Update label from nested function called from Task API in JavaFX - javafx

Question

I am performing some background task using this class
class Download extends Task{
protected Object call() throws Exception {
try {
updateMessage("Establishing Connection");
DownloadHelper downloadHelper = new DownloadHelper();
downloadHelper.performTask();
return null;
} catch (IOException | ParseException ex) {
logger.error(ExceptionUtils.getStackTrace(ex));
throw ex;
}
}
}
This Task in turn calls DownloadHelper to perform some task.
class DownloadHelper{
public DownloadHelper(){
}
public void performTask(){
----
----
}
}
Is there a way to update the status message of the Task API (updateMessage()) from the DownloadHelper class.?

Answer 1

The expedient approach is to pass a reference to the Download task as a parameter to the DownloadHelper constructor. To minimize coupling, you can instead pass a reference to your implementation of updateMessage() as a parameter of type Consumer, "an operation that accepts a single input argument and returns no result."
DownloadHelper helper = new DownloadHelper(this::updateMessage);
Your helper's implementation of performTask() can then ask the updater to accept() messages as needed.
Consumer<String> updater;
public DownloadHelper(Consumer<String> updater) {
this.updater = updater;
}
public void performTask() {
updater.accept("Helper message");
}
A related example is seen here.
import java.util.function.Consumer;
import javafx.application.Application;
import javafx.beans.InvalidationListener;
import javafx.beans.Observable;
import javafx.concurrent.Task;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
/**
* #see https://stackoverflow.com/q/45708923/230513
*/
public class MessageTest extends Application {
#Override
public void start(Stage primaryStage) {
primaryStage.setTitle("MessageTest");
StackPane root = new StackPane();
Label label = new Label();
root.getChildren().add(label);
Scene scene = new Scene(root, 320, 120);
primaryStage.setScene(scene);
primaryStage.show();
Download task = new Download();
task.messageProperty().addListener((Observable o) -> {
label.setText(task.getMessage());
});
Thread thread = new Thread(task);
thread.setDaemon(true);
thread.start();
}
private static class Download extends Task<String> {
#Override
protected String call() throws Exception {
updateMessage("Establishing connection");
DownloadHelper helper = new DownloadHelper(this::updateMessage);
helper.performTask();
return "MessageTest";
}
#Override
protected void updateMessage(String message) {
super.updateMessage(message);
}
}
private static class DownloadHelper {
Consumer<String> updater;
public DownloadHelper(Consumer<String> updater) {
this.updater = updater;
}
public void performTask() {
updater.accept("Helper message");
}
}
public static void main(String[] args) {
launch(args);
}
}