Adding test case for the clear operation

mvvmfx/src/test/java/de/saxsys/mvvmfx/utils/notifications/ClearNotificationCenterTest.java

@@ -0,0 +1,48 @@
+/*******************************************************************************
+ * Copyright 2015 Alexander Casall, Manuel Mauky
+ *
+ * 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 de.saxsys.mvvmfx.utils.notifications;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mockito;
+
+/**
+ * Test-case of <a href="https://github.com/sialcasa/mvvmFX/issues/585">Issue #585</a>.
+ */
+class ClearNotificationCenterTest {
+    private static final String TEST_NOTIFICATION = "test_notification";
+
+    private NotificationCenter defaultCenter;
+
+    NotificationObserver notificationObserver;
+
+    @BeforeEach
+    public void init() {
+        notificationObserver = Mockito.mock(NotificationObserver.class);
+        defaultCenter = new DefaultNotificationCenter();
+    }
+
+    @Test
+    void testClearObservers() {
+        defaultCenter.subscribe(TEST_NOTIFICATION, notificationObserver);
+
+        defaultCenter.clear();
+
+        Mockito.verifyNoMoreInteractions(notificationObserver);
+
+        defaultCenter.publish(TEST_NOTIFICATION);
+    }
+}