创建ROS msg和srv

1.关于msg和srv

    msg: msg files are simple text files that describe the fields of a ROS message. They are used to generate source code for messages in different languages.

    srv: an srv file describes a service. It is composed of two parts: a request and a response.

    msg files are stored in themsg directory of a package, and srv files are stored in the srv directory.

    msgs are just simple text files with a field type and field name per line. The fieldtypes you can use are:
    int8, int16, int32, int64 (plusuint*)
    float32, float64
    string
    time, duration
    other msg files
    variable-length array[] and fixed-lengtharray[C]

    Thereis also a special type in ROS:Header, the header contains a timestamp and coordinate frame information that are commonly used in ROS. You will frequently see the first line in a msg file have Header header.

    Hereis an example of a msg that uses a Header, a string primitive, and two othermsgs :

    Header header
    string child_frame_id
    geometry_msgs/PoseWithCovariance pose
    geometry_msgs/TwistWithCovariance twist  

   srv files are just like msg files, except they contain two parts: arequest and a response. The two parts are separated by a '---' line. Here is anexample of a srv file:

    int64 A
    int64 B
    ---
    int64 Sum

   In the above example,A and B are the request, and Sum is the response. 

2.使用msg

  roscd beginner_tutorials
  mkdir msg
  echo "int64 num" > msg/Num.msg

  接下来修改package.xml文件，在相关位置添加如下内容：

message_generation
message_runtime

  接下来修改CMakeLists.txt文件，在如下相应位置添加粗体的内容：

  find_package(catkin REQUIRED COMPONENTS
    roscpp
    rospy
    std_msgs
    message_generation
  )

  catkin_package(
  # INCLUDE_DIRS include
  # LIBRARIES beginner_tutorials
  # CATKIN_DEPENDS roscpp rospy std_msgs
  # DEPENDS system_lib
    CATKIN_DEPENDS message_runtime
  )

  # add_message_files(
  #  FILES
  #  Message1.msg
  #  Message2.msg
  # )
  add_message_files(
      FILES
      Num.msg
  )

 # generate_messages(
 #   DEPENDENCIES
 #   std_msgs
 # )
  generate_messages(
      DEPENDENCIES
      std_msgs
  )

  文件保存后，执行如下命令可以看到相应的信息：

  rosmsg show beginner_tutorials/Num

  执行输出：

  int64 num

  而执行rosmsg show Num命令会输出：

  [beginner_tutorials/Num]:
  int64 num

3.使用srv

  roscd beginner_tutorials
  mkdir srv

  接下来复制一个现成的srv文件：

  roscp rospy_tutorials AddTwoInts.srvsrv/AddTwoInts.srv

  从rospy_tutorials包里将 AddTwoInts.srv复制为当前目录下的srv/AddTwoInts.srv文件。

  接下来package.xml的修改跟msg里的一样，而CMakeLists.txt的修改如下红色标记：

  find_package(catkin REQUIRED COMPONENTS
    roscpp
    rospy
    std_msgs
    message_generation
  )

  # add_service_files(
  #  FILES
  #  Service1.srv
  #  Service2.srv
  # )
  add_service_files(
      FILES
      AddTwoInts.srv
  )

  文件保存后，使用如下命令可查看到刚创建的srv相关信息：

 guochongxin@slam:~/ros/catkin_ws/src/beginner_tutorials$rossrv show beginner_tutorials/AddTwoInts
  int64 a
  int64 b
  ---
  int64 sum

 guochongxin@slam:~/ros/catkin_ws/src/beginner_tutorials$ rossrv showAddTwoInts
[beginner_tutorials/AddTwoInts]:
  int64 a
  int64 b
  ---
  int64 sum

  [rospy_tutorials/AddTwoInts]:
  int64 a
  int64 b
  ---
  int64 sum

4.关于msg和srv相同的操作步骤

  在CMakeLists.txt文件中，都需要如下内容：

  generate_messages(
      DEPENDENCIES
      std_msgs
  )

  确认后，接下来需要执行如下命令重新编译安装以便生成相应的新内容：

  roscd beginner_tutorials
  cd ../..
  catkin_make install

  编译无误并安装更新相应的文件。

5.回顾

    rospack = ros+pack(age) : provides information related to ROS packages
    roscd = ros+cd :changesdirectoryto a ROS package or stack
    rosls= ros+ls :listsfilesin a ROS package
    roscp = ros+cp :copiesfiles from/to a ROS package
    rosmsg = ros+msg : provides informationrelated to ROS message definitions
    rossrv = ros+srv : provides informationrelated to ROS service definitions
    catkin_make : makes (compiles) a ROSpackage
    rosmake =ros+make : makes (compiles) a ROS package (if you're not using a catkinworkspace)

参考网址：http://wiki.ros.org/ROS/Tutorials/CreatingMsgAndSrv