ROS學習筆記(一)：自己動手寫一個ROS程序

創建一個工作區（workspace）

工作區可以作為一個獨立的項目進行編譯，存放ROS程序的源文件、編譯文件和執行文件。建立工作區的方法如下：

 

$ mkdir -p ~/catkin_ws/src
$ cd ~/catkin_ws/src
$ catkin_init_workspace

雖然這時候工作區是空的，但是我們依然可以進行編譯：

 

 

$ cd ~/catkin_ws/
$ catkin_make

 

這時候，會在當前文件夾下生成devel，build這兩個子文件夾，在devel文件夾下能看到幾個setup.*sh文件。

接下來把工作區在bash中注冊

 

$ source devel/setup.bash

要驗證是否已經在bash中注冊可以使用如下命令：

 

 

$ echo $ROS_PACKAGE_PATH
/home/youruser/catkin_ws/src:/opt/ros/indigo/share:/opt/ros/indigo/stacks

如果能看到自己工作區的文件路徑就說明已經成功了。

 

創建一個ROS工程包（Package）

在一個工作區內，可能會包含多個ROS工程包。而最基本ROS工程包中會包括CmakeLists.txt和Package.xml這兩個文件，其中Package.xml中主要包含本項目信息和各種依賴（depends），而CmakeLists.txt中包含了如何編譯和安裝代碼的信息。

 

首先切換到工作區：

 

 $ cd ~/catkin_ws/src

現在可以使用catkin_create_pkg命令去創建一個叫beginner_tutorials的包，這個包依靠std_msgs、roscpp、rospy。

 

 

 $ catkin_create_pkg beginner_tutorials std_msgs rospy roscpp

接下來在工作區編譯這個工程包。

 

 

 $ cd ~/catkin_ws
 $ catkin_make

 

一個簡單的發布（Publisher）、訂閱(Subscriber)程序

 

寫一個發布（Publisher）節點

節點（node）是連接到ROS網絡中可執行的基本單元。我們在這創建一個發布者---“talker”節點，這個節點持續對外發布消息。

首先我們要把目錄切換到我們的beginner_tutorials工程包中

 

 $ cd ~/catkin_ws/src/beginner_tutorials

因為我們已經編譯過這個工程包了，所以會在beginner_tutorials文件夾下看到CmakeList.txt、package.xml文件和include、src這兩個目錄。接下來進入src子目錄

 

 

 $ cd src

在src目錄中創建一個talker.cpp文件，里面的內容如下：

 

 

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

 #include <sstream>
 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;
 }

寫一個訂閱（Subscriber）節點

還是在src目錄下，創建一個listener.cpp文件。內容如下：

 

 #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;
 }

編譯創建的節點
在編譯我們創建的節點之前，我們還需要編輯Cmakelist.txt文件（注意：是beginner_tutorials項目包下的CMakelist文件），告訴編輯器我們需要編輯什么文件，需要什么依賴。

 

 

 $ gedit CMakeLists.txt

在文件末尾添加如下語句：

 

 

 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)

將目錄切換到工作區目錄，並執行catkin_make運行命令：

 

 

 $ cd ~/catkin_ws
 $ catkin_make

不出意外的話，會出現如下界面：

 

至此，程序已經創建完成，而接下來我們要檢查一下我們創建的程序是否正確。

測試程序的正確性

首先，我們得要啟動ROS核心程序roscore。

 $ roscore

在使用我們的程序之前，需要先把程序注冊

 $ cd ~/catkin_ws
 $ source ./devel/setup.bash

運行talker節點：

 $ rosrun beginner_tutorials talker

這時候會看到如下信息：

 [INFO] [WallTime: 1314931831.774057] hello world 1314931831.77
 [INFO] [WallTime: 1314931832.775497] hello world 1314931832.77
 [INFO] [WallTime: 1314931833.778937] hello world 1314931833.78
 [INFO] [WallTime: 1314931834.782059] hello world 1314931834.78
 [INFO] [WallTime: 1314931835.784853] hello world 1314931835.78
 [INFO] [WallTime: 1314931836.788106] hello world 1314931836.79

這就表示發布（Publisher）節點已經正確的運行了。

接下來運行listener節點：

 $ rosrun beginner_tutorials listener

這時候會看到如下信息：

 [INFO] [WallTime: 1314931969.258941] /listener_17657_1314931968795I heard hello world 1314931969.26
 [INFO] [WallTime: 1314931970.262246] /listener_17657_1314931968795I heard hello world 1314931970.26
 [INFO] [WallTime: 1314931971.266348] /listener_17657_1314931968795I heard hello world 1314931971.26
 [INFO] [WallTime: 1314931972.270429] /listener_17657_1314931968795I heard hello world 1314931972.27
 [INFO] [WallTime: 1314931973.274382] /listener_17657_1314931968795I heard hello world 1314931973.27
 [INFO] [WallTime: 1314931974.277694] /listener_17657_1314931968795I heard hello world 1314931974.28
 [INFO] [WallTime: 1314931975.283708] /listener_17657_1314931968795I heard hello world 1314931975.28

這說明訂閱節點（listener）已經成功的接收到了發布節點（talker）發布的信息。至此，整個程序結束！