ROS Clients and Subscribers

Writing the arm_mover node, you practiced generating custom messages, publishing to a topic, building ROS services servers, setting parameters, and creating launch files. You almost have a complete overview of ROS, but you still have to learn ROS clients to request services from client nodes, as well as ROS subscribers.

ROS Clients

A service client defined inside a service client node can request services from a service server node. In C++, ROS clients frequently have the following format, although other parameters and arguments are possible:

ros::ServiceClient client = n.serviceClient<package_name::service_file_name>("service_name");

The client object is instantiated from the ros::ServiceClient class. This object allows you to request services by calling the client.call() function.

To communicate with the ROS Master in C++, you need a NodeHandle. The node handle n will initialize the node.

The package_name::service_file_name indicates the name of the service file located in the srv directory of the package.

The service_name argument indicates the name of the service which is defined in the service server node.

ROS Subscribers

A subscriber enables your node to read messages from a topic, allowing useful data to be streamed to the node. In C++, ROS subscribers frequently have the following format, although other parameters and arguments are possible:

ros::Subscriber sub1 = n.subscribe("/topic_name", queue_size, callback_function);

The sub1 object is a subscriber object instantiated from the ros::Subscriber class. This object allows you to subscribe to messages by calling the subscribe() function.

To communicate with the ROS Master in C++, you need a NodeHandle. The node handle n will initialize the node.

The "/topic_name" indicates the topic to which the Subscriber should listen.

The queue_size determines the number of messages that can be stored in a queue. If the number of messages published exceeds the size of the queue, the oldest messages are dropped. As an example, if the queue_size is set to 100 and the number of messages stored in the queue is equal to 100, we will have to start deleting old messages to make room in the queue for new messages. This means that we are unable to process messages fast enough and we probably need to increase the queue_size.

The callback_function is the name of the function that will be run each incoming message. Each time a message arrives, it is passed as an argument to callback_function. Typically, this function performs a useful action with the incoming data. Note that unlike service handler functions, the callback_function is not required to return anything.