uORB Subscription callbacks with WorkItem scheduling on publication (#12207)

src/modules/uORB/uORBDeviceNode.hpp

@@ -45,6 +45,7 @@ namespace uORB
 class DeviceNode;
 class DeviceMaster;
 class Manager;
+class SubscriptionCallback;
 }
 
 /**
@@ -231,6 +232,12 @@ public:
 	 */
 	uint64_t copy_and_get_timestamp(void *dst, unsigned &generation);
 
+	// add item to list of work items to schedule on node update
+	bool register_callback(SubscriptionCallback *callback_sub);
+
+	// remove item from list of work items
+	void unregister_callback(SubscriptionCallback *callback_sub);
+
 protected:
 
 	pollevent_t poll_state(cdev::file_t *filp) override;
@@ -269,6 +276,7 @@ private:
 	uint8_t     *_data{nullptr};   /**< allocated object buffer */
 	hrt_abstime   _last_update{0}; /**< time the object was last updated */
 	volatile unsigned   _generation{0};  /**< object generation count */
+	List<uORB::SubscriptionCallback *>	_callbacks;
 	uint8_t   _priority;  /**< priority of the topic */
 	bool _published{false};  /**< has ever data been published */
 	uint8_t _queue_size; /**< maximum number of elements in the queue */