JavaFX: How to scroll to a specific line in the textarea - javafx

Question

package sample;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.TextArea;
import javafx.stage.Stage;
import java.util.stream.IntStream;
public class Main extends Application {
#Override
public void start(Stage primaryStage) throws Exception{
final StringBuilder sb = new StringBuilder();
IntStream.range(1, 100).forEach(i -> sb.append("Line " + i + "\n"));
TextArea ta = new TextArea();
ta.setText(sb.toString());
//how to I get line 30 at top of the visible textarea
double someValue = 0;
ta.setScrollTop(someValue);
primaryStage.setTitle("Hello World");
primaryStage.setScene(new Scene(ta, 300, 300));
primaryStage.show();
}
public static void main(String[] args) {
launch(args);
}
}
How do I get line 30 at top of the visible textarea?
I think 'someValue' should be relative to the total height which can be scrolled
But what is the total height that can be scrolled
This is the result I want to achieve:

Answer 1

This is a bit tricky. We could just determine each line height and call ta.setScrollTop((line - 1) * lineHeight);, but we do not know what line spacing TextArea uses.
But I found that TextAreaSkin contains public methods for determining bounds for any selected character, we just need to know its index.
#Override
public void start(Stage primaryStage) throws Exception{
final StringBuilder sb = new StringBuilder();
IntStream.range(1, 100).forEach(i -> sb.append("Line " + i + "\n"));
TextArea ta = new TextArea();
ta.setText(sb.toString());
// TextArea did not setup its skin yet, so we can't use it right now.
// We just append our task to the user tasks queue.
Platform.runLater(() -> {
// Define desired line
final int line = 30;
// Index of the first character in line that we look for.
int index = 0;
// for this example following line will work:
// int index = ta.getText().indexOf("Line " + line);
// for lines that do not contain its index we rely on "\n" count
int linesEncountered = 0;
boolean lineFound = false;
for (int i = 0; i < ta.getText().length(); i++) {
// count characters on our way to our desired line
index++;
if(ta.getText().charAt(i) == '\n') {
// next line char encountered
linesEncountered++;
if(linesEncountered == line-1) {
// next line is what we're looking for, stop now
lineFound = true;
break;
}
}
}
// scroll only if line found
if(lineFound) {
// Get bounds of the first character in the line using internal API (see comment below the code)
Rectangle2D lineBounds = ((com.sun.javafx.scene.control.skin.TextAreaSkin) ta.getSkin()).getCharacterBounds(index);
// Scroll to the top-Y of our line
ta.setScrollTop(lineBounds.getMinY());
}
});
primaryStage.setTitle("Hello World");
primaryStage.setScene(new Scene(ta, 300, 300));
primaryStage.show();
}
This solution works on Java 8, on 9+ TextAreaSkin was moved to the public package, so everything you need to make it work is to replace com.sun.javafx.scene.control.skin.TextAreaSkin with javafx.scene.control.skin.TextAreaSkin