How to thread a specific computationally expensive callback
Hello ROS community.
I am building a specific application where I have two subscriber callbacks as written below:
def callback1(msg):
X = msg.X
def callback2(msg):
desX = msg.desX
publish(desX)
while duration < 1 sec:
if equal(X, desX):
return True
return False
The callbacks are oversimplified for easier understanding, and I also assume X is some global variable (or object attribute), so that both callbacks can access it.
callback1 is a sensor measurement coming in at 20 Hz.
callback2 is called on user request, but let's assume it can be called at more than 1 Hz.
In a single threaded mode, this will not work, since callback2 blocks callback1 and the sensor measurement is not read properly. I would like to implement this multi-threaded, but with following requirements:
- callback1 is blocking, i.e. two of them cannot be performed at the same time,
- callback2 is preemptable, i.e. if another call is made the previous one will be canceled,
- It should be done in C++.
My initial idea was to have a separate callback queue for callback2 and have the callback1 in the global queue in the single-threaded callback manner, like mentioned here under point 2.
I would like to know how to implement this given the additional requirement that the callback2 is preemptable. Also, any other design recommendations are more than welcome!
Why not have the two callbacks simply update the global/member variables and then implement the logic in a main loop or a roscpp Timer ? You could also use a ROS Action for callback2 (instead of a plain subscriber)
In case of using a timer, what would be a good design? Have a timer which periodically checks if equal(X, desX), and takes into consideration the timestamp to publish False on exceeding the max duration? Also, regarding placing the logic in main, are there any pr's of doing that?
You can do a
ros::Timerthat triggers every so often and does that check or you can have a while loop running at someros::Rate. They are similar, although the ROS wiki recommends Timers.And here some more info: http://wiki.ros.org/roscpp/Overview/T... and http://wiki.ros.org/roscpp_tutorials/...
Thanks a lot, this solution works fine.