Difference between Observable.defer and Observable.create in java rx

java reactivex
Xitrum picture Xitrum · Mar 30, 2016 · Viewed 16.7k times · Source

Can someone explain me the difference between defer and create methods in Observable? I failed to understand when I should use defer and when should I use create ..

REFERENCES:

Defer: http://reactivex.io/documentation/operators/defer.html

Create: http://reactivex.io/documentation/operators/create.html

Thank you

Answer

Bob Spryn picture Bob Spryn · Jan 24, 2017

So the distinction seems to be: defer is good when you have something that creates/returns an observable already, but you don’t want it to that process to happen until subscription.

create is good when you need to manually wrap an async process and create an observable. That creation is also deferred until subscription.

To put it another way:

defer is an operator that enables deferred composition of observable sequences.

create is a custom implementation of observable sequence (where creation is deferred until subscription).

So if you have a situation where you might use just to create an Observable from some results/value or you have a network API layer that returns an Observable of the request, but you don't want that request to kick off until subscription. defer would be good for those scenarios.

If you have a network API layer that doesn't return an Observable for a request, but which you need an Observable interface to, you might use create. That Observable sequence still wouldn't be created until subscription though. If you wanted that network call to kick off regardless of subscription, then you would use a different mechanism, like a Subject, potentially that replays.

Related questions

RxJava 2.0 - How to convert Observable to Publisher

How to convert Observable to Publisher in RxJava version 2? In the first version we have the https://github.com/ReactiveX/RxJavaReactiveStreams project that do exactly what I need. But How can I do it in RxJava 2?

Read More
Single vs Observable in Java RX

In the new release, reactiveX introduced Single, as a variant of Observable http://reactivex.io/documentation/single.html Which is nice to have since in my current use-case, I have multiple executions, and each of them only returns a single …

Read More
io.reactivex.exceptions.UndeliverableException The exception could not be delivered to the consumer because it has already canceled/disposed

Getting an UndeliverableException while using completable public Completable createBucketWithStorageClassAndLocation() { return Completable.complete() .doFinally(() -> { Bucket bucket = storage.create( BucketInfo.newBuilder(googleUploadObjectConfiguration.bucketName()) .setStorageClass(storageClass) .setLocation(googleUploadObjectConfiguration.locationName()) .build()); }).doOnError(error -> LOG.error(error.getMessage())); } The exception is …

Read More