mavlink: add function to copy only if updated

The MavlinkOrbSubscription only had an interface to either always copy
or copy based on a timestamp. This commit adds a copy interface if the
topic has been updated.

src/modules/mavlink/mavlink_orb_subscription.cpp

@@ -111,6 +111,29 @@ MavlinkOrbSubscription::update(void* data)
 	return !orb_copy(_topic, _fd, data);
 }
 
+bool
+MavlinkOrbSubscription::update_if_changed(void *data)
+{
+	bool updated;
+	if (orb_check(_fd, &updated)) {
+		return false;
+	}
+
+	if (!updated) {
+		return false;
+	}
+
+	if (orb_copy(_topic, _fd, data)) {
+		if (data != nullptr) {
+			/* error copying topic data */
+			memset(data, 0, _topic->o_size);
+		}
+		return false;
+	}
+
+	return true;
+}
+
 bool
 MavlinkOrbSubscription::is_published()
 {

src/modules/mavlink/mavlink_orb_subscription.h

@@ -54,11 +54,13 @@ public:
 	~MavlinkOrbSubscription();
 
 	/**
-	 * Check if subscription updated and get data.
+	 * Check if subscription updated based on timestamp.
 	 *
-	 * @return true only if topic was updated and data copied to buffer successfully.
+	 * @return true only if topic was updated based on a timestamp and
-	 * If topic was not updated since last check it will return false but still copy the data.
+	 * copied to buffer successfully.
-	 * If no data available data buffer will be filled with zeroes.
+	 * If topic was not updated since last check it will return false but
+	 * still copy the data.
+	 * If no data available data buffer will be filled with zeros.
 	 */
 	bool update(uint64_t *time, void *data);
 
@@ -69,11 +71,20 @@ public:
 	 */
 	bool update(void *data);
 
+	/**
+	 * Check if the subscription has been updated.
+	 *
+	 * @return true if there has been an update which has been
+	 * copied successfully.
+	 */
+	bool update_if_changed(void *data);
+
 	/**
 	 * Check if the topic has been published.
 	 *
 	 * This call will return true if the topic was ever published.
 	 * @return true if the topic has been published at least once.
+	 * If no data is available the buffer will be filled with zeros.
 	 */
 	bool is_published();
 	orb_id_t get_topic() const;