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boostshared pointer方式进行publish调用，实现zero copy。

1. 特点

1,1 nodelets间数据传输zero copy，有效避免数据copy和网络传输代价

1.2 支持pulgin的方式动态加载

1.3 使用C++ ROS的接口方式

2. nodelet 基类

2.1 定义基类nodelet::Nodelet， 任何nodelet继承自它可以使用plugin的方式动态加载。

2.2 提供命名空间，参数可remap

2.3 nodelet的manager的过程可以加载更多的nodelets. 同一个manager过程的nodelet数据传输zero copy .

3. 基本使用

参数的使用方式详见代码： https://github.com/ros/nodelet_core/blob/indigo-devel/nodelet/src/nodelet.cpp

参数解析 +nodelet::Loader /NodeletInterface

nodelet load pkg/Type manager - Launch a nodelet of type pkg/Type on manager manager //向manager中loader nodelet
nodelet standalone pkg/Type   - Launch a nodelet of type pkg/Type in a standalone node //程序复用，相当启动一个普通node
nodelet unload name manager   - Unload a nodelet a nodelet by name from manager //从manager移除nodelet
nodelet manager               - Launch a nodelet manager node  //创建mananger

4. 例程

(改自https://github.com/ros/common_tutorials/blob/hydro-devel/nodelet_tutorial_math/src/plus.cpp)

4.1 cmakeLists

cmake_minimum_required(VERSION 2.8.3)
project(nodelet_test_pkg)

find_package(catkin required COMPONENTS nodelet roscpp std_msgs)#

## Setup include directories
include_directories(${catkin_INCLUDE_DIRS})

catkin_package(
)

add_library(nodelet_test_lib plus.cpp)
target_link_libraries(nodelet_test_lib ${catkin_LIBRARIES})

4.2 package

<package>
  <name>nodelet_test_pkg</name>
  <version>0.0.0</version>
  <description>Nodelet test.</description>
  <maintainer email="huasheng_zyh@163.com">kint zhao</maintainer>
  <license>BSD</license>

  <buildtool_depend>catkin</buildtool_depend>

  <build_depend>nodelet</build_depend>
  <build_depend>roscpp</build_depend>
  <build_depend>std_msgs</build_depend>

  <run_depend>nodelet</run_depend>
  <run_depend>roscpp</run_depend>
  <run_depend>std_msgs</run_depend>

  <export>
    <nodelet plugin="${prefix}/nodelet_test_plugin.xml"/>
  </export>
</package>

4.3 pulgin

<library path="lib/libnodelet_test_lib">
  <class name="nodelet_ns/Plus" type="nodelet_ns::Plus" base_class_type="nodelet::Nodelet">
    <description> 
      A node to add a value and republish.
    </description>
  </class>
</library>

<launch>
  <node pkg="nodelet" type="nodelet" name="manager_1"  args="manager" output="screen"/>

  <node pkg="nodelet" type="nodelet" name="test1" args="load nodelet_ns/Plus manager_1" output="screen"/>
  <node pkg="nodelet" type="nodelet" name="test2" args="load nodelet_ns/Plus manager_1" output="screen"/>
  <node pkg="nodelet" type="nodelet" name="test3" args="load nodelet_ns/Plus manager_1" output="screen"/>

  <node pkg="nodelet" type="nodelet" name="manager_2"  args="manager" output="screen"/>
  <node pkg="nodelet" type="nodelet" name="test4" args="load nodelet_ns/Plus manager_2" output="screen"/>
  <node pkg="nodelet" type="nodelet" name="test5" args="load nodelet_ns/Plus manager_2" output="screen"/>

  <node pkg="nodelet" type="nodelet" name="test6" args="standalone nodelet_ns/Plus " output="screen"/>
		  
</launch>

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


#include <math.h> //fabs

namespace nodelet_ns
{

class Plus : public nodelet::Nodelet
{
public:
  Plus()
  : value_(0)
  {}

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

  void 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);
    pub.publish(output);
  }

  ros::Publisher pub;
  ros::Subscriber sub;
  double value_;
};

PLUGINLIB_DECLARE_CLASS(nodelet_ns,Plus,nodelet_ns::Plus,nodelet::Nodelet);//*******
}