MavlinkOrbSubscription API reworked

src/modules/mavlink/mavlink_orb_subscription.h

@@ -53,7 +53,21 @@ public:
 	MavlinkOrbSubscription(const orb_id_t topic);
 	~MavlinkOrbSubscription();
 
-	bool update(const hrt_abstime t, void* data);
+	/**
+	 * Check if subscription updated and get data.
+	 *
+	 * @return true only if topic was updated and data copied to buffer successfully.
+	 * 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 zeroes.
+	 */
+	bool update(uint64_t *time, void* data);
+
+	/**
+	 * Copy topic data to given buffer.
+	 *
+	 * @return true only if topic data copied successfully.
+	 */
+	bool update(void* data);
 
 	/**
 	 * Check if the topic has been published.
@@ -68,8 +82,6 @@ private:
 	const orb_id_t _topic;		///< topic metadata
 	int _fd;			///< subscription handle
 	bool _published;		///< topic was ever published
-	hrt_abstime _last_check;	///< time of last check
-	bool _updated;			///< updated on last check
 };