Could not find a connection between 'ee_link' and 'world' because they are not part of the same tree.Tf has two or more unconnected trees.

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Dear all,

Willing to avoid the use of Transformer::waitForTransform() (as I find the loop it implies does not look clean), I used Transformer::addTransformsChangedListener() so as to create a callback on tf changed. Here is my code:

#include "ros/ros.h"
#include <tf/transform_listener.h>


////////////////////////////////////////////////////////////////////////////////
// Callback when tf is updated.
struct MyTfCommunicator
{
public:
    void connectOnTfChanged()
    {
        m_tf_connection = m_tf_listener.addTransformsChangedListener(boost::bind(&MyTfCommunicator::onTfChanged, this));
    }
    void disconnectOnTfChanged()
    {
        if (m_tf_connection.connected())
            m_tf_listener.removeTransformsChangedListener(m_tf_connection);
    }

protected:
    void onTfChanged()
    {
        try
        {
            // Read ee_link's position.
            tf::StampedTransform transform;
            m_tf_listener.lookupTransform("/ee_link", "/world", ros::Time(0), transform);

            // Read desired position.

            // Compute the error.
            ROS_INFO("EE pos.x: [%f]", transform.getOrigin().x());
        }
        catch (tf::TransformException &ex)
        {
            ROS_ERROR("%s", ex.what());
            ros::Duration(1.0).sleep();
        }
    }

    tf::TransformListener m_tf_listener;
    boost::signals::connection m_tf_connection;
};


////////////////////////////////////////////////////////////////////////////////
// This node computes the effector's positional and rotational errors and publishes them.
int main(int argc, char **argv)
{
    // Pass argc, arv to init() to allow cmd line remapping.
    ros::init(argc, argv, "lpc_error");

    // Handle to access ROS.
    ros::NodeHandle n;

    // Setup on tf update callback and spin.
    MyTfCommunicator tfCommunicator;

    tfCommunicator.connectOnTfChanged();
    ros::spin();
    tfCommunicator.disconnectOnTfChanged();

    // Exit.
    return 0;
}

This code returns me the following error (I used it against the UR5 model from ROS-I) before actually displaying a real position message 10 or 20 seconds after my node started:

[ERROR] [1407495099.316372953, 56679.089000000]: "world" passed to lookupTransform argument source_frame does not exist.

[ERROR] [1407495100.333225598, 56680.089000000]: Could not find a connection between 'ee_link' and 'world' because they are not part of the same tree.Tf has two or more unconnected trees.

[ERROR] [1407495100.333225598, 56680.089000000]: Could not find a connection between 'ee_link' and 'world' because they are not part of the same tree.Tf has two or more unconnected trees.

... (10 - 20 seconds)

[ INFO] [1407495286.595620292, 56859.293000000]: EE pos.x: [-0.642255]

[ INFO] [1407495286.595733987, 56859.293000000]: EE pos.x: [-0.642255]

[ INFO] [1407495286.595796002, 56859.293000000]: EE pos.x: [-0.642255]

...

I can understand a few error messages while waiting for the buffers to fill up but in my case I have to wait for 10 or 20 seconds before I get the real position message displaying relevant data. This looked weird to me so I created a hacky code shown below and which uses a callback based on a usual subscriber (I no longer use the listener's callback), this code looks like this:

#include "ros/ros.h"
#include "tf2_msgs/TFMessage.h"


void onTfChanged(const tf2_msgs::TFMessage::ConstPtr& msg)
{
    // Compute the error.
    ROS_INFO("EE pos.x: [%f]", msg->transforms[0].transform.translation.x);
}


////////////////////////////////////////////////////////////////////////////////
// This node computes the effector's positional and rotational errors and publishes them.
int main(int argc, char **argv)
{
    // Pass argc, arv to init() to allow cmd line remapping.
    ros::init(argc, argv, "lpc_error");

    // Handle to access ROS.
    ros::NodeHandle n;

    // Setup on tf update callback and spin.
    ros::Subscriber sub = n.subscribe("tf", 3, onTfChanged);

    ros::spin();

    // Exit.
    return 0;
}

Now everything works fine and I get the expected position message without the 10-20 ... (more)