feat: implement HasValidation

pom.xml

@@ -3,7 +3,7 @@
 	<modelVersion>4.0.0</modelVersion>
 	<groupId>com.flowingcode.vaadin.addons</groupId>
 	<artifactId>twincolgrid</artifactId>
-	<version>3.0.1-SNAPSHOT</version>
+	<version>3.1.0-SNAPSHOT</version>
 	<name>TwinColGrid add-on</name>
 	<description>Dual list component based on Vaadin Grids</description>
 	<url>https://www.flowingcode.com/en/open-source/</url>

src/main/java/com/flowingcode/vaadin/addons/twincolgrid/TwinColGrid.java

@@ -2,7 +2,7 @@
  * #%L
  * TwinColGrid add-on
  * %%
- * Copyright (C) 2017 - 2022 Flowing Code
+ * Copyright (C) 2017 - 2025 Flowing Code
  * %%
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,6 +26,7 @@
 import com.vaadin.flow.component.ComponentUtil;
 import com.vaadin.flow.component.HasComponents;
 import com.vaadin.flow.component.HasSize;
+import com.vaadin.flow.component.HasValidation;
 import com.vaadin.flow.component.HasValue;
 import com.vaadin.flow.component.HasValue.ValueChangeEvent;
 import com.vaadin.flow.component.HasValueAndElement;
@@ -79,7 +80,7 @@
 @CssImport(value = "./styles/twin-col-grid-button.css")
 @CssImport(value = "./styles/twincol-grid.css")
 public class TwinColGrid<T> extends VerticalLayout
-    implements HasValueAndElement<ValueChangeEvent<Set<T>>, Set<T>>, HasComponents, HasSize {
+    implements HasValueAndElement<ValueChangeEvent<Set<T>>, Set<T>>, HasComponents, HasSize, HasValidation {
 
   private static final class TwinColModel<T> implements Serializable {
     final Grid<T> grid;
@@ -142,6 +143,8 @@ public enum Orientation {
 
   private Span fakeButtonContainerLabel = new Span();
 
+  private Span errorMessageSpan = new Span();
+
   private Orientation orientation = Orientation.HORIZONTAL;
 
   private boolean autoResize = false;
@@ -184,7 +187,10 @@ public TwinColGrid(@NonNull Grid<T> availableGrid, @NonNull Grid<T> selectionGri
 
     available = new TwinColModel<>(availableGrid, "twincol-grid-available");
     selection = new TwinColModel<>(selectionGrid, "twincol-grid-selection");
-
+
+    errorMessageSpan.setClassName("twincol-grid-error-message");
+    selection.layout.addComponentAtIndex(1, errorMessageSpan);
+
     setClassName("twincol-grid");
 
     setMargin(false);
@@ -677,6 +683,26 @@ public boolean isRequiredIndicatorVisible() {
     return getElement().getAttribute("required") != null;
   }
 
+  @Override
+  public void setErrorMessage(String errorMessage) {
+    errorMessageSpan.setText(errorMessage);
+  }
+
+  @Override
+  public String getErrorMessage() {
+    return errorMessageSpan.getText();
+  }
+
+  @Override
+  public void setInvalid(boolean invalid) {
+    getElement().setAttribute("invalid", invalid);
+  }
+
+  @Override
+  public boolean isInvalid() {
+    return getElement().getAttribute("invalid") != null;
+  }
+
   @Override
   public void setReadOnly(final boolean readOnly) {
     getAvailableGrid().setSelectionMode(readOnly ? SelectionMode.NONE : SelectionMode.MULTI);
@@ -956,4 +982,5 @@ private void updateOrientationOnResize(int width, int height) {
       this.withOrientation(Orientation.HORIZONTAL);
     }
   }
+
 }

src/main/java/com/flowingcode/vaadin/addons/twincolgrid/TwinColGridListAdapter.java

@@ -2,7 +2,7 @@
  * #%L
  * TwinColGrid add-on
  * %%
- * Copyright (C) 2017 - 2022 Flowing Code
+ * Copyright (C) 2017 - 2025 Flowing Code
  * %%
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,6 +19,7 @@
  */
 package com.flowingcode.vaadin.addons.twincolgrid;
 
+import com.vaadin.flow.component.HasValidation;
 import com.vaadin.flow.component.HasValue;
 import com.vaadin.flow.component.HasValue.ValueChangeEvent;
 import com.vaadin.flow.shared.Registration;
@@ -40,9 +41,9 @@
  */
 @SuppressWarnings("serial")
 @RequiredArgsConstructor
-class TwinColGridListAdapter<T> implements HasValue<ValueChangeEvent<List<T>>, List<T>> {
+class TwinColGridListAdapter<T> implements HasValue<ValueChangeEvent<List<T>>, List<T>>, HasValidation {
 
-  private interface IDelegate {
+  private interface IDelegate extends HasValidation {
     boolean isEmpty();
 
     void clear();

src/main/resources/META-INF/resources/frontend/styles/twincol-grid.css

@@ -2,7 +2,7 @@
  * #%L
  * TwinColGrid add-on
  * %%
- * Copyright (C) 2017 - 2022 Flowing Code
+ * Copyright (C) 2017 - 2025 Flowing Code
  * %%
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -44,6 +44,19 @@
   color: var(--lumo-error-text-color);
 }
 
+.twincol-grid .twincol-grid-selection .twincol-grid-error-message {
+    display: none;
+}
+
+.twincol-grid[invalid] .twincol-grid-selection .twincol-grid-error-message {
+	font-size: var(--vaadin-input-field-error-font-size, var(--lumo-font-size-xs));
+    line-height: var(--lumo-line-height-xs);
+    font-weight: var(--vaadin-input-field-error-font-weight, 400);
+    color: var(--vaadin-input-field-error-color, var(--lumo-error-text-color));
+    display: block;
+    padding-bottom: 4px;
+}
+
 .twincol-grid .fake-button-container-label {
   font-size: var(--lumo-font-size-s);
   margin-bottom: calc(var(--vaadin-button-margin, var(--lumo-space-xs))* -1);

src/test/java/com/flowingcode/vaadin/addons/twincolgrid/BoundDemo.java

@@ -2,7 +2,7 @@
  * #%L
  * TwinColGrid add-on
  * %%
- * Copyright (C) 2017 - 2022 Flowing Code
+ * Copyright (C) 2017 - 2025 Flowing Code
  * %%
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -58,7 +58,9 @@ public BoundDemo() {
     twinColGrid.setCaption("TwinColGrid demo with Binder and row select without checkbox");
 
     final Binder<Library> binder = new Binder<>();
-    binder.forField(twinColGrid.asList()).asRequired().bind(Library::getBooks, Library::setBooks);
+    binder.forField(twinColGrid.asList())
+      .asRequired("You must add at least one book.")
+      .bind(Library::getBooks, Library::setBooks);
     binder.setBean(library);
 
     add(twinColGrid);