Hi everyone

I am quite new to ROS-Gazebo and am still learning how to use this platform. Nowadays, I meet with a puzzle and have no idea to solve it. Hope anyone here can offer me some suggestions or inspirations for addressing the issue.

Previously, I use another simulator called ARGoS, which use range and bearing system for multi-robot communication. After transfering my code to ros, I use publish or subscribe topic for inter-robot communication. However, in my project I need to remove some robots from the environment to simulate partial failure of the group. The issue arises from here. The callback function is called automatically through spinonce() function. But how can I know whether the topic I subscribe exist or not? Or how can I know whether the callback function is called or not?

I appreciate any message from anyone.