Create new mixin interface HasLabel(#3241) (#8424)

Add new HasLabel interface to be used by components that supports label
definitions

Fixes #3241

Co-authored-by: Pekka Hyvönen <pekka@vaadin.com>

flow-server/src/main/java/com/vaadin/flow/component/HasLabel.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2000-2020 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.vaadin.flow.component;
+
+import com.vaadin.flow.dom.ElementConstants;
+
+/**
+ * A component that supports label definition.
+ * <p>
+ * The default implementations set the label of the component to the given text for
+ * {@link #getElement()}. Override all methods in this interface if the text
+ * should be added to some other element.
+ *
+ *
+ * @author Vaadin Ltd
+ * @since
+ */
+public interface HasLabel extends HasElement{
+    /**
+     * Set the label of the component to the given text.
+     *
+     * @param label
+     *            the label text to set or {@code null} to clear
+     */
+    default void setLabel(String label) {
+        getElement().setProperty(ElementConstants.LABEL_PROPERTY_NAME, label);
+    }
+
+    /**
+     * Gets the label of the component.
+     *
+     * @return the label of the component or {@code null} if no label has
+     *         been set
+     */
+    default String getLabel() {
+        return getElement().getProperty(ElementConstants.LABEL_PROPERTY_NAME, null);
+    }
+}

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

@@ -51,6 +51,10 @@ public class ElementConstants {
      * The style property for color.
      */
     public static final String STYLE_COLOR = "color";
+    /**
+     * The label property.
+     */
+    public static final String LABEL_PROPERTY_NAME = "label";
 
     private ElementConstants() {
         // Constants only

flow-server/src/test/java/com/vaadin/flow/component/HasLabelTest.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2000-2020 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.vaadin.flow.component;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
+import org.junit.Test;
+
+
+public class HasLabelTest {
+
+	@Tag(Tag.DIV)
+    private static class TestComponent extends Component
+            implements HasLabel {
+
+    }
+
+    @Test
+    public void withoutLabelComponent_getLabelReturnsNull() {
+       TestComponent component = new TestComponent();
+
+       assertNull(component.getLabel());
+    }
+
+    @Test
+    public void withNullLabel_getLabelReturnsNull() {
+       TestComponent component = new TestComponent();
+       component.setLabel(null);
+       assertNull(component.getLabel());
+    }
+
+    @Test
+    public void withEmptyLabel_getLabelReturnsEmptyString() {
+       TestComponent component = new TestComponent();
+       component.setLabel("");
+       assertEquals("",component.getLabel());
+    }
+
+
+    @Test
+    public void setLabel() {
+        TestComponent component = new TestComponent();
+        component.setLabel("test label");
+
+        assertEquals("test label", component.getLabel());
+    }
+
+}