JavaFX: Autoscroll of TextArea with appendText but without Listeners

Question

I've read the other articles on Stackoverflow on this topic. But none of them matches my problem exactly.

I want to log an algorithm in a TextArea. In each iteration of the algorithm the text in this TextArea should be expanded via appendText().

My first problem is: Where should I create the new Thread and how can the both threads communicate with each other (GUI and algorithm)?

In my actual design I have three important classes: The view, which holds the TextArea, the controller, which calls the algorithm and the algorithm, which takes a number of iterations and the TextArea (to call the appendText()-method on severage places in the code).

In this design the controller calls the algorithm, the algorithm iterates n times with a for-loop and after it terminates, the GUI shows the changes. But I want the GUI to show the changes simultaneously, when the algorithm calls the appendText()-method.

And my second problem is the autoscroll of the TextArea. After each appendText-call the TextArea should be scrolled completely down. But I think the solution of this problem is the same solution of my first problem.

I would be very grateful for some help.

Answer 1

It's pretty much impossible to answer your question completely without (a lot) more information, but the general approach I would use would be for the algorithm to have a callback to process a message, which could be invoked by each step of the algorithm. From the controller, pass an implementation of the callback which updates the text area with the message, on the FX Application Thread.

Something like:

public class Algorithm {

    private Consumer<String> statusCallback ;

    public Algorithm(Consumer<String> statusCallback) {
        this.statusCallback = statusCallback ;
    }

    public Algorithm() {
        // by default, callback does nothing:
        this(s -> {});
    }

    public void performAlgorithm() {

        while (! finished() ) {
            doNextStep();
            String statusMessage = getStatus();
            statusCallback.accept(statusMessage);
        }
    }
}

and then

public class Controller {

    private View view = ... ;

    public void startAlgorithm() {
        Algorithm algorithm = new Algorithm(s -> Platform.runLater(view.appendStatus(s)));
        Thread t = new Thread(algorithm::performAlgorithm);
        t.setDaemon(true);
        t.start();
    }
}

For the View you then do the following (note that you can scroll down with textArea.setScrollTop(Double.MAX_VALUE);):

public class View {
    private TextArea textArea ;

    public View() {
        textArea = new TextArea();
        // ...
    }

    public void appendStatus(String status) {
        if (!textArea.getText().isEmpty()) {
            textArea.appendText("\n");
        }
        textArea.appendText(status);
        textArea.setScrollTop(Double.MAX_VALUE);
    }
}

This should work as long as your algorithm doesn't create too many status updates too fast (so that they flood the FX Application Thread and prevent it doing its normal work).