Bug: `scrollBy` calls succeeding `moveTo` calls are negated by `moveTo` calls

[vkhoroshko]

Hello,

We're using CodeArea and trying to combine moveTo operation and setEstimatedScrollY operation after that.
Example:

codeArea.moveTo(startParagraph, 0);
codeArea.setEstimatedScrollY(startParagraphYOffset + delta);

We need to scroll a little to give more context to a user (i.e. few lines after caret position) cause when VirtualizedScrollPane is used and current scroll postion is at the top - moveTo just scrolls to specified paragraph that will be at the bottom of visible area (i.e. no lines after that).

If I comment moveTo instruction - than second instruction works. As I understand - we need to wait till moveTo really worked and only try to set scroll position after that. We tried Platform.runLater but without success.

Could you please suggest whether there is a possible solution to this issue?

Thanks in advance.

[JordanMartinez]

Which version are you using?

Could you moveTo a paragraph or two farther down and then back up? That'll work for most of your use cases except for the last paragraph line, which wouldn't be able to scroll farther down anyways.

VirtualizedScrollPane has scrollBy(double delta) but it's not exposed as an API in any version right now and I haven't done any bugfixing until #356 gets merged.

[vkhoroshko]

Hello Jordan,

Thank you for the response. We're using the 0.7-M2 version.
We tried your suggestion also:

codeArea.moveTo(indexRange.getEnd(), 0);
codeArea.moveTo(indexRange.getStart(), 0);

Unfortunately, it doesn't work. It still works like only second instruction is active (i.e. move to start).

The only way we could make it working is to use a timer with 100 milliseconds delay (just the minimum number that worked correctly in our case):

codeArea.moveTo(indexRange.getEnd(), 0);       

new Timer().schedule(new TimerTask() {
     public void run() {
          Platform.runLater(() -> {
              codeArea.moveTo(indexRange.getStart(), 0);
           });
      }
}, 100);

With this timer trick the setEstimatedScrollY instruction works just fine also - and this is more preferable solution cause with two moveTo operations you also need to hide a caret temporary (i.e. for user to do not notice that caret is moved twice).

But we don't like this solution really because it's unclear what's the delay should really be.

Maybe we can listen to a specific event to be acknowledged when moveTo operation completes?
We tried to play with listener for the beingUpdated property but without success.

[JordanMartinez]

Mmm... It sounds like exposing the aforementioned API for scrollBy would be the best thing to do. Anything else I propose or you come up with will only be a workaround for that method.

[JordanMartinez]

After Tomas merges my code in Flowless, I can update #391 to include more of those additional scrolling methods.

[JordanMartinez]

@vkhoroshko The current master branch now includes scrollBy. Will that fully address this issue (I merely skimmed this to see if it's been fully addressed now).

[JordanMartinez]

The snapshot version of Flowless now includes additional scrolling methods. So, I believe this issue has been resolved.
Please reopen if you think otherwise.

[grenche]

I'm using scrollYBy() to solve this same issue and I also need to use the timer to get it to register the scrollYBy() call. Additionally, it seems like after moving the caret, allParToVisibleParIndex needs the timer otherwise the index returned will be empty.

The timer trick fixes both of these issues, but because of it, the additional scrolling is pretty jerky. Is there a way to adjust the scroll position without the timer?

Specifically I want the caret to be in the middle of the viewport if possible. A method like showParagraphAtTop, but in the middle would be super useful, but I can't seem find anything like that.

[JordanMartinez]

@grenche Could you provide a TestFX test that easily reproduces the issue every time?

[JordanMartinez]

Seems like it would be helpful to add an API like moveTo(location).andThenScrollBy(xAmount, yAmount)

[grenche]

Here's a demo for the problem with scrollYBy().

import javafx.application.Platform;
import org.fxmisc.flowless.VirtualizedScrollPane;
import org.fxmisc.richtext.CodeArea;
import org.fxmisc.richtext.LineNumberFactory;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

import java.util.Timer;
import java.util.TimerTask;

public class BugFix extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        final CodeArea codeArea = new CodeArea();
        codeArea.setParagraphGraphicFactory(LineNumberFactory.get(codeArea));
        codeArea.replaceText(getText());
        codeArea.showParagraphAtTop(0);

        final BorderPane root = new BorderPane(new VirtualizedScrollPane<>(codeArea));
        final Scene scene = new Scene(root, 600, 400);
        primaryStage.setScene(scene);
        primaryStage.show();

        // With the first method, the scroll happens with a small delay (more obvious with a longer delay or styled text)
        // and with the second one there is no scroll at all.
        moveCaretWithTimer(codeArea);
//        moveCaretWithoutTimer(codeArea);
    }

    private String getText() {
        final StringBuilder sb = new StringBuilder();

        for (int i = 0; i < 200; i++) {
            sb.append("This is a line of text.\n");
        }

        return sb.toString();
    }

    private void moveCaretWithTimer(CodeArea codeArea) {
        codeArea.moveTo(100, 0);
        codeArea.requestFollowCaret();

        new Timer().schedule(new TimerTask() {
            public void run() {
                Platform.runLater(() -> {
                    codeArea.scrollYBy(100);
                });
            }
        }, 100);
    }

    private void moveCaretWithoutTimer(CodeArea codeArea) {
        codeArea.moveTo(100, 0);
        codeArea.requestFollowCaret();
        codeArea.scrollYBy(100);
    }
}

You can replace codeArea.scrollYBy(100); with System.out.println(codeArea.allParToVisibleParIndex(codeArea.getCurrentParagraph()).get()) in both methods and you should get 24 for the first one and an exception for the second one.

[JordanMartinez]

Thanks! I can further investigate this tomorrow.