SubscribableState
A SubscribableState object allows you to subscribe an Observer to receive updates from a specific source - as with Subscribable. Each updated value will be provided through the given Observer.onNext method.
The SubscribableState also maintains the current value of the property for synchronous access
Updates can be immediate, or asynchronous depending on the underlying source.
To stop observing updates, use the Disposable.dispose method of the returned Disposable.