Demonstrate via demo the usage of StyledTextArea#paragraphGraphicFactory

richtextfx-demos/src/main/java/org/fxmisc/richtext/demo/lineindicator/ArrowFactory.java

@@ -0,0 +1,35 @@
+package org.fxmisc.richtext.demo.lineindicator;
+
+import javafx.beans.value.ObservableValue;
+import javafx.scene.Node;
+import javafx.scene.paint.Color;
+import javafx.scene.shape.Polygon;
+import org.reactfx.value.Val;
+
+import java.util.function.IntFunction;
+
+/**
+ * Given the line number, return a node (graphic) to display to the left of a line.
+ */
+public class ArrowFactory implements IntFunction<Node> {
+    private final ObservableValue<Integer> shownLine;
+
+    ArrowFactory(ObservableValue<Integer> shownLine) {
+        this.shownLine = shownLine;
+    }
+
+    @Override
+    public Node apply(int lineNumber) {
+        Polygon triangle = new Polygon(0.0, 0.0, 10.0, 5.0, 0.0, 10.0);
+        triangle.setFill(Color.GREEN);
+
+        ObservableValue<Boolean> visible = Val.map(shownLine, sl -> sl == lineNumber);
+
+        triangle.visibleProperty().bind(
+            Val.flatMap(triangle.sceneProperty(), scene -> {
+                return scene != null ? visible : Val.constant(false);
+        }));
+
+        return triangle;
+    }
+}

richtextfx-demos/src/main/java/org/fxmisc/richtext/demo/lineindicator/LineIndicatorDemo.java

@@ -0,0 +1,52 @@
+/*
+ * Written as a StackOverflow answer (2015) by Tomas Mikula,
+ * which Jordan Martinez then extracted into this demo.
+ *
+ * The author dedicates this file to the public domain.
+ */
+
+package org.fxmisc.richtext.demo.lineindicator;
+
+import javafx.application.Application;
+import javafx.geometry.Pos;
+import javafx.scene.Node;
+import javafx.scene.Scene;
+import javafx.scene.layout.HBox;
+import javafx.scene.layout.StackPane;
+import javafx.stage.Stage;
+import org.fxmisc.richtext.CodeArea;
+import org.fxmisc.richtext.LineNumberFactory;
+import org.fxmisc.richtext.StyledTextArea;
+
+import java.util.function.IntFunction;
+
+/**
+ * Demonstrates the usage of {@link StyledTextArea#paragraphGraphicFactoryProperty()}.
+ */
+public class LineIndicatorDemo extends Application {
+
+    public static void main(String[] args) {
+        launch(args);
+    }
+
+    @Override
+    public void start(Stage primaryStage) {
+        CodeArea codeArea = new CodeArea();
+
+        IntFunction<Node> numberFactory = LineNumberFactory.get(codeArea);
+        IntFunction<Node> arrowFactory = new ArrowFactory(codeArea.currentParagraphProperty());
+        IntFunction<Node> graphicFactory = line -> {
+            HBox hbox = new HBox(
+                    numberFactory.apply(line),
+                    arrowFactory.apply(line));
+            hbox.setAlignment(Pos.CENTER_LEFT);
+            return hbox;
+        };
+        codeArea.setParagraphGraphicFactory(graphicFactory);
+        codeArea.replaceText("The green arrow will only be on the line where the caret appears.\n\nTry it.");
+        codeArea.moveTo(0, 0);
+
+        primaryStage.setScene(new Scene(new StackPane(codeArea), 600, 400));
+        primaryStage.show();
+    }
+}