ROS nodelet初探

1. 简介

• nodelet官方教程:http://wiki.ros.org/nodelet
• 中文教程:http://www.ncnynl.com/archives/201702/1324.html

nodelet是为了减少ros node之间的消息传输(进程间通讯), 将多个算法"结点(nodelet)"跑到一个进程中, 这样避免了数据传输, (因为进程内部内存是共享的, 传数据的话传个指针就行了), 从而实现的零拷贝(zero copy).

nodelet和node很大程度上保持一致, 对外来说, nodelet几乎可以视为node, nodelet之间仍然是消息传输(只是内部实现由原始数据的传输改成了指针传输), 将node转换为nodelet只需要少量的改动.

2. 编写nodelet

nodelet的pkg组织架构和node有点相似, 包括: include, src, package.xml, CMakeLists.txt, 和 nodelet_plugins.xml.

1. 创建pkg: catkin_create_pkg sample_nodelet nodelet roscpp rospy. 注意要依赖nodelet包.
2. 在include, src中创建一个类, 该类继承nodelet::Nodelet, 最好在类的外面加一个namespace, 可以以package名字做namespace. 类中有一个onInit()函数, 相当于main()函数.

//sample_nodelet.h
#ifndef __SAMPLE_NODELET_H__
#define __SAMPLE_NODELET_H__
#include <nodelet/nodelet.h>
namespace example_pkg
{
    class SampleNodelet: public nodelet::Nodelet
    {
    public:
        virtual void onInit();
    };
}
#endif //__SAMPLE_NODELET_H__

3. 在cpp中实现onInit()函数, 编写自己的代码. 有个特别的地方是 cpp中还需要加一行

• PLUGINLIB_EXPORT_CLASS(example_pkg::SampleNodelet, nodelet::Nodelet)

//sample_nodelet.cpp
#include <pluginlib/class_list_macros.h>
#include <ros/ros.h>
#include <sample_nodelet.h>

PLUGINLIB_EXPORT_CLASS(example_pkg::SampleNodelet, nodelet::Nodelet)

namespace example_pkg
{
    void SampleNodelet::onInit()
    {
        NODELET_DEBUG("Initializing nodelet...");
        ROS_INFO("Nodelet is Ok for test!!");
    }
}

4. CMakelist中添加library. 这里我们发现, nodelet的pkg中没有main()没有可执行程序, 而是生成一个lib, 让ros调用.

# CMakeLists.txt
cmake_minimum_required(VERSION 2.8.3)
project(sample_nodelet)

find_package(catkin REQUIRED COMPONENTS
  nodelet
  roscpp
  rospy
)

catkin_package(
 INCLUDE_DIRS include
 LIBRARIES ${PROJECT_NAME}
 CATKIN_DEPENDS nodelet roscpp rospy
 # DEPENDS system_lib
)

include_directories(
    include
    ${catkin_INCLUDE_DIRS}
)

add_library(${PROJECT_NAME} src/sample_nodelet.cpp)

add_dependencies(${PROJECT_NAME} ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})

5. nodelet_plugins.xml是nodelet特有的, 文件名可以自己定, 最好用这个名字. xml文件里面指定nodelet的lib的路径, 以及nodelet类的名字.

<library path="lib/libsample_nodelet">
  <class name="example_pkg/SampleNodelet" type="example_pkg::SampleNodelet" base_class_type="nodelet::Nodelet">
  <description>
  This is my nodelet.
  </description>
  </class>
</library>

6. package.xml中增加export项, 把上一个xml文件的位置配置进去.

<?xml version="1.0"?>
<package>
  <name>sample_nodelet</name>
  <version>0.0.0</version>
  <description>The sample_nodelet package</description>
  <maintainer email="symao@todo.todo">symao</maintainer>
  <license>TODO</license>
  <buildtool_depend>catkin</buildtool_depend>
  <build_depend>nodelet</build_depend>
  <build_depend>roscpp</build_depend>
  <build_depend>rospy</build_depend>
  <run_depend>nodelet</run_depend>
  <run_depend>roscpp</run_depend>
  <run_depend>rospy</run_depend>

  <!-- The export tag contains other, unspecified, tags -->
  <export>
    <nodelet plugin="${prefix}/nodelet_plugins.xml" />
  </export>
</package>

至此, nodelet pkg的编写就基本完成. 可以用catkin_make直接编译

3. 运行nodelet

3.1 以launch文件运行

编写launch文件, 如下. 该launch打开两个nodelet, 更改第二个nodelet的name和args.

<launch>
  <node pkg="nodelet" type="nodelet" name="standalone_nodelet"  args="manager" output="screen"/>
  <node pkg="nodelet" type="nodelet" name="SampleNodelet" args="load example_pkg/SampleNodelet standalone_nodelet" output="screen">
  </node>
</launch>

3.2 命令行运行

#启动nodelet管理器
roscore
rosrun nodelet nodelet manager __name:=nodelet_manager
#启动自己的nodelet
rosrun nodelet nodelet load nodelet_tutorial_math/Plus nodelet_manager __name:=nodelet1 nodelet1/in:=foo _value:=1.1

这里(http://www.ncnynl.com/archives/201702/1325.html)有一个简单的实例, 可以看一下.