Callbacks

A callback is a segment of code that will run only when a certain condition is satisfied. For the Swarm Platform, the callbacks we use are triggered by the VRPN communications protocol used by the Camera System. Whenever the Client receives a new packet of data from the Camera System the callback will be triggered.

Registering a Callback

Before we can do anything we need to set up a callback that executes when new data is present.

1. Use the VRPN Library to create a Connection and Tracker_Remote object pointers (code in vrpn.cpp):

vrpn_Connection *connection;
vrpn_Tracker_Remote *trackerHand;
vrpn_Tracker_Remote *tracker;
vrpn_Tracker_Remote *tracker2;
vrpn_Tracker_Remote *tracker3;
vrpn_Tracker_Remote *tracker4;

Note: We need a Tracker_Remote object for each constellation we want to track (Hand and each Crazyflie in the Swarm)

2. Now we create a function to:

1. Register the connection to the Server computer
2. Assign each Tracker_Remote object to a valid constellation name on the Server computer
3. Link that constellation to a callback (code in vrpn.cpp):

void vrpn_init(std::string connectionName, void (*callbackHand)(void*, const vrpn_TRACKERCB),
      void (*callback)(void*, const vrpn_TRACKERCB), void (*callback2)(void*, const vrpn_TRACKERCB),
          void (*callback3)(void*, const vrpn_TRACKERCB), void (*callback4)(void*, const vrpn_TRACKERCB), char *key) {

	connection = vrpn_get_connection_by_name(connectionName.c_str());
#if USE_HAND
	trackerHand = new vrpn_Tracker_Remote("hand", connection);
#endif

	tracker = new vrpn_Tracker_Remote("Crazyflie21", connection);
	
	tracker2 = new vrpn_Tracker_Remote("Crazyflie2", connection);

	tracker3 = new vrpn_Tracker_Remote("Crazyflie22", connection);

	tracker4 = new vrpn_Tracker_Remote("Crazyflie23", connection);
#if USE_HAND
	trackerHand->register_change_handler(0, callbackHand);
#endif
	tracker->register_change_handler(0, callback);
	tracker2->register_change_handler(0, callback2);
	tracker3->register_change_handler(0, callback3);
	tracker4->register_change_handler(0, callback4);

	usleep(2000);
}

Note 1: We pass in the following as inputs to the function:

• string connectionName is the IP address of the Computer running the Camera System
• A callback pointer for each constellation we want to track (These will be established in the Using a Callback section)

Note 2: Notice the names of each tracker match the names of the constellations in the Camera System Software (shown below):

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