function observeState() {
          if (observer.next) {
            observer.next(getState());
          }
        }
The minimal observable subscription method.
@param {Object} observer Any object that can be used as an observer.
The observer object should have a `next` method.
@returns {subscription} An object with an `unsubscribe` method that can
be used to unsubscribe the observable from the store, and prevent further
emission of values from the observable.