src/main/java/io/reactivex/internal/operators/single/SingleToObservable.java
public static <T> SingleObserver<T> create(Observer<? super T> downstream) {
        return new SingleToObservableObserver<T>(downstream);
    }
public static <T> SingleObserver<T> create(Observer<? super T> downstream) {
        return new SingleToObservableObserver<T>(downstream);
    }
Creates a {@link SingleObserver} wrapper around a {@link Observer}.
<p>History: 2.0.1 - experimental
@param <T> the value type
@param downstream the downstream {@code Observer} to talk to
@return the new SingleObserver instance
@since 2.2