I'm a beginner to ROS and I'm trying to tell the timer callback function to check whether a call_async function has retrieved its result, if it does, change the state to STOPPED; this works when I set the timer frequency to 1 Hz, but it fails to change the state (or at least when I checked the state) when I set the frequency higher at 100 Hz; I'm not sure what is triggering this, because it seems like the timer loops too quickly for the state to be updated, a mechanism of which I know little of.

Please help, many thanks!