參考連結：ROS與C++入門教程-使用Timers類

ROS與C++入門教程-使用Timers類

說明：

• 本教程介紹了roscpp定時器，它允許您安排一個回調發生週期性。

什麼是Timers(定時器)?

• Timers能讓你以一定的頻率來執行
• 他們是比ros::Rate更加靈活和有用的形式，ros::Rate在編寫簡單釋出節點和訂閱節點用到。
• 注意：定時器不是實時的執行緒/核心替換，也不能保證它們的準確度，因為系統負載/功能會有很大的變化。
• 更多可以參考: roscpp Timers overview

使用定時器

• 建立定時器象建立訂閱一樣：

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

• 回撥函式：

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

完整例子

• 上面為簡單的例子，這裡做一個帶多個定時器的例子。
• 程式碼如下：

#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;
}
 

程式碼分析：

• 程式碼：

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

• 增加兩個定時器，一個100毫秒執行一次，一個1秒執行一次。
• 執行效果：

[ 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
 

TimerEvent結構體

• ros::TimerEvent結構體提供當前定時器的時間資訊
• 定義如下：

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;
};