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## descriptive title for the tutorial
## title = Understanding Timers
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## description = This tutorial explains roscpp Timers, which allow you to schedule a callback to happen periodically.
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== What are Timers? ==
Timers let you schedule a callback to happen at a specified rate.  They are a more flexible and useful form of the `ros::Rate` used in the [[ROS/Tutorials/WritingPublisherSubscriber(c++)|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.

See also: [[roscpp/Overview/Timers|roscpp Timers overview]]

== Using Timers ==
Creating a Timer works very similar to creating a Subscriber.

{{{#!cplusplus
ros::Timer timer = n.createTimer(ros::Duration(0.1), timerCallback);
}}}

Timer callbacks take the form:
{{{#!cplusplus
void timerCallback(const ros::TimerEvent& e);
}}}

== A full example ==
Now that you've seen the basics, let's go through a larger example, with multiple Timers.

=== The code ===
<<GetTaggedCode(https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/timers/timers.cpp,cplusplus,FULLTEXT,unindent,global_lines,no_tag_newlines)>>

=== The code explained ===
I'll ignore parts that have been explained in previous tutorials, which really just leaves two lines:
<<GetTaggedCode(https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/timers/timers.cpp,cplusplus,CREATE_TIMERS,unindent,global_lines,no_tag_newlines)>>
Here we create two timers, one which fires every 100 milliseconds, and one which fires every second.  If you run this program, it should output something like the following:
{{{
[ 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
...
}}}

== The TimerEvent Structure ==
The `ros::TimerEvent` structure provides you information about the timing of the current timer.  Here is its definition:
{{{#!cplusplus
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;
};
}}}

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