MRS ROS example

This package was created as an example of how to write basic ROS nodelets. The package is written in C++ and features custom MRS libraries and msgs.

Functionality

• Desired waypoints are loaded as a matrix from config file
• Service fly_to_first_waypoint prepares the UAV by flying to the first waypoint
• Service start_waypoint_following causes the UAV to start tracking the waypoints
• Service stop_waypoint_following stops adding new waypoints. Flight to the current waypoint is not interrupted.

Package structure

See ROS packages

• src directory contains all source files
• include directory contains all header files. It is good practice to separate them from source files.
• launch directory contains .launch files which are used to parametrize the nodelet. Command-line arguments, as well as environment variables, can be loaded from the launch files, the nodelet can be put into the correct namespace (each UAV has its namespace to allow multi-robot applications), config files are loaded, and parameters passed to the nodelet. See .launch files
• config directory contains parameters in .yaml files. See .yaml files
• package.xml defines properties of the package, such as package name and dependencies. See package.xml

Example features

• Nodelet initialization
• Subscriber, publisher, and timer initialization
• Service servers and clients initialization
• Loading parameters with mrs_lib::ParamLoader class
• Loading Eigen matrices with mrs_lib::ParamLoader class
• Checking nodelet initialization status in every callback
• Checking whether subscribed messages are coming
• Throttling text output to a terminal
• Thread-safe access to variables using std::lock_scope()
• Using ConstPtr when subscribing to a topic to avoid copying large messages
• Storing and accessing matrices in Eigen classes
• Remapping topics in the launch file

Coding style

For easy orientation in the code, we have agreed to follow the ROS C++ Style Guide when writing our packages.

Naming variables

• Member variables are distinguished from local variables by underscore at the end:
  • position_x - local variable
  • position_x_ - member variable
• Also, we distinguish parameters which are loaded as parameters by underscore at the beginning
  • _simulation_ - parameter
• Descriptive variable names are used. The purpose of the variable should be obvious from the name.
  • sub_odom_uav_ - member subscriber to uav odometry msg type
  • pub_reference_ - member publisher of reference msg type
  • srv_server_start_waypoints_following_ - member service server for starting following of waypoints
  • WaypointFlier::callbackTimerCheckSubscribers() - callback of timer which checks subscribers
  • mutex_odom_uav_ - mutex locking access to variable containing odometry of the UAV

Good practices

• Nodelet everything! Nodelets compared to nodes do not need to send whole messages. Multiple nodelets running under the same nodelet manager form one process and messages can be passed as pointers.
• Do not use raw pointers! Smart pointers from <memory> free resources automatically, thus preventing memory leaks.
• Lock access to member variables! Nodelets are multi-thread processes, so it is our responsibility to make our code thread-safe.
  • Use c++17 scoped_lock which unlocks the mutex after leaving the scope. This way, you can't forget to unlock the mutex.

  {
    std::scoped_lock lock(mutex_odom_uav_);
    odom_uav_ = *msg;
  }

• Use ros::Time::waitForValid() after creating node handle ros::NodeHandle nh("~")
• When a nodelet is initialized, the method onInit() is called. In the method, the subscribers are initialized, and callbacks are bound to them. The callbacks can run even before the onInit() method ends, which can lead to some variables being still not initialized, parameters not loaded, etc. This can be prevented by using an is_initialized_, initializing it to false at the beginning of onInit() and setting it to true at the end. Every callback should check this variable and continue only when it is true.
• Use mrs_lib::ParamLoader class to load parameters from launch files and config files. This class checks whether the parameter was actually loaded, which can save a lot of debugging. Furthermore, loading matrices into config files becomes much simpler.
• For printing debug info to terminal use ROS_INFO(), ROS_WARN(), ROS_ERROR() macros. Do not spam the terminal by printing a variable every time a callback is called, use for example ROS_INFO_THROTTLE(1.0, "dog") to print dog not more often than every second. Other animals can also be used for debugging purposes.
• If you need to execute a piece of code periodically, do not use sleep in a loop, or anything similar. The ROS API provides ros::Timer class for this purposes, which executes a callback every time the timer expires.
• Always check whether all subscribed messages are coming. If not, print a warning. Then you know the problem is not in your nodelet and you know to look for the problem in topic remapping or the node publishing it.