Event when BLE notification has been enabled/disabled

Hi :)

How do I get an event when a BLE notification has been enabled or disabled by the central? I guess the configuration and I will be able to read that. But an event on configuration changes would be neat to power hardware on/off correspondingly.

I am using SimpleBLEPeripeiral as my baseline and would like to know when CHAR4 is enabled or disabled.

Thanks :)