feat: Add Signal binding to class name list

flow-server/src/main/java/com/vaadin/flow/dom/ClassList.java

@@ -18,6 +18,8 @@
 import java.io.Serializable;
 import java.util.Set;
 
+import com.vaadin.signals.Signal;
+
 /**
  * Representation of the class names for an {@link Element}.
  *
@@ -44,4 +46,32 @@ default boolean set(String className, boolean set) {
         }
     }
 
+    /**
+     * Binds the presence of the given class name to the provided signal so that
+     * the class is added when the signal value is {@code true} and removed when
+     * the value is {@code false}.
+     * <p>
+     * Passing {@code null} as the {@code signal} removes any existing binding
+     * for the given class name. When unbinding, the current presence of the
+     * class is left unchanged.
+     * <p>
+     * While a binding for the given class name is active, manual calls to
+     * {@link #add(Object)}, {@link #remove(Object)} or
+     * {@link #set(String, boolean)} for that name will throw a
+     * {@code com.vaadin.flow.dom.BindingActiveException}. Bindings are
+     * lifecycle-aware and only active while the owning {@link Element} is
+     * attached; they are deactivated while the element is detached.
+     * <p>
+     * Bulk operations that indiscriminately replace or clear the class list
+     * (for example {@link #clear()} or setting the {@code class} attribute via
+     * {@link Element#setAttribute(String, String)}) clear all bindings.
+     *
+     * @param name
+     *            the class name to bind, not {@code null} or blank
+     * @param signal
+     *            the boolean signal to bind to, or {@code null} to unbind
+     * @since 25.0
+     */
+    void bind(String name, Signal<Boolean> signal);
+
 }

flow-server/src/main/java/com/vaadin/flow/dom/impl/ImmutableClassList.java

@@ -23,6 +23,7 @@
 import java.util.Iterator;
 
 import com.vaadin.flow.dom.ClassList;
+import com.vaadin.signals.Signal;
 
 /**
  * Immutable class list implementation.
@@ -63,4 +64,15 @@ public Iterator<String> iterator() {
     public int size() {
         return values.size();
     }
+
+    /**
+     * {@inheritDoc}
+     * <p>
+     * Text nodes do not support binding a {@link Signal} to a stylesheet class,
+     * because they do not support styling in general.
+     */
+    @Override
+    public void bind(String name, Signal<Boolean> signal) {
+        throw new UnsupportedOperationException(CANT_MODIFY_MESSAGE);
+    }
 }

flow-server/src/main/java/com/vaadin/flow/internal/nodefeature/ElementClassList.java

@@ -15,8 +15,17 @@
  */
 package com.vaadin.flow.internal.nodefeature;
 
+import java.io.Serializable;
+import java.util.HashMap;
+import java.util.Map;
+
 import com.vaadin.flow.dom.ClassList;
+import com.vaadin.flow.dom.Element;
+import com.vaadin.flow.dom.ElementEffect;
 import com.vaadin.flow.internal.StateNode;
+import com.vaadin.flow.shared.Registration;
+import com.vaadin.signals.BindingActiveException;
+import com.vaadin.signals.Signal;
 
 /**
  * Handles CSS class names for an element.
@@ -28,11 +37,16 @@
  */
 public class ElementClassList extends SerializableNodeList<String> {
 
+    private Map<String, SignalBinding> bindingsByName;
+
     private static class ClassListView extends NodeList.SetView<String>
             implements ClassList {
 
+        private final ElementClassList elementClassList;
+
         private ClassListView(ElementClassList elementClassList) {
             super(elementClassList);
+            this.elementClassList = elementClassList;
         }
 
         @Override
@@ -41,7 +55,7 @@ protected void validate(String className) {
                 throw new IllegalArgumentException("Class name cannot be null");
             }
 
-            if ("".equals(className)) {
+            if (className.isEmpty()) {
                 throw new IllegalArgumentException(
                         "Class name cannot be empty");
             }
@@ -50,6 +64,104 @@ protected void validate(String className) {
                         "Class name cannot contain spaces");
             }
         }
+
+        private void internalSetPresence(String name, boolean set) {
+            // Directly mutate the underlying NodeList to bypass SetView
+            // add/remove overrides which enforce BindingActiveException for
+            // manual updates.
+            ElementClassList list = this.elementClassList;
+            int index = list.indexOf(name);
+            if (set) {
+                if (index == -1) {
+                    // append at the end
+                    list.add(list.size(), name);
+                }
+            } else {
+                if (index != -1) {
+                    list.remove(index);
+                }
+            }
+        }
+
+        private Map<String, SignalBinding> getBindings() {
+            return elementClassList.getBindings();
+        }
+
+        private boolean isBound(String name) {
+            return elementClassList.isBound(name);
+        }
+
+        private StateNode getNode() {
+            return elementClassList.getNode();
+        }
+
+        @Override
+        public void bind(String name, Signal<Boolean> signal) {
+            validate(name);
+            if (signal == null) {
+                // Unbind: remove existing binding and leave the current class
+                // presence as-is
+                if (isBound(name)) {
+                    SignalBinding old = getBindings().remove(name);
+                    if (old != null && old.registration != null) {
+                        old.registration.remove();
+                    }
+                }
+                return;
+            }
+
+            // Replace any existing binding
+            SignalBinding existing = getBindings().remove(name);
+            if (existing != null && existing.registration != null) {
+                existing.registration.remove();
+            }
+            Element owner = Element.get(getNode());
+            Registration registration = ElementEffect.bind(owner, signal,
+                    (element, value) -> internalSetPresence(name,
+                            Boolean.TRUE.equals(value)));
+            SignalBinding binding = new SignalBinding(signal, registration,
+                    name);
+            getBindings().put(name, binding);
+        }
+
+        @Override
+        public boolean add(String className) {
+            if (isBound(className)) {
+                throw new BindingActiveException("Class name '" + className
+                        + "' is bound and cannot be modified manually");
+            }
+            return super.add(className);
+        }
+
+        @Override
+        public boolean remove(Object className) {
+            if (className instanceof String name) {
+                if (isBound(name)) {
+                    throw new BindingActiveException("Class name '" + name
+                            + "' is bound and cannot be modified manually");
+                }
+            }
+            return super.remove(className);
+        }
+
+        @Override
+        public void clear() {
+            clearBindings();
+            super.clear();
+        }
+
+        // Bulk operations in AbstractCollection ultimately delegate to
+        // add/remove
+        // which are guarded above. No need to override
+        // addAll/removeAll/retainAll
+        // unless optimization is required.
+
+        /**
+         * Clears all signal bindings.
+         */
+        public void clearBindings() {
+            elementClassList.clearBindings();
+        }
     }
 
     /**
@@ -70,4 +182,31 @@ public ElementClassList(StateNode node) {
     public ClassList getClassList() {
         return new ClassListView(this);
     }
+
+    private Map<String, SignalBinding> getBindings() {
+        if (bindingsByName == null) {
+            bindingsByName = new HashMap<>();
+        }
+        return bindingsByName;
+    }
+
+    private boolean isBound(String name) {
+        return bindingsByName != null && bindingsByName.containsKey(name);
+    }
+
+    private void clearBindings() {
+        if (bindingsByName == null || bindingsByName.isEmpty()) {
+            return;
+        }
+        for (SignalBinding binding : bindingsByName.values()) {
+            if (binding.registration != null) {
+                binding.registration.remove();
+            }
+        }
+        bindingsByName.clear();
+    }
+
+    private record SignalBinding(Signal<Boolean> signal,
+            Registration registration, String name) implements Serializable {
+    }
 }