How to recognize when the user is lost in openni_tracker? [closed]

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Previously I asked a question about /tf topic. Unfortunately it seems my question is not obvious, or the answer simply did not work!

I use openni_tracker to publish /tf message. This topic will start publishing when the calibration for a user is finished, and will stop publishing when the person is lost. It just send the message of "user 1 lost" to the terminal. But even when the user is lost, none of these functions return false.

bool tf::TransformListener::canTransform (const std::string &target_frame, const std::string &source_frame, const ros::Time &time, std::string *error_msg=NULL) const
bool tf::TransformListener::waitForTransform (const std::string &target_frame, const std::string &source_frame, const ros::Time &time, const ros::Duration &timeout, const ros::Duration &polling_sleep_duration=ros::Duration(0.01), std::string *error_msg=NULL) const

These functions are doing well at the start of the program when I need to recognize when the person is calibrated and tracking started. I wonder how can I recognize in my program that the person is lost. Is it any way to access the text message that is printed in the terminal?