Add HeartbeatMonitor convenience class

Users monitoring a heartbeat can use it to determine if a new state
has been reached.

src/main/java/org/springframework/cloud/client/discovery/event/HeartbeatMonitor.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2013-2015 the original author or authors.
+ *
+ * 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 org.springframework.cloud.client.discovery.event;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+/**
+ * @author Dave Syer
+ */
+public class HeartbeatMonitor {
+
+	private AtomicReference<Object> latestHeartbeat = new AtomicReference<>();
+
+	/**
+	 * @param value the latest heartbeat
+	 * @return true if the state changed
+	 */
+	public boolean update(Object value) {
+		Object last = this.latestHeartbeat.get();
+		if (!value.equals(last)) {
+			return this.latestHeartbeat.compareAndSet(last, value);
+		}
+		return false;
+	}
+
+}

src/main/java/org/springframework/cloud/client/discovery/event/ParentHeartbeatEvent.java

@@ -16,25 +16,16 @@
 
 package org.springframework.cloud.client.discovery.event;
 
-import org.springframework.context.ApplicationEvent;
-
 /**
- * Heartbeat Event that a Parent ApplicationContext can send to a child Context.
- * Useful when config server is located via DiscoveryClient
+ * Heartbeat Event that a Parent ApplicationContext can send to a child Context. Useful
+ * when config server is located via DiscoveryClient
  * @author Spencer Gibb
  */
 @SuppressWarnings("serial")
-public class ParentHeartbeatEvent extends ApplicationEvent {
-
-	private final Object value;
+public class ParentHeartbeatEvent extends HeartbeatEvent {
 
 	public ParentHeartbeatEvent(Object source, Object value) {
-		super(source);
-		this.value = value;
-	}
-
-	public Object getValue() {
-		return this.value;
+		super(source, value);
 	}
 
 }

src/test/java/org/springframework/cloud/client/discovery/event/HeartbeatMonitorTests.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2013-2015 the original author or authors.
+ *
+ * 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 org.springframework.cloud.client.discovery.event;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+/**
+ * @author Dave Syer
+ */
+public class HeartbeatMonitorTests {
+
+	private HeartbeatMonitor monitor = new HeartbeatMonitor();
+
+	@Test
+	public void onAndOff() {
+		assertTrue(this.monitor.update("foo"));
+		assertFalse(this.monitor.update("foo"));
+	}
+
+	@Test
+	public void toggle() {
+		assertTrue(this.monitor.update("foo"));
+		assertTrue(this.monitor.update("bar"));
+	}
+
+}