Some explanation of how reactivex Observable works

observable.java

// An observable is an helper class that creates a Subscriber and pass it
// an OnScubscribe method.
public class Observable<T> {
  public Observable(OnSubscribe<T> func) {
    onSubscribe = func;
  }

  public Subscription subscribe(Observer<? super T> observer) {
    if (observer instance Subscriber) {
      return subscribe((Subscriber<? super T>) observer);
    }
    return subscribe(new Subscriber<T>() { 
      @Override public void onNext(T args) {
        observer.onNext(args);
      }
      @Override public void onCompleted() {
        observer.onCompleted();
      }
      @Override public void onError(Throwable e) {
        observer.onError(e);
      }
    });
  }
  // public Subscription subscribe() -> same thing as above
  // public Subscription subscribe(Action1<? super T> onNext) -> same thing as above

  public Subscription subscribe(Subscriber<? super T> subscriber) {
    if (!(subscriber instanceof SafeSubscriber)) {
      subscriber = new SafeSubscriber<T>(subscriber);
    }

    onSubscribe.call(subscriber);
    return subscriber;
  }

  public static <T> Observable<T> just(final T value) {
    return new Observable<T>(new OnSubscribe<T>() {
      @Override public void call(Subscriber<? super T> s) {
        s.onNext(value);
        s.onComplete();
      }
    });
  });
  
  // An operator convert the output of the previous Subscriber
  // to the input of the next Subscriber
  public interface Operator<R, T> extends Func1<Subscriber<? super R>,
                                         Subscriber<? super T>> {}
  
  public <R> Observable<R> lift(Operator<? extends R, ? super T> operator) {
    return new Observable<R>(new OnSubscribe<R>() {
      @Override public void call(Subsciber<? super R> sr) {
        Subscriber<? super T> st = operator.call(sr);
        onSubscribe.call(st);
      })
    });
  }
  
  // Operators
  public <R> Observable<R> map(Func1<? super T, ? extends R> func) {
    return lift(new OperatorMap<T, R>(func));
  }
}