Get/Set the System.Object object that event callbacks for this joint will be sent to. Care should be taken with any System.Object assigned as a callback target that isn't a UnityEngine.Object as this assignment will not in itself keep the object alive and can be garbage collected. To avoid this, you should have at least a single reference to the object in your code. This includes the following events: - A JointThresholdEvent with call IJointThresholdCallback.