Add the ability to store user data in the Element

connector/src/main/java/devtoolsfx/connector/LocalElement.java

@@ -4,15 +4,16 @@
 import devtoolsfx.scenegraph.*;
 import devtoolsfx.util.ClassInfoCache;
 import devtoolsfx.util.SceneUtils;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
 import javafx.scene.Node;
 import javafx.stage.Window;
 import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
 
-import java.util.List;
-import java.util.Objects;
-import java.util.stream.Collectors;
-
 /**
  * The {@link Element} implementation that directly wraps a link to the scene graph
  * node and must not leak the JVM process (e.g. by transferring it via the network).
@@ -25,6 +26,7 @@ public final class LocalElement implements Element {
     private final Vertex vertex;
     private final @Nullable NodeProperties nodeProperties;
     private final @Nullable WindowProperties windowProperties;
+    private final Map<Object, Object> properties = new ConcurrentHashMap<>();
     private final @Nullable Node node;
 
     private LocalElement(int uid,
@@ -89,6 +91,11 @@ public boolean hasChildren() {
         return windowProperties;
     }
 
+    @Override
+    public Map<Object, Object> getProperties() {
+        return this.properties;
+    }
+
     @Override
     public boolean isWindowElement() {
         return windowProperties != null;

connector/src/main/java/devtoolsfx/scenegraph/Element.java

@@ -1,7 +1,7 @@
 package devtoolsfx.scenegraph;
 
 import java.util.List;
-
+import java.util.Map;
 import javafx.scene.Node;
 import javafx.stage.Window;
 import org.jspecify.annotations.NullMarked;
@@ -58,6 +58,17 @@ public interface Element {
     @Nullable
     WindowProperties getWindowProperties();
 
+    /**
+     * Returns the map of properties associated with this element.
+     * <p>
+     * This map can be used to store user-defined data or metadata related to
+     * the element. The returned map is mutable, and callers may add, modify,
+     * or remove entries as needed.
+     *
+     * @return a mutable map of user-defined properties
+     */
+    Map<Object, Object> getProperties();
+
     /**
      * Checks whether the element is a wrapper around {@link Node}.
      */

gui/src/main/java/devtoolsfx/gui/util/DummyElement.java

@@ -4,11 +4,12 @@
 import devtoolsfx.scenegraph.Element;
 import devtoolsfx.scenegraph.NodeProperties;
 import devtoolsfx.scenegraph.WindowProperties;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
 
-import java.util.List;
-
 /**
  * Dummy class to represent the root of a scene graph element tree.
  */
@@ -17,6 +18,8 @@ public final class DummyElement implements Element {
 
     private final String name;
 
+    private final Map<Object, Object> properties = new ConcurrentHashMap<>();
+
     public DummyElement(String name) {
         this.name = name;
     }
@@ -61,6 +64,11 @@ public boolean hasChildren() {
         return null;
     }
 
+    @Override
+    public Map<Object, Object> getProperties() {
+        return this.properties;
+    }
+
     @Override
     public boolean isWindowElement() {
         return false;