1.编写好listener.cpp和talker.cpp后catkin_make报错：

The dependency target "_generate_messages_cpp" of target "talker" does not
  exist.

解决方法：将CMakeLists.txt中的cmake_minimum_required(VERSION 版本号)改为cmake_minimum_required(VERSION 2.8.3)

2.生成新乌龟，ROS版本noetic。

rosservice call /spawn "x:8.0 y:8.0 theta:0.0 name:'turtle2'"
ERROR: Incompatible arguments to call service:
Not enough arguments:
 * Given: ["x:8.0 y:8.0 theta:0.0 name:'turtle2'"]
 * Expected: ['x', 'y', 'theta', 'name']
Provided arguments are:
 * x:8.0 y:8.0 theta:0.0 name:'turtle2' (type str)

解决方法：rosservice call /spawn '8.0' '8.0' '0.0' 'turtle2'