Creates a new Observer that will notify observers exactly once when the promise is resolved.
In the event of an error, the observer will be notified with the error.
Note that the promise is only created when the first observer is added so it is lazy.
Creates a new Observer that will notify observers exactly once when the promise is resolved. In the event of an error, the observer will be notified with the error. Note that the promise is only created when the first observer is added so it is lazy.