Topics subscription


Overview

Gazebo communicates on TCP/IP sockets, which allows separate programs to interface with Gazebo. Boost ASIO is used by Gazebo to manage the communication layer, and Google Protobufs are used as the message passing and serialization library. Messages are sent on named channels called topics via publishers. On the other side of a topic are subscribers, which receive callbacks when messages arrive. In summary, to send messages one must publish messages using a publisher on a named topic, and to receive messages one must subscribe to a named topic using a subscriber.

The easiest way to communicate with Gazebo over TCP/IP sockets is to link against the Gazebo libraries, and use the provided functions.

The Gazebo transport system is documented here and messages are documented here.

A list of all the topics in use on a running system can be found with the following command (make sure Gazebo is running first):

gz topic -l

Example

This example subscribes to a WorldStatistics message and assumes that you can link against Gazebo.

Download listener.cc and CMakeLists.txt and put them into to a folder called listener at your home directory and then compile the example:

mkdir ~/listener
cd ~/listener
wget https://bitbucket.org/osrf/gazebo/raw/default/examples/stand_alone/listener/listener.cc
wget https://bitbucket.org/osrf/gazebo/raw/default/examples/stand_alone/listener/CMakeLists.txt
mkdir build
cd build
cmake ..
make

Run the listener program when an instance of Gazebo is already running.

cd ~/listener/build
./listener

The output should be similar to

sim_time {
  sec: 1104
  nsec: 855000000
}
pause_time {
  sec: 0
  nsec: 0
}
real_time {
  sec: 1108
  nsec: 263362269
}
paused: false
iterations: 1104855
sim_time {
  sec: 1105
  nsec: 55000000
}
pause_time {
  sec: 0
  nsec: 0
}
real_time {
  sec: 1108
  nsec: 464165998
}
paused: false
iterations: 1105055

Explanation of the code

Load gazebo and run the transport system.

gazebo::setupClient(_argc, _argv);

Next create a Node, which provides functions to create publishers and subscribers.

gazebo::transport::NodePtr node(new gazebo::transport::Node());
node->Init();

Create a subscriber on the ''world_stats'' topic. Gazebo publishes a stream of stats on this topic.

gazebo::transport::SubscriberPtr sub = node->Subscribe("~/world_stats", cb);

You'll need to create a callback function that will print the message to the console, which we called cb in the previous line.

void cb(ConstWorldStatisticsPtr &_msg)
{
  std::cout << _msg->DebugString();
}

At this point you'll have to create a wait loop while messages come in, or do some other processing. Here is simple wait loop.

while (true)
  gazebo::common::Time::MSleep(10);

Once you're done, finalize the transport system.

gazebo::shutdown();