1、转移到之前教程在catkin工作空间所创建的package路径下:
$ roscd package_name/
例子：$ roscd practice/
2、在package路径下创建src目录:
$ mkdir src
如果已经有了就不用创建了
3、在 package里创建src/talker.cpp文件，
$ gedit talker.cpp
并粘贴如下代码：
#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; }
代码解释
-
现在，我们来分段解释代码.
#include "ros/ros.h"
-
ros/ros.h是一个实用的头文件，它引用了ROS系统中大部分常用的头文件，使用它会使得编程很简便。
#include "std_msgs/String.h"
- 这引用了std_msgs/String 消息, 它存放在std_msgs package里，是由String.msg文件自动生成的头文件。
- 需要更详细的消息定义,参考msg页面.
ros::init(argc, argv, "talker");
- 初始化ROS。它允许ROS通过命令行进行名称重映射——目前，这不是重点。
- 同样，我们也在这里指定我们节点的名称——必须唯一。
- 这里的名称必须是一个base name，不能包含/。
ros::NodeHandle n;
- 为这个进程的节点创建一个句柄。
- 第一个创建的NodeHandle会为节点进行初始化，最后一个销毁的会清理节点使用的所有资源。
ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);
- 告诉master我们将要在chatter topic上发布一个std_msgs/String的消息。
- 这样master就会告诉所有订阅了chatter topic的节点，将要有数据发布。
- 第二个参数是发布序列的大小。
- 在这样的情况下，如果我们发布的消息太快，缓冲区中的消息在大于1000个的时候就会开始丢弃先前发布的消息。
NodeHandle::advertise()
-
返回一个 ros::Publisher对象,它有两个作用:
- 它有一个publish()成员函数可以让你在topic上发布消息；
- 如果消息类型不对,它会拒绝发布。
ros::Rate loop_rate(10);
- ros::Rate对象可以允许你指定自循环的频率。
- 它会追踪记录自上一次调用Rate::sleep()后时间的流逝，并休眠直到一个频率周期的时间。
- 在这个例子中，我们让它以10hz的频率运行。
int count = 0;
while (ros::ok())
{
-
roscpp会默认安装一个SIGINT句柄，它负责处理Ctrl-C键盘操作——使得ros::ok()返回FALSE。
-
ros::ok()返回false，如果下列条件之一发生：
- SIGINT接收到(Ctrl-C)
- 被另一同名节点踢出ROS网络
- ros::shutdown()被程序的另一部分调用
- 所有的ros::NodeHandles都已经被销毁
- 一旦ros::ok()返回false, 所有的ROS调用都会失效。
std_msgs::String msg;
std::stringstream ss;
ss << "hello world " << count;
msg.data = ss.str();
- 我们使用一个由msg file文件产生的‘消息自适应’类在ROS网络中广播消息。
- 现在我们使用标准的String消息，它只有一个数据成员"data"。
- 当然你也可以发布更复杂的消息类型。
chatter_pub.publish(msg);
- 现在我们已经向所有连接到chatter topic的节点发送了消息。
ROS_INFO("%s", msg.data.c_str());
- ROS_INFO和类似的函数用来替代printf/cout.
- 参考rosconsole documentation以获得更详细的信息。
ros::spinOnce();
- 在这个例子中并不是一定要调用ros::spinOnce()，因为我们不接受回调。
- 然而，如果你想拓展这个程序，却又没有在这调用ros::spinOnce()，你的回调函数就永远也不会被调用。
- 所以，在这里最好还是加上这一语句。
loop_rate.sleep();
-
这条语句是调用ros::Rate对象来休眠一段时间以使得发布频率为10hz。
-
对上边的内容进行一下总结：
-
初始化ROS系统
-
在ROS网络内广播我们将要在chatter topic上发布std_msgs/String消息
-
以每秒10次的频率在chatter上发布消息
-
接下来我们要编写一个节点来接收消息。
-
编写订阅器节点
源代码
在package目录下创建src/listener.cpp文件，
$ gedit listener.cpp
并粘贴如下代码：
#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; }
代码解释
- 下面我们将逐条解释代码，当然，之前解释过的代码就不再赘述了。
void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
ROS_INFO("I heard: [%s]", msg->data.c_str());
}
- 这是一个回调函数，当消息到达chatter topic的时候就会被调用。
- 消息是以 boost shared_ptr指针的形式传输，这就意味着你可以存储它而又不需要复制数据
ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);
- 告诉master我们要订阅chatter topic上的消息。
- 当有消息到达topic时，ROS就会调用chatterCallback()函数。
- 第二个参数是队列大小，以防我们处理消息的速度不够快，在缓存了1000个消息后，再有新的消息到来就将开始丢弃先前接收的消息。
- NodeHandle::subscribe()返回ros::Subscriber对象,你必须让它处于活动状态直到你不再想订阅该消息。
- 当这个对象销毁时，它将自动退订上的消息。
- 有各种不同的NodeHandle::subscribe()函数，允许你指定类的成员函数，甚至是Boost.Function对象可以调用的任何数据类型。roscpp overview 提供了更为详尽的信息。
ros::spin();
-
ros::spin()进入自循环，可以尽可能快的调用消息回调函数。
-
如果没有消息到达，它不会占用很多CPU，所以不用担心。一旦ros::ok()返回FALSE，ros::spin()就会立刻跳出自循环。
-
这有可能是ros::shutdown()被调用，或者是用户按下了Ctrl-C，使得master告诉节点要shutdown。
-
也有可能是节点被人为的关闭。
-
还有其他的方法进行回调，但在这里我们不涉及。
-
想要了解，可以参考roscpp_tutorials package里的一些demo应用。
-
需要更为详尽的信息，参考roscpp overview。
-
下边，我们来总结一下:
- 初始化ROS系统
- 订阅chatter topic
- 进入自循环，等待消息的到达
- 当消息到达，调用chatterCallback()函数
编译节点
- 之前教程中使用catkin_create_pkg创建了package.xml 和 CMakeLists.txt 文件。
- 生成的CMakeLists.txt看起来应该是这样(在Creating Msgs and Srvs教程中的修改和未被使用的注释和例子都被移除了):
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(DIRECTORY msg FILES Num.msg) add_service_files(DIRECTORY srv FILES AddTwoInts.srv) ## Generate added messages and services generate_messages(DEPENDENCIES std_msgs) ## Declare a catkin package catkin_package()
-
在CMakeLists.txt文件末尾加入几条语句:
include_directories(include ${catkin_INCLUDE_DIRS})
add_executable(talker src/talker.cpp)
target_link_libraries(talker ${catkin_LIBRARIES})add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES}) -
结果，CMakeLists.txt文件看起来像这样:
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 ${catkin_LIBRARIES}) add_dependencies(listener beginner_tutorials_generate_messages_cpp)
-
这会生成两个可执行文件, talker 和 listener, 默认存储到devel space目录
-
具体是在~/catkin_ws/devel/lib/中.
-
现在要为可执行文件添加对生成的消息文件的依赖：
# add_dependencies(${PROJECT_NAME} ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})add_dependencies(talker beginner_tutorials_generate_messages_cpp)
- 这样就可以确保自定义消息的头文件在被使用之前已经被生成。因为catkin把所有的package并行的编译，所以如果你要使用其他catkin工作空间中其他package的消息，你同样也需要添加对他们各自生成的消息文件的依赖。
- 当然，如果在Groovy版本下，你可以使用下边的这个变量来添加对所有必须的文件依赖:
add_dependencies(talker ${catkin_EXPORTED_TARGETS})
-
你可以直接调用可执行文件，也可以使用rosrun来调用他们。他们不会被安装到'/bin'路径下，因为那样会改变系统的PATH环境变量。如果你确定要将可执行文件安装到该路径下，你需要设置安装目标，请参考catkin/CMakeLists.txt
-
需要关于CMakeLists.txt更详细的信息，请参考catkin/CMakeLists.txt
-
现在运行 catkin_make:
# In your catkin workspace
$ catkin_make