Observer Metodo：onNext, onCompleted, and onError

原文链接：http://reactivex.io/documentation/observable.html
onNext, onCompleted, and onError
The Subscribe method is how you connect an observer to an Observable. Your observer implements some subset of the following methods:

onNext
An Observable calls this method whenever the Observable emits an item. This method takes as a parameter the item emitted by the Observable.
onError
An Observable calls this method to indicate that it has failed to generate the expected data or has encountered some other error. It will not make further calls to onNext or onCompleted. The onError method takes as its parameter an indication of what caused the error.
onCompleted
An Observable calls this method after it has called onNext for the final time, if it has not encountered any errors.

Unsubscribing
In some ReactiveX implementations, there is a specialized observer interface, Subscriber, that implements an unsubscribe method. You can call this method to indicate that the Subscriber is no longer interested in any of the Observables it is currently subscribed to. Those Observables can then (if they have no other interested observers) choose to stop generating new items to emit.

The results of this unsubscription will cascade back through the chain of operators that applies to the Observable that the observer subscribed to, and this will cause each link in the chain to stop emitting items. This is not guaranteed to happen immediately, however, and it is possible for an Observable to generate and attempt to emit items for a while even after no observers remain to observe these emissions.