Know if the other nodes are ready

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Hi, I have a python node (a state machine) launching a launch file (using the python api) containing lots of nodes (sensors, navigation modes, ...) In order for the state machine to be operational, I need all (or at least most) of the nodes to be "ready".

For now, I just add some fixed delays to make sure that all nodes are ready : it works (at least I didn't noticed a case were I know it caused a problem), but it adds some "useless" delay to the startup process of the robot.

Is there a way to find out if a given node (or all nodes) are "ready"? That way, I could gain a significant amount of time in the power-up sequence of the robot.

Note that I can change the way things are launched if needed (for now, a bash script start the state machine (with rosrun), which in turn launches a launch file containing all the other nodes). The nodes are a mix between python2 and C++ nodes, some I wrote, other that are standard nodes

Thanks a lot in advance

Felix