Make SeparatorBorder public API and move it from GEF to Draw2D

Use Case: Border between figures directly adjacent to one another. See
the UMLClass example, for reference.

org.eclipse.draw2d.examples/src/org/eclipse/draw2d/examples/uml/UMLClassFigure.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2003, 2023 IBM Corporation and others.
+ * Copyright (c) 2003, 2024 IBM Corporation and others.
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License 2.0 which is available at
@@ -18,15 +18,15 @@
 import org.eclipse.swt.graphics.Font;
 import org.eclipse.swt.graphics.Image;
 
+import org.eclipse.draw2d.Border;
 import org.eclipse.draw2d.Figure;
-import org.eclipse.draw2d.Graphics;
-import org.eclipse.draw2d.IFigure;
 import org.eclipse.draw2d.Label;
 import org.eclipse.draw2d.LineBorder;
 import org.eclipse.draw2d.MarginBorder;
+import org.eclipse.draw2d.PositionConstants;
+import org.eclipse.draw2d.SeparatorBorder;
 import org.eclipse.draw2d.ToolbarLayout;
 import org.eclipse.draw2d.geometry.Insets;
-import org.eclipse.draw2d.geometry.Rectangle;
 
 public class UMLClassFigure extends Figure {
 
@@ -37,18 +37,6 @@ public class UMLClassFigure extends Figure {
 	static Font BOLD = new Font(null, "", 10, SWT.BOLD); //$NON-NLS-1$
 
 	public UMLClassFigure() {
-		class SeparatorBorder extends MarginBorder {
-			SeparatorBorder() {
-				super(3, 5, 3, 5);
-			}
-
-			@Override
-			public void paint(IFigure figure, Graphics graphics, Insets insets) {
-				Rectangle where = getPaintRectangle(figure, insets);
-				graphics.drawLine(where.getTopLeft(), where.getTopRight());
-			}
-		}
-
 		Label header = new Label("ClassName", classImage); //$NON-NLS-1$
 		header.setFont(BOLD);
 		header.setBorder(new MarginBorder(3, 5, 3, 5));
@@ -59,15 +47,17 @@ public void paint(IFigure figure, Graphics graphics, Insets insets) {
 		layout.setStretchMinorAxis(false);
 		attributes.add(new Label("name : String")); //$NON-NLS-1$
 		attributes.add(new Label("ID : String")); //$NON-NLS-1$
-		attributes.setBorder(new SeparatorBorder());
+		Border attributeBorder = new SeparatorBorder(new Insets(3, 5, 3, 5), PositionConstants.TOP);
+		attributes.setBorder(attributeBorder);
 
 		Figure methods = new Figure();
 		layout = new ToolbarLayout();
 		methods.setLayoutManager(layout);
 		layout.setStretchMinorAxis(false);
 		methods.add(new Label("foo() : int")); //$NON-NLS-1$
 		methods.add(new Label("bar() : char")); //$NON-NLS-1$
-		methods.setBorder(new SeparatorBorder());
+		Border methodBorder = new SeparatorBorder(new Insets(3, 5, 3, 5), PositionConstants.TOP);
+		methods.setBorder(methodBorder);
 
 		setBorder(new LineBorder());
 		setLayoutManager(new ToolbarLayout());

org.eclipse.draw2d/src/org/eclipse/draw2d/SeparatorBorder.java

@@ -0,0 +1,86 @@
+/*******************************************************************************
+ * Copyright (c) 2000, 2024 IBM Corporation and others.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0.
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     IBM Corporation - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.draw2d;
+
+import org.eclipse.swt.graphics.Color;
+
+import org.eclipse.draw2d.geometry.Insets;
+import org.eclipse.draw2d.geometry.Rectangle;
+
+/**
+ * This border behaves similar to a {@link LineBorder} with width {@code 1},
+ * except that individual sides are drawn. The sides are specified via
+ * {@link PositionConstants#LEFT}, {@link PositionConstants#RIGHT},
+ * {@link PositionConstants#TOP} and {@link PositionConstants#BOTTOM}. Multiple
+ * sides can be specified by logically or-ing these constants.
+ *
+ * @since 3.16
+ */
+public final class SeparatorBorder extends MarginBorder {
+	private final int position;
+	private Color color;
+
+	/**
+	 * The sides are specified via {@link PositionConstants#LEFT},
+	 * {@link PositionConstants#RIGHT}, {@link PositionConstants#TOP},
+	 * {@link PositionConstants#BOTTOM}. Multiple sides can be specified by
+	 * logically or-ing these constants.
+	 *
+	 * @param insets The Insets for the border
+	 * @param sides  The integer-encoded sides that should be drawn.
+	 * @see PositionConstants#LEFT
+	 * @see PositionConstants#RIGHT
+	 * @see PositionConstants#TOP
+	 * @see PositionConstants#BOTTOM
+	 */
+	public SeparatorBorder(Insets insets, int sides) {
+		super(insets);
+		this.position = sides;
+	}
+
+	/**
+	 * If {@code null} is given as a parameter, the current foreground color of the
+	 * graphics object is used while painting.
+	 *
+	 * @param color The color used for drawing each side.
+	 */
+	public void setColor(Color color) {
+		this.color = color;
+	}
+
+	@Override
+	public void paint(IFigure f, Graphics g, Insets i) {
+		Rectangle r = getPaintRectangle(f, i);
+		if (color != null) {
+			g.setForegroundColor(color);
+		}
+		g.setLineWidth(1);
+		g.setLineStyle(Graphics.LINE_SOLID);
+		if ((position & PositionConstants.LEFT) > 0) {
+			g.drawLine(r.getTopLeft(), r.getBottomLeft());
+		}
+		if ((position & PositionConstants.RIGHT) > 0) {
+			r.width--;
+			g.drawLine(r.getTopRight(), r.getBottomRight());
+			r.width++;
+		}
+		if ((position & PositionConstants.TOP) > 0) {
+			g.drawLine(r.getTopLeft(), r.getTopRight());
+		}
+		if ((position & PositionConstants.BOTTOM) > 0) {
+			r.height--;
+			g.drawLine(r.getBottomLeft(), r.getBottomRight());
+			r.height++;
+		}
+	}
+}