#7911 Implement Component.isAttached(). (#8810)

Closes #7911
# Conflicts:
#	flow-server/src/test/java/com/vaadin/flow/component/ComponentTest.java

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

@@ -431,6 +431,19 @@ protected void onDetach(DetachEvent detachEvent) {
         // NOOP by default
     }
 
+    /**
+     * Checks whether this component is currently attached to a UI.
+     * <p>
+     * When {@link UI#close()} is called, the UI and the components are not detached immediately;
+     * the UI cleanup is performed at the end of the current request which also detaches the
+     * UI and its components.
+     *
+     * @return true if the component is attached to an active UI.
+     */
+    public boolean isAttached() {
+        return getElement().getNode().isAttached();
+    }
+
     /**
      * Sets the value of the given component property.
      *

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

@@ -56,6 +56,7 @@
 
 import elemental.json.Json;
 import net.jcip.annotations.NotThreadSafe;
+import org.mockito.Mockito;
 
 @NotThreadSafe
 public class ComponentTest {
@@ -758,6 +759,56 @@ public void testSecondAttach() {
         Assert.assertFalse(initialAttach.get());
     }
 
+    /**
+     * Tests {@link Component#isAttached}.
+     */
+    @Test
+    public void testIsAttached() {
+        UI ui = new UI();
+        // ui is initially attached
+        Assert.assertTrue(ui.isAttached());
+
+        TestComponentContainer parent = new TestComponentContainer();
+        TestComponentContainer child = new TestComponentContainer();
+        TestComponent grandChild = new TestComponent();
+        child.track();
+        grandChild.addAttachListener(
+                event -> Assert.assertTrue(grandChild.isAttached()));
+        grandChild.addDetachListener(
+                event -> grandChild.getDetachEvents().incrementAndGet());
+
+        parent.add(child);
+        child.add(grandChild);
+        Assert.assertFalse(parent.isAttached());
+        Assert.assertFalse(child.isAttached());
+        Assert.assertFalse(grandChild.isAttached());
+
+        ui.add(parent);
+        Assert.assertTrue(parent.isAttached());
+        Assert.assertTrue(child.isAttached());
+        Assert.assertTrue(grandChild.isAttached());
+
+        ui.remove(parent);
+        Assert.assertFalse(parent.isAttached());
+        Assert.assertFalse(child.isAttached());
+        Assert.assertFalse(grandChild.isAttached());
+
+        ui.add(parent);
+        Assert.assertTrue(parent.isAttached());
+        Assert.assertTrue(child.isAttached());
+        Assert.assertTrue(grandChild.isAttached());
+
+        // Mock closing of UI after request handled
+        ui.getInternals().setSession(Mockito.mock(VaadinSession.class));
+        ui.close();
+        ui.getInternals().setSession(null);
+
+        Assert.assertFalse(parent.isAttached());
+        Assert.assertFalse(child.isAttached());
+        Assert.assertFalse(grandChild.isAttached());
+        Assert.assertFalse(ui.isAttached());
+    }
+
     @Test(expected = IllegalArgumentException.class)
     public void wrapNullComponentType() {
         new Element("div").as(null);