本帖最后由 对折之内 于 2015-3-10 20:55 编辑
打开一个终端，进入到之前建立的beginner_tutorials包下面：cd ~/catkin_ws/src/beginner_tutorials
找不到路径的，记得用之前的方法配置环境变量。
编辑文件src/listener.cpp：
将下面的代码复制到文件中：
[mw_shl_code=c,true]#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;
}[/mw_shl_code]
保存退出。下面看一下代码的解释：
- void chatterCallback(const std_msgs::String::ConstPtr& msg)
- {
- ROS_INFO("I heard: [%s]", msg->data.c_str());
- }
从名字就可以看出，这是一个回调函数。当一个消息到达chatter话题时，这个回调函数将会被调用。
ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);
订阅chatter话题，当一个新的消息到达时，ROS将会调用chatterCallback()函数。第二个参数是对列的长度，如果我们处理消息的速度不够快，会将收到的消息缓冲下来，一共可以缓冲1000条消息，满1000之后，后面的到达的消息将会覆盖前面的消息。 subscribe()函数将会返回一个ros::Subscriber类型的对象，当订阅对象被销毁以后，它将会自动从chatter话题上撤销。
ros::spin()进入了一个循环，可以尽快的调用消息的回调函数。不要担心，如果它没有什么事情可做时，它也不会浪费太多的CPU。当ros: k()返回false时，ros::spin()将会退出。这就意味着，当ros::shutdown()被调用，或按下CTRL+C等情况，都可以退出。 下面总结一下写一个订阅者的步骤：（1）初始化ROS系统（2）订阅chatter话题（3）Spin，等待消息的到来（4）当一个消息到达时，chatterCallback()函数被调用。 下面看一下如何构建节点。这时候你的CMakeLists.txt看起来应该是下面这个样子，包括前面所做的修改，注释部分可以除去： [mw_shl_code=html,true]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()
[/mw_shl_code]
将下面几行代码添加到CMakeLists.txt的最后。最终你的CMakeLists.txt文件看起来样该是下面这个样子： [mw_shl_code=html,true]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)
[/mw_shl_code]
这将会创建两个可执行文件，talker和listener。它们将会产生在~/catkin_ws/devel/lib/share/<package name>目录下，下面开始构建，在你的工作空间根目录下输入：
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发表于 2015-3-10 20:51