There is no approach by which hystrix will notify the user about state change. Hystrix provides some variables in event notifier but it doesn't contain events pertaining to state( HALF_OPEN or CLOSE).
What one can do is create a mechanism that calls continuously or with delay to check the state of hystrix command. The hystrix instance can be fetched by:
HystrixCircuitBreaker circuitBreaker = HystrixCircuitBreaker.Factory.getInstance(HystrixCommandKey.Factory.asKey(name));
and post this we do respective functions to check the state of the circuitBreaker like :
circuitBreaker!=null, circuitBreaker.allowRequest(), circuitBreaker.isOpen();
So your service can use the state returned by these.