# 11.2 编写Subscriber Node

## 11.2.1 **源代码**

在 `beginner_tutorials package` 目录下创建 `src/listener.cpp` 文件，并粘贴如下代码：

```cpp
#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;
}
```

## 11.2.2 代码说明

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。

```cpp
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，当接收到 `chatter`话题的时候就会被调用。消息是以 [boost shared\_ptr](http://www.boost.org/doc/libs/1_37_0/libs/smart_ptr/shared_ptr.htm) 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。

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

告诉 `master` 我们要订阅 `chatter` 话题上的消息。**当有消息发布到这个话题时，**`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，当缓存达到 1000 条消息后，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 chatter 话题的消息。**

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](http://wiki.ros.org/roscpp/Overview) 提供了更为详尽的信息。

```cpp
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` 告诉节点要终止运行。也有可能是节点被人为关闭的。

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](http://wiki.ros.org/roscpp_tutorials) package 里的一些 demo 应用。需要更为详尽的信息，可以参考 [roscpp overview](http://wiki.ros.org/roscpp/Overview)。

下边，我们来总结一下:

* 初始化ROS系统
* 订阅 `chatter` 话题
* 进入自循环，等待消息的到达
* 当消息到达，调用 `chatterCallback()` 函数