Topic reading inside a callback

callback

asked 2014-12-10 10:46:23 -0500

arenillas

223 ●31 ●39 ●46 https://github.com/are...

updated 2014-12-10 11:22:07 -0500

[EDIT]

I have changed my question because I think what I am trying to do is bit different from what was explained here before.

I have a Callback and I need to read a different topic inside this callback. I don't know if it is possible, but I have to ensure that some part of the code is only executed if the other topic has a particular value.

Any case, I don't know if I can read a topic or execute a particular Callback while I am just executing one callback

Thank you.

edit retag flag offensive close merge delete

Comments

What is addCallback? Do you want to make another subscription or just "do the same thing as a callback would have" - in the latter case: It's just a function that you can call.

dornhege ( 2014-12-10 11:04:56 -0500 )edit

I have changed the question because It was totally confusing.

arenillas ( 2014-12-10 11:22:32 -0500 )edit

add a comment

4

answered 2014-12-10 13:08:50 -0500

ajain

281 ●13 ●19 ●25

updated 2014-12-11 11:47:36 -0500

I would suggest having 2 separate callbacks. And the code that requires certain variable to be set is executed only if respective flags are true. Also, I'd suggest using mutex on flags to avoid any race conditions. For eg.

class A 
{
    bool flag;
    std::mutex m;
    void callback1(...)
    {
         //Set the flag
         m.lock();
         flag = true;
         m.unlock();
         .
         .
         .
    }
    void callback2(...)
    {
        //Use flag to check if you wanna execute the code
       m.lock();
       bool isFlagTrue = flag;
       m.unlock();
       if (isFlagTrue)
       {
             //Put your code here ...
       }
};

I hope that's what you're looking for.

EDIT:

As Kramer suggested in his comment below, only one callback happens at a time. So you don't have to use mutex unless you have multi-threaded node process.

edit flag offensive delete link

Comments

2

FYI: unless the node has multi-threaded spinning, there isn't a need for the mutex. When a node uses single-threaded spinning, only one callback executes at any time (see: Callbacks Overview).

kramer ( 2014-12-10 18:28:10 -0500 )edit

You are correct. I'll add that as an edit to the answer. Thanks for correcting me.

ajain ( 2014-12-11 11:46:10 -0500 )edit

add a comment

0

answered 2014-12-10 12:06:44 -0500

dornhege

31395 ●134 ●284 ●498

I won't work like that.

A callback is called back, i.e. it will be triggered iff there is data available from a publisher. You cannot request data from a callback.

The correct way to solve the problem is to synchronize the callbacks, i.e. collect data from both and only trigger your specific code if data from both is OK. For some common use cases, there already exist ROS methods, e.g. Stamped data from one callback and a transformation from TF.

edit flag offensive delete link

add a comment

3

answered 2014-12-10 12:06:17 -0500

Erwan R.

697 ●24 ●37 ●46

Hello

You can't know the order in which messages will be received, so callbacks should be robust to information from other topics. From my experience of ROS, the best way of doing that consist in having callbacks that store data in node's class attributes, so they are accessible from other callbacks. You should also check if the data you need is available (the attribute has a consistent value) or skip processing if not. At most, callbacks should be independent from each other.

edit flag offensive delete link

add a comment

Topic reading inside a callback

Comments

3 Answers

Comments

Question Tools

Stats

Related questions

Topic reading inside a callback edit

Comments

3 Answers

Comments

Question Tools

Stats

Related questions

Topic reading inside a callback