ROS與C++入門教程-寫簡單釋出節點和訂閱節點

說明：

• 介紹用C++編寫簡單釋出節點和訂閱節點

準備

• 第三節已經建立的基本的工作空間catkin_ws和教程包beginner_tutorials

編寫釋出節點

• 進入教程目錄,並新建源目錄和釋出節點talker.cpp（程式碼看參考github），實現釋出hello world

$ roscd beginner_tutorials
$ mkdir src
$ touch src/talker.cpp
$ vim src/talker.cpp

• 內容如下：

#include "ros/ros.h"
#include "std_msgs/String.h"

#include <sstream>

/**
 * This tutorial demonstrates simple sending of messages over the ROS system.
 */
int main(int argc, char **argv)
{
  /**
   * The ros::init() function needs to see argc and argv so that it can perform
   * any ROS arguments and name remapping that were provided at the command line.
   * For programmatic remappings you can use a different version of init() which takes
   * remappings directly, but for most command-line programs, passing argc and argv is
   * the easiest way to do it.  The third argument to init() is the name of the node.
   *
   * You must call one of the versions of ros::init() before using any other
   * part of the ROS system.
   */
  ros::init(argc, argv, "talker");

  /**
   * NodeHandle is the main access point to communications with the ROS system.
   * The first NodeHandle constructed will fully initialize this node, and the last
   * NodeHandle destructed will close down the node.
   */
  ros::NodeHandle n;

  /**
   * The advertise() function is how you tell ROS that you want to
   * publish on a given topic name. This invokes a call to the ROS
   * master node, which keeps a registry of who is publishing and who
   * is subscribing. After this advertise() call is made, the master
   * node will notify anyone who is trying to subscribe to this topic name,
   * and they will in turn negotiate a peer-to-peer connection with this
   * node.  advertise() returns a Publisher object which allows you to
   * publish messages on that topic through a call to publish().  Once
   * all copies of the returned Publisher object are destroyed, the topic
   * will be automatically unadvertised.
   *
   * The second parameter to advertise() is the size of the message queue
   * used for publishing messages.  If messages are published more quickly
   * than we can send them, the number here specifies how many messages to
   * buffer up before throwing some away.
   */
  ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);

  ros::Rate loop_rate(10);

  /**
   * A count of how many messages we have sent. This is used to create
   * a unique string for each message.
   */
  int count = 0;
  while (ros::ok())
  {
    /**
     * This is a message object. You stuff it with data, and then publish it.
     */
    std_msgs::String msg;

    std::stringstream ss;
    ss << "hello world " << count;
    msg.data = ss.str();

    ROS_INFO("%s", msg.data.c_str());

    /**
     * The publish() function is how you send messages. The parameter
     * is the message object. The type of this object must agree with the type
     * given as a template parameter to the advertise<>() call, as was done
     * in the constructor above.
     */
    chatter_pub.publish(msg);

    ros::spinOnce();

    loop_rate.sleep();
    ++count;
  }


  return 0;
}
 

程式碼分析：

• 程式碼：

#include "ros/ros.h"

• 解釋：

  • 匯入ROS系統包含核心公共標頭檔案

• 程式碼：

#include "std_msgs/String.h"

• 解釋：

  • 匯入std_msgs/String訊息標頭檔案，這個是由std_msgs包的string.msg檔案自動生成。
  • std_msgs這是標準的訊息包，可參考std_msgs ，更多的標準訊息定義，可參考msg

• 程式碼：

ros::init(argc, argv, "talker");
 

• 解釋：

  • 初始化ROS，指定節點名稱為“talker”，節點名稱要保持唯一性。名稱定義參考

• 程式碼：

ros::NodeHandle n;

• 解釋：

  • 例項化節點

• 程式碼：

ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);

• 解釋：

  • 釋出一個訊息型別為std_msgs/String，命名為chatter的話題。
  • 定義訊息佇列大小為1000，即超過1000條訊息之後，舊的訊息就會丟棄。

• 程式碼：

ros::Rate loop_rate(10);

• 解釋：

  • 指定釋出訊息的頻率，這裡指10Hz，也即每秒10次
  • 通過 Rate::sleep()來處理睡眠的時間來控制對應的釋出頻率。

• 程式碼：

  int count = 0;
  while (ros::ok())
  {

• 解釋：

  • 預設roscpp會植入一個SIGINT處理機制，當按下Ctrl-C，就會讓ros::ok()返回false,那迴圈就會結束。
  • ros::ok() 返回false的幾種情況：
    • SIGINT收到(Ctrl-C)訊號
    • 另一個同名節點啟動，會先中止之前的同名節點
    • ros::shutdown()被呼叫
    • 所有的ros::NodeHandles被銷燬

• 程式碼：

std_msgs::String msg;

std::stringstream ss;
ss << "hello world " << count;
msg.data = ss.str();

• 解釋：

  • 例項化訊息msg, 定義字串流“hello world”並賦值給ss, 最後轉成為字串賦值給msg.data

• 程式碼：

chatter_pub.publish(msg);

• 解釋：

  • 實際釋出的函式

• 程式碼：

ROS_INFO("%s", msg.data.c_str());

• 解釋：

  • 輸出除錯資訊，更多除錯相關參考Verbosity Levels, rosconsole

• 程式碼：

ros::spinOnce();

• 解釋：
  • 不是必需的，但是保持增加這個呼叫，是好習慣。
  • 如果程式裡也有訂閱話題，就必需，否則回撥函式不能起作用。

• 程式碼：

loop_rate.sleep();

• 解釋：
  • 根據之前ros::Rate loop_rate(10)的定義來控制釋出話題的頻率。定義10即為每秒10次

編寫訂閱節點

• 進入beginner_tutorials,新建src/listener.cpp檔案，原始碼參考

cd beginner_tutorials
touch src/listener.cpp
vim src/listener.cpp

• 新增程式碼如下：

#include "ros/ros.h"
#include "std_msgs/String.h"

/**
 * This tutorial demonstrates simple receipt of messages over the ROS system.
 */
void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());
}

int main(int argc, char **argv)
{
  /**
   * The ros::init() function needs to see argc and argv so that it can perform
   * any ROS arguments and name remapping that were provided at the command line.
   * For programmatic remappings you can use a different version of init() which takes
   * remappings directly, but for most command-line programs, passing argc and argv is
   * the easiest way to do it.  The third argument to init() is the name of the node.
   *
   * You must call one of the versions of ros::init() before using any other
   * part of the ROS system.
   */
  ros::init(argc, argv, "listener");

  /**
   * NodeHandle is the main access point to communications with the ROS system.
   * The first NodeHandle constructed will fully initialize this node, and the last
   * NodeHandle destructed will close down the node.
   */
  ros::NodeHandle n;

  /**
   * The subscribe() call is how you tell ROS that you want to receive messages
   * on a given topic.  This invokes a call to the ROS
   * master node, which keeps a registry of who is publishing and who
   * is subscribing.  Messages are passed to a callback function, here
   * called chatterCallback.  subscribe() returns a Subscriber object that you
   * must hold on to until you want to unsubscribe.  When all copies of the Subscriber
   * object go out of scope, this callback will automatically be unsubscribed from
   * this topic.
   *
   * The second parameter to the subscribe() function is the size of the message
   * queue.  If messages are arriving faster than they are being processed, this
   * is the number of messages that will be buffered up before beginning to throw
   * away the oldest ones.
   */
  ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);

  /**
   * ros::spin() will enter a loop, pumping callbacks.  With this version, all
   * callbacks will be called from within this thread (the main one).  ros::spin()
   * will exit when Ctrl-C is pressed, or the node is shutdown by the master.
   */
  ros::spin();

  return 0;
}

程式碼分析：

• 程式碼：

void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());
}

• 解釋：
  • 定義回撥函式chatterCallback，當收到chatter話題的訊息就會呼叫這個函式。
  • 訊息通過boost shared_ptr（共享指標）來傳遞。
  • 但收到訊息，通過ROS_INFO函式顯示到終端

• 程式碼：

ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);

• 解釋：

  • 定義訂閱節點，名稱為chatter，指定回撥函式chatterCallback
  • 但收到訊息，則呼叫函式chatterCallback來處理。
  • 引數1000，定義佇列大小，如果處理不夠快，超過1000，則丟棄舊的訊息。

• 程式碼：

ros::spin();

• 解釋：
  • 呼叫此函式才真正開始進入迴圈處理，直到 ros::ok()返回false才停止。

構建節點

• 在之前使用catkin_create_pkg建立包beginner_tutorials，會得到 package.xml 和 CMakeLists.txt兩個檔案
• 增加如下程式碼到CMakeLists.txt：

include_directories(include ${catkin_INCLUDE_DIRS})

add_executable(talker src/talker.cpp)
target_link_libraries(talker ${catkin_LIBRARIES})
add_dependencies(talker beginner_tutorials_generate_messages_cpp)

add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})
add_dependencies(listener beginner_tutorials_generate_messages_cpp)

• 完整程式碼類似：

cmake_minimum_required(VERSION 2.8.3)
project(beginner_tutorials)

## Find catkin and any catkin packages
find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)

## Declare ROS messages and services
add_message_files(FILES Num.msg)
add_service_files(FILES AddTwoInts.srv)

## Generate added messages and services
generate_messages(DEPENDENCIES std_msgs)

## Declare a catkin package
catkin_package()

## Build talker and listener
include_directories(include ${catkin_INCLUDE_DIRS})

add_executable(talker src/talker.cpp)
target_link_libraries(talker ${catkin_LIBRARIES})
add_dependencies(talker beginner_tutorials_generate_messages_cpp)

add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})
add_dependencies(listener beginner_tutorials_generate_messages_cpp)

• 建立兩個執行檔案， talker and listener，預設在

~/catkin_ws/devel/lib/<package name>

• 注意，您必須為訊息生成目標新增可執行目標的依賴項：

add_dependencies(talker beginner_tutorials_generate_messages_cpp)

• 這確保此包的訊息頭在使用前生成。如果你使用的資訊在你的catkin工作空間的其他包裡面，你需要新增依賴各自生成目標，因為catkin會平行構建的所有專案。

target_link_libraries(talker ${catkin_LIBRARIES})

• 進入工作空間編譯

$ cd ~/catkin_ws
$ catkin_make  

執行節點

• 新開終端執行：

$ roscore

• 新開終端執行talker：

source ~/catkin_ws/devel/setup.bash
rosrun beginner_tutorials talker

• 新開終端執行listener：

source ~/catkin_ws/devel/setup.bash
rosrun beginner_tutorials listener

• talker終端效果：

[ INFO] I published [Hello there! This is message [0]]
[ INFO] I published [Hello there! This is message [1]]
[ INFO] I published [Hello there! This is message [2]]

• listener終端效果：

[ INFO] Received [Hello there! This is message [20]]
[ INFO] Received [Hello there! This is message [21]]
[ INFO] Received [Hello there! This is message [22]]