1 to 3).iterator.map(i => (i, j)).toVector
    }

    val seqVar = Var(computeSequence())

    // increment j and recompute sequence
    def increment() = {
      j += 1
      seqVar.set(computeSequence())
    }

    div(
      button(
        "increment",
        onClick --> Observer { _: Any => increment() }
      ),
      table(
        children <-- {
          seqVar.signal.split(_._1) { case (key, _, badSig) =>
            val goodSig = seqVar.signal.map(_.find(_._1 == key).get)

            tr(
              td(
                button(
                  key.toString,
                  onClick --> Observer { _: Any => toggleVisibility(key) }
                )
              ),
              td(
                child <-- visibleSetVar.signal.map(_.contains(key)).map { isVisible =>
                  if (isVisible)
                    div(
                      child.text <-- badSig.map(_._2.toString), // keeps an old value
                      "|",
                      child.text <-- goodSig.map(_._2.toString) // works as expected
                    )
                  else
                    div("hidden")
                }
              )
            )
          }
        }
      )
    )
  }
}

The behavior (latest laminar):
Each row of the table has a show/hide toggle button which shows a hypothetical component.
When a component is shown (attached to the dom) and the increment button is pressed, everything works as expected (both numbers in the row are updated).
When a component is hidden (not attached to the dom), the increment button is pressed, then the component is shown, only the value from goodSig is the current one and the value from badSig is old.
In other words the values comming from the badSig are updated only when the element is mounted.

Behavior of different laminar versions:
Versions 15.0.0, 16.0.0 and 17.0.0-M6 exhibit the described behavior
My local version 0.14.2 with applied fix for #95 exhibits the same behavior.
Version 0.14.2 from maven behaves such that none of the values are updated when hidden (although the behavior of the goodSig and badSig differs in terms of the first mount).

Best regards,
Tomas Stanek

[yurique]

I tried reproducing it here:
https://scribble.ninja/u/yurique/dhzaiieqevjkokiyajlxyrhilmh

And it does reproduce.
Also, if you uncomment the badSig.signal.debugLogEvents() - it starts to behave correctly.

[raquo]

Thanks for reporting with a nice reproduction!

In 0.14.x, this is the expected behaviour, however in v15 and up, this child signal should have synced up with the parent signal when the former was re-started (when its isVisible changed from false to true). The fact that this does not happen is indeed a bug.

The root of this problem in v15 and up is that SplitChildSignal (the type of your badSig) did not have proper re-syncing implemented, and the standard re-syncing logic did not work for it, because technically, internally, its parent is a special timing stream, not the signal that the users would expect semantically.

The fix is not binary compatible with Airstream v17.0.0-M6 but is binary compatible with Laminar v17.0.0-M6, and probably with most other libraries that depend on Airstream v17.0.0-M6.

I will publish Airstream v17.0.0-M8 a bit later, but for now, you can try to publishLocal the latest Airstream master, and try using it with Laminar v17.0.0-M6 and other dependencies that work with it.

[TomasStanek]

Thank you for your quick response.
I have tried the local version and it solves the issue partially.

Steps to reproduce:
open the page
click button 1 to show the component
click increment
click button 1 to hide the component
click increment
click button 1 to show the component
-- component 1 has both numbers consistent -- correct so far
click button 2 to show the component
-- component 2 shows 0|2 (same for component 3)

[raquo]

Ah, indeed, the problem was a bit more complicated than I thought, and my initial fix was not quite right. I have reworked the solution now, and with the latest commit, it works as expected.

[TomasStanek]

The latest commit works for me.
Thanks!