ROS2 Journey IV

1. Background

Services是另一种节点间通信的方式。Service基于call-and-response 模型，对应着的是topic的publisher- subscriber模型。对比来说，topic可以让节点持续的更新数据，services知识在被client请求时才传数据。

Tasks

1. Setup

打开/turtlesim and /teleop_turtle

运行ros2 service list

每个节点有6个service

2. ros2 service type

ros2 service type <service_name>

ros2 service type /clear

这里的Empty指当请求时不接收任何数据。

ros2 service list -t可以查看service的类型。

ros2 service find std_srvs/srv/Empty

可以返回所有Empty类型的services。

ros2 interface show <type_name>

如果想要手动请求service，需要首先知道输入的结构。就可以用interface来查看。

接下来我们就可以call。

ros2 service call /clear std_srvs/srv/Empty

轨迹被清除掉了。

ros2 service call /spawn turtlesim/srv/Spawn "{x: 2, y: 2, theta: 0.2, name: ''}"

生成了一个新的turtle。

Summary

Nodes can communicate using services in ROS 2. Unlike a topic - a one way communication pattern where a node publishes information that can be consumed by one or more subscribers - a service is a request/response pattern where a client makes a request to a node providing the service and the service processes the request and generates a response.

You generally don’t want to use a service for continuous calls; topics or even actions would be better suited.