Is it possible to show a Node during FX thread - multithreading

Question

I'm making a program which reads text files. What I would like to do is show an arbitrary node (Alert or other Node) which is created in separate thread before or during the file reading. I tried using Task and Platform.runLater() like this:
if (filetoopen != null)
{
Platform.runLater(new Runnable() {
#Override
void run() {
Alert alert=new Alert(Alert.AlertType.INFORMATION)
alert.setHeaderText('TEST')
}
})
//method to read the file
Tools.convertFromFile(filetoopen,newredactor)
lastDirectory = filetoopen.getParentFile()
}
I'd like to show an Alert or progress bar of reading the file, but the Control initializes after the reading is finished. So, is it possible to show a Node with a progress bar while the file is being read? Or the Runnable I create will always be executed in the end?
Edit: an attempt with Task:
class Alerter extends Task{
Alerter(File f,Editor e)
{
file=f
editor=e
}
File file
Editor editor
#Override
protected Object call() throws Exception {
Dialog dialog=new Dialog()
DialogPane dp=dialog.getDialogPane()
dp.setHeaderText('TEST')
dp.getButtonTypes().add(new ButtonType('Cancel',ButtonBar.ButtonData.CANCEL_CLOSE))
dialog.setOnCloseRequest(new javafx.event.EventHandler<DialogEvent>() {
#Override
void handle(DialogEvent event) {
dialog.close()
}
})
dialog.show()
Tools.convertFromFile(file,editor)
return null
}
}
The dialog still initializes after Tools.convertFromFile.

Answer 1

There are two threading rules in JavaFX (and in almost every other UI toolkit):
Changes to the scene graph (i.e. creating new scenes or windows, or changing the state of nodes already displayed) must be done on the FX Application Thread.
Long-running processes should be performed on a background thread (i.e. not the FX Application Thread), otherwise the UI will become unresponsive.
Your first code block violates the second rule (probably, you haven't shown much context) and your second code block violates the first rule.
So basically you need to:
Show the dialog from the FX Application Thread
Start a new thread which processes the file in the background
From the new thread, schedule any changes to the new UI on the FX Application Thread
When processing the file finishes, update the UI on the FX Application Thread
You can use Platform.runLater(...) to schedule code to run on the FX Application Thread, but the Task class provides more convenient API for these updates.
So:
// set up and show dialog:
ProgressBar progressBar = new ProgressBar();
DialogPane dialogPane = new DialogPane();
dialogPane.getButtonTypes().setAll(ButtonType.OK);
dialogPane.setHeaderText("Processing file");
dialogPane.setContent(progressBar);
dialogPane.lookupButton(ButtonType.OK).setDisable(true);
Dialog dialog = new Dialog();
dialog.setDialogPane(dialogPane);
dialog.show();
// create task:
Task<Void> task = new Task<Void>() {
#Override
public Void call() throws Exception {
Tools.convertFromFile(file, editor);
// can call updateProgress(...) here to update the progress periodically
return null ;
}
};
// update progress bar with progress from task:
progressBar.progressProperty().bind(task.progressProperty());
// when task completes, update dialog:
task.setOnSucceeded(event -> {
dialogPane.lookupButton(ButtonType.OK).setDisable(false);
progressBar.progressProperty().unbind();
progressBar.setProgress(1);
dialogPane.setHeaderText("Processing complete");
});
// handles errors:
task.setOnFailed(event -> {
dialogPane.lookupButton(ButtonType.OK).setDisable(false);
progressBar.progressProperty().unbind();
progressBar.setProgress(0);
dialogPane.setHeaderText("An error occurred");
});
// run task in background thread:
Thread thread = new Thread(task);
thread.start();
Note here that your Tools.convertFromFile(...) method is called from a background thread, so it must not update the UI (or at least any calls in that method that do update the UI must be wrapped in Platform.runLater(...)).
Here is a complete SSCCE (which just sleeps as a demo of a long-running process):
import java.util.Random;
import javafx.application.Application;
import javafx.concurrent.Task;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ButtonType;
import javafx.scene.control.Dialog;
import javafx.scene.control.DialogPane;
import javafx.scene.control.ProgressBar;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
public class TaskWithProgressDemo extends Application {
#Override
public void start(Stage primaryStage) {
Button button = new Button("Start process");
button.setOnAction(e -> {
button.setDisable(true);
// set up and show dialog:
ProgressBar progressBar = new ProgressBar();
DialogPane dialogPane = new DialogPane();
dialogPane.getButtonTypes().setAll(ButtonType.OK);
dialogPane.setHeaderText("Processing file in progress");
dialogPane.setContent(progressBar);
dialogPane.lookupButton(ButtonType.OK).setDisable(true);
Dialog<Void> dialog = new Dialog<Void>();
dialog.setDialogPane(dialogPane);
dialog.show();
// create task:
Task<Void> task = new Task<Void>() {
#Override
public Void call() throws Exception {
Random rng = new Random();
for (int i = 0 ; i <= 100 ; i++) {
Thread.sleep(rng.nextInt(40));
updateProgress(i, 100);
}
if (rng.nextBoolean()) {
System.out.println("Simulated error");
throw new Exception("An unknown error occurred");
}
return null ;
}
};
// update progress bar with progress from task:
progressBar.progressProperty().bind(task.progressProperty());
// when task completes, update dialog:
task.setOnSucceeded(event -> {
dialogPane.lookupButton(ButtonType.OK).setDisable(false);
button.setDisable(false);
progressBar.progressProperty().unbind();
progressBar.setProgress(1);
dialogPane.setHeaderText("Processing complete");
});
// handles errors:
task.setOnFailed(event -> {
dialogPane.lookupButton(ButtonType.OK).setDisable(false);
button.setDisable(false);
progressBar.progressProperty().unbind();
progressBar.setProgress(0);
dialogPane.setHeaderText("An error occurred");
});
// run task in background thread:
Thread thread = new Thread(task);
thread.start();
});
StackPane root = new StackPane(button);
root.setPadding(new Insets(20));
primaryStage.setScene(new Scene(root));
primaryStage.show();
}
public static void main(String[] args) {
launch(args);
}
}

Answer 2

So I finally figured it out. I had to move both my file loading code and progress update to a Task, so it wouldn't block FX thread. The indicator shows progress of loading a file.
Edit: to achieve progress display in a separate non-blocking window, must use a new Stage instead of anything else.