does anybody have a good breakdown on multithreadi...

# javafx

[groostav]

does anybody have a good breakdown on multithreading in javafx? I understand that scene graph changes must be done on the event thread, and I guess we should assume that "unless your sure, just do it on the UI thread", but is it safe for me to do something like:

launch(Dispatchers.JavaFx){
  val viewData = snapshotViewModelState()
  val newView = async(Dispatchers.Default){
    val vbox = VBox()
    vbox.children += makeCoolElements(viewData)
  }

  this.view.content = newView
}

or

launch(Dispatchers.JavaFx){
  val viewData = snapshotViewModelState()
  val fxmlLoader = javafx.fxml.FXMLLoader()

  val newView = async(Dispatchers.Default){
    fxmlLoader.load(findresource("/com/mycompany/reasonably-standard-fxml-that-doesnt-contain-stages.fxml")) //this is actually annoying since defining an alert in FXML would be handy
  }

  this.view.content = newView
}

that is to say, is it safe for me to call constructors and/or mutate view objects off the UI thread if I'm certain they're not on a scene graph?

[melatonina]

Yes, it's safe because they are not on a scene graph, so you are not changing the scene graph.
I do it routinely in my application. Creating significant part of the UI on secondary dispatcher (Default, usually), then switching back to the Dispatchers.JavaFX to add the to the scene graph, improves the responsiveness and it's correct.
You can just create a suspend function and switch dispatcher using withContext, depending on what you are doing.
I have three functions called io, computation and javafx which do just that:

suspend fun <T> computation(
    block: suspend CoroutineScope.() -> T
): T =
    withContext(Dispatchers.Default, block = block)

suspend fun <T> io(
    block: suspend CoroutineScope.() -> T
): T =
    withContext(<http://Dispatchers.IO|Dispatchers.IO>, block = block)


suspend fun <T> javafx(
    block: suspend CoroutineScope.() -> T
): T =
    withContext(Dispatchers.JavaFx, block = block)

Here I populate a ScrollPane with a content which is created by scanning the filesystem:

suspend fun populate(repository: SampleRepository) {
    val buttons = io {
        repository.mapAllSampleGroups().map { group ->
            VBox().apply {
                spacing = 5.0
                padding = insets(5.0)
                addClass(ClipBrowserStylesheet.card2)
                children.setAll(
                    Label(group.name),
                    HBox().apply {
                        spacing = 5.0
                        padding = insets(5.0)
                        children.setAll(
                            group.samples.mapIndexed { index, sample ->
                                Button("${index + 1}").apply {
                                    setOnAction {
                                        onClick?.invoke(sample)
                                    }
                                }
                            }
                        )
                    }
                )
            }
        }
    }

    content = FlowPane().apply {
        setGapsAndPadding(5.0)
        children.addAll(buttons)
    }
}