ROS学习笔记83（roscpp 理解定时器）

原文链接： http://wiki.ros.org/roscpp_tutorials/Tutorials/Timers

1 什么是定时器

定时器定以一定的速率定时的调用回调函数。They are a more flexible and useful form of the ros::Rate used in the Writing a Simple Publisher and Subscriber tutorial.

Note that Timers are not a realtime thread/kernel replacement, and make no guarantees about how accurate they are, which can vary wildly because of system load/capabilities.

2 使用定时器

创建一个定时器的工作原理非常类似于创建一个订阅者:

ros::Timer timer = n.createTimer(ros::Duration(0.1), timerCallback);

回调函数的格式如下:

void timerCallback(const ros::TimerEvent& e);

3 一个完整的例子

ow that you've seen the basics, let's go through a larger example, with multiple Timers.

#include "ros/ros.h"

/**
 * This tutorial demonstrates the use of timer callbacks.
 */

void callback1(const ros::TimerEvent&)
{
  ROS_INFO("Callback 1 triggered");
}

void callback2(const ros::TimerEvent&)
{
  ROS_INFO("Callback 2 triggered");
}

int main(int argc, char **argv)
{
  ros::init(argc, argv, "talker");
  ros::NodeHandle n;

  /**
   * Timers allow you to get a callback at a specified rate.  Here we create
   * two timers at different rates as a demonstration.
   */
  ros::Timer timer1 = n.createTimer(ros::Duration(0.1), callback1);
  ros::Timer timer2 = n.createTimer(ros::Duration(1.0), callback2);

  ros::spin();

  return 0;
}

3.2 程序解释

忽略那些在前面的教程，只是留下两行被解释部分：

ros::Timer timer1 = n.createTimer(ros::Duration(0.1), callback1);
ros::Timer timer2 = n.createTimer(ros::Duration(1.0), callback2);

 在这里，我们创建两个定时器，其中一个触发间隔为100毫秒，并且其中一个触发每一秒。如果你运行这个程序，它应该输出类似如下：

[ INFO] 1251854032.362376000: Callback 1 triggered
[ INFO] 1251854032.462840000: Callback 1 triggered
[ INFO] 1251854032.562464000: Callback 1 triggered
[ INFO] 1251854032.662169000: Callback 1 triggered
[ INFO] 1251854032.762649000: Callback 1 triggered
[ INFO] 1251854032.862853000: Callback 1 triggered
[ INFO] 1251854032.962642000: Callback 1 triggered
[ INFO] 1251854033.063118000: Callback 1 triggered
[ INFO] 1251854033.162221000: Callback 1 triggered
[ INFO] 1251854033.262749000: Callback 1 triggered
[ INFO] 1251854033.262864000: Callback 2 triggered
[ INFO] 1251854033.362643000: Callback 1 triggered
[ INFO] 1251854033.463158000: Callback 1 triggered
...

4 The TimerEvent Structure

The ros::TimerEvent structure provides you information about the timing of the current timer. Here is its definition:

struct TimerEvent
{
  Time last_expected;                     ///< In a perfect world, this is when the last callback should have happened
  Time last_real;                         ///< When the last callback actually happened

  Time current_expected;                  ///< In a perfect world, this is when the current callback should be happening
  Time current_real;                      ///< This is when the current callback was actually called (Time::now() as of the beginning of the callback)

  struct
  {
    WallDuration last_duration;           ///< How long the last callback ran for, always in wall-clock time
  } profile;
};