Mercury is a lightweight C++ publisher-subscriber library developed for use in the VEX Robotics Competition. The library is a PROS template that follows in the footsteps of frameworks like ROS by enabling students to write simple messaging code that lets the different subsystems of a robot communicate with each other in a thread-safe, asynchronous manner.
Publisher-subscriber, or pub-sub, systems are a way to allow communication between the various parts of a system.
Let's say you had a robot with a button sensor mounted to the front of its chassis, and you wanted to stop the robot from driving forward when the button was pressed. Using pub-sub messaging, you would create a publisher for the button sensor and a subscriber for the chassis motors. The publisher would be set up to publish a certain value representing the button's state (pressed or not pressed) to a specific topic. The subscriber would then subscribe to that topic, and any time a value was published to the topic, it would receive that value and perform some operation with it.
This may seem like a lot of work for just one button, but if you wanted to add another subsystem that checked for that same button press, all you would need to do was add another subscriber. Mercury simplifies this even further by taking care of the messaging for you. All you do is specify your publishers and subscribers.
To use Mercury, all you need to do is download the latest ZIP file from the Releases page. Then, using the PROS integrated terminal, run
pros c fetch mercury@x.x.x.zipreplacing 'x.x.x' with the required version number.
Finally, navigate to your PROS project and run
pros c apply mercuryCreating a Publisher is as simple as a single line of code:
mercury::Publisher<int> publisher("topic_name");This line creates a Publisher with integer message type and topic name "topic_name". If you want a different message type, just swap out the int for any other type.
Creating a Subscriber is a bit more involved. First, you need a function that takes in a single parameter, like this:
void subscriber_callback(int data) {
printf("%d\n", data);
}This function will be called on the messages that the subscriber will be receiving, so make sure the type of the parameter matches the type of the topic being subscribed to. Then, make a Subscriber with this line:
mercury::Subscriber<int> subscriber("topic_name", subscriber_callback);This Subscriber is subscribed to the "topic_name" topic we made with that previous Publisher, and it is set up with the subscriber_callback function.
Now, whenever you want to communicate between the Publisher and the Subscriber, just write:
publisher.publish(data)and the Subscriber will receive the data integer.