ROS Tutorials （3）

创建一个ROS msg 和 srv

msg和srv简介：

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

msg是一个每行是一个域类型和域名的简单文本文件，可用的域类型有：

• int8, int16, int32, int64 (plus uint*)
• loat32, float64
• string
• time, duration
• other msg files
• variable-length array[] and fixed-length array[C]
  在ROS中有一个特殊的类型：Header，它包含一个在ＲＯＳ中经常用到的时间戳记timestamp 和协调框架信息 coordinate frame information 。

---

使用msg

１．在一个包中创建一个新的msg，使用如下代码：

$ roscd beginner_tutorials
$ mkdir msg
$ echo "int64 num" > msg/Num.msg```
这样msg文件中只有一行内容，为 **int64 num**。
２．修改**package.xml**中的内容，取消以下两行代码的注释：
```xml
message_generation
message_runtime```
***注：we need "message_generation", while at runtime, we only need "message_runtime". ***
３．打开**CMakeLists.txt**文件，将以下代码的注释取消：

find_package(catkin REQUIRED COMPONENTS
roscpp
rospy
std_msgs
message_generation
)```
注：Despite its name, message_generation works for both msg and srv.

catkin_package( 
... 
CATKIN_DEPENDS message_runtime ... 
...
)```

add_message_files(
FILES
Num.msg
)```

generate_messages( 
DEPENDENCIES 
std_msgs
)```
上述代码中的*Num.msg*是我们所创建的msg的文件名。
４．显示**msg**

$ rosmsg show beginner_tutorials/Num```
或者是

$ rosmsg show Num```
结果会显示出**msg**中的内容：**int64 num**。
********
####使用srv
１．创建一个**srv**文件:
我们用**roscp**命令，从一个包中复制文件到另一个包中：

$ roscp [package_name] [file_to_copy_path] [copy_path]```
２．打开CMakeLists.txt文件，将以下代码的注释取消：

add_service_files( 
FILES 
AddTwoInts.srv
)```
上述代码中的*AddTwoInts.srv*是我们所创建的srv文件名。
３．显示**srv**

$ rossrv show beginner_tutorials/AddTwoInts```
或者是

$ rossrv show AddTwoInts```
结果会显示出**srv**中的内容：

int64 a
int64 b

---

int64 sum```

注意：
*If you are building C++ nodes which use your new messages, you will also need to declare a dependency between your node and your message, as described in the catkin msg/srv build documentation. *