参考链接:
http://wiki.ros.org/nodelet
http://wiki.ros.org/nodelet/Tutorials/Running a nodelet

http://wiki.ros.org/nodelet/Tutorials/Porting nodes to nodelets
https://blog.csdn.net/zyh821351004/article/details/52143309

ROS的数据通信是以XML-RPC的方式,在graph结构中以topic,service和param的方式传输数据,天生的数据交互存在一定的延时和阻塞。Nodelet 包就是改善这一状况设计的, 使得多个算法运行在同一个过程中,并且算法间数据传输无需拷贝就可实现。 详见http://wiki.ros.org/nodelet。 简单的讲就是可以将以前启动的多个node捆绑在一起manager,使得同一个manager里面的topic的数据传输更快,数据通讯中roscpp采用boost shared pointer方式进行publish调用,实现zero copy。

以下为本人自己在学习相关资料,自己做的一个测试用例,供参考。

1 创建工作空间
makedir nodelet_ws/src

2 创建nodelet_test包
catkin_create_pkg nodelet_test roscpp std_msgs nodelet

3 创建nodelet_test/src/math_plus.cpp文件
math_plus.cpp

#include <pluginlib/class_list_macros.h>
#include <nodelet/nodelet.h>
#include <ros/ros.h>
#include <std_msgs/Float64.h>

namespace math_test
{
    /* code */
    class math_plus: public nodelet::Nodelet
    {
    public:
        math_plus()
        {

        }
        virtual ~math_plus()
        {

        }
    
    private:
        virtual void onInit()
        {
            ros::NodeHandle& private_nh = getPrivateNodeHandle();
            private_nh.getParam("value", value_);
            plus_result_pub_ = private_nh.advertise<std_msgs::Float64>("out", 10);
            plus_param_sub_ = private_nh.subscribe("in", 10, &math_plus::plus_process_callback, this);
        }

        void plus_process_callback(const std_msgs::Float64::ConstPtr& input)
        {
            std_msgs::Float64Ptr output(new std_msgs::Float64());
            output->data = input->data + value_;
            NODELET_DEBUG("Adding %f to get %f", value_, output->data);
            plus_result_pub_.publish(output);
        }

    private:
        ros::Publisher plus_result_pub_;
        ros::Subscriber plus_param_sub_;
        double value_;
    };
}

PLUGINLIB_EXPORT_CLASS(math_test::math_plus, nodelet::Nodelet);

math_plus.cpp解析:
(1)通过 plus_param_sub_ = private_nh.subscribe(“in”, 10, &math_plus::plus_process_callback, this)订阅名为math_plus/in topic的数据
(2)与通过参数服务器(private_nh.getParam(“value”, value_))获取到的参数value_进行相加
(3)将相加后的结果通过plus_result_pub_.publish(output)以topic的形式发布出去

插件注册:
PLUGINLIB_EXPORT_CLASS(math_test::math_plus, nodelet::Nodelet);

4 math_plus_plugin.xml

<library path="lib/libmath_plus">
    <class name="nodelet_test/math_plus" type="math_test::math_plus" base_class_type="nodelet::Nodelet">
        <description>
            A node to add a value and republish.
        </description>
    </class>
</library>

5 CMakeList.txt

cmake_minimum_required(VERSION 2.8.3)
project(nodelet_test)

find_package(catkin REQUIRED COMPONENTS
  nodelet
  roscpp
  std_msgs
)

catkin_package(
)

include_directories(
  include ${catkin_INCLUDE_DIRS}
  ${catkin_INCLUDE_DIRS}
)

add_library(math_plus
  src/math_plus.cpp
)
add_dependencies(math_plus ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})
target_link_libraries(math_plus
  ${catkin_LIBRARIES}
)

6 package.xml

<?xml version="1.0"?>
<package format="2">
  <name>nodelet_test</name>
  <version>0.0.0</version>
  <description>The nodelet_test package</description>

  <maintainer email="zyn@todo.todo">zyn</maintainer>
  <license>TODO</license>
  <buildtool_depend>catkin</buildtool_depend>

  <build_depend>nodelet</build_depend>
  <build_depend>roscpp</build_depend>
  <build_depend>std_msgs</build_depend>
  <build_export_depend>nodelet</build_export_depend>
  <build_export_depend>roscpp</build_export_depend>
  <build_export_depend>std_msgs</build_export_depend>
  <exec_depend>nodelet</exec_depend>
  <exec_depend>roscpp</exec_depend>
  <exec_depend>std_msgs</exec_depend>

  <!-- The export tag contains other, unspecified, tags -->
  <export>
    <!-- Other tools can request additional information be placed here -->
    <nodelet plugin="${prefix}/math_plus_plugin.xml"/>
  </export>
</package>

7 launch文件
math_plus_launch.launch

<launch>
    <node pkg="nodelet" type="nodelet" name="math_plus_manage" args="manager" output="screen"/>

    <node pkg="nodelet" type="nodelet" name="math_plus" args="load nodelet_test/math_plus math_plus_manage" output="screen">
        <param name="value" type="double" value="10"/>
    </node>

</launch>

步骤 3 4 5 6 7之间的联系:
(1)步骤4 中的libmath_plus 由步骤5 中的add_library(math_plus src/math_plus.cpp)生成
(2)步骤4 中 的 type 和 base_class_type 由步骤3中PLUGINLIB_EXPORT_CLASS(math_test::math_plus, nodelet::Nodelet)确定
(3)步骤6 中 的math_plus_plugin.xml 为步骤 4 中创建的文件
(4)步骤7 中 的nodelet_test/math_plus 与步骤4 中的 class name=“nodelet_test/math_plus” 相对应
(5)步骤7 中的 为步骤3 中的 value参数提供数据,数据值为double型数值10

math_plus_launch.launch文件:

(1)启动两个notelet节点,但参数不同分别为args="manager"和args=“load nodelet_test/math_plus math_plus_manage”,可以浅显的认为参数manager对应的节点充当管理层,另一个或多个节点充当被管理者
(2)参数args=“load nodelet_test/math_plus math_plus_manage” 中,
load :加载
nodelet_test/math_plus:插件类名称
math_plus_manage:插件所属的管理层名称

nodelet可以同时创建多个管理层,可以根据需求将nodelet分配到对应的manage层中,如
example.launch

<launch>
    <node pkg="nodelet" type="nodelet" name="math_plus_manage" args="manager" output="screen"/>

    <node pkg="nodelet" type="nodelet" name="math_plus" args="load nodelet_test/math_plus math_plus_manage" output="screen">
        <param name="value" type="double" value="10"/>
    </node>

    <node pkg="nodelet" type="nodelet" name="math_plus1" args="load nodelet_test/math_plus math_plus_manage" output="screen">
    </node>
    <node pkg="nodelet" type="nodelet" name="math_plus2" args="load nodelet_test/math_plus math_plus_manage" output="screen">
    </node>
    <node pkg="nodelet" type="nodelet" name="math_plus3" args="load nodelet_test/math_plus math_plus_manage" output="screen">
    </node>

    <node pkg="nodelet" type="nodelet" name="math_plus_manage_2" args="manager" output="screen"/>

    <node pkg="nodelet" type="nodelet" name="math_plus4" args="load nodelet_test/math_plus math_plus_manage_2" output="screen">
    </node>
    <node pkg="nodelet" type="nodelet" name="math_plus5" args="load nodelet_test/math_plus math_plus_manage_2" output="screen">
    </node>

    <node pkg="nodelet" type="nodelet" name="math_plus6" args="standalone nodelet_test/math_plus" output="screen">
    </node>

</launch>

example.launch中表明,math_plus、math_plus1、math_plus2、math_plus3对应的nodelet所属manager为math_plus_manage,math_plus4、math_plus5所属manager为math_plus_manage_2,test6独立存在,他们之间的数据交互按manager进行划分,示意如下:

在这里插入图片描述

8

运行测试
(1)启动nodelet
roslaunch nodelet_test math_plus_launch.launch

(2)发布/math_plus/in topic 数值为 1
rostopic pub -r 1 /math_plus/in std_msgs/Float64 1

(3)rostopic echo /math_plus/out查看输出数据

zyn@ubuntu:~/zyn_test/nodelet_ws$  rostopic echo /math_plus/out
data: 11.0
---
data: 11.0
---
data: 11.0
---
data: 11.0
---
data: 11.0
---
data: 11.0
---
data: 11.0
---
data: 11.0
---
data: 11.0
---

launch文件中的参数值为10, 发布的topic数据为1 ,相加后的结果为 11