Understanding RxJava basics

Question

I am just doing my first steps with RxJava, making use of this tutorial. I understand what an Observable and what a Subscriber is. But I have problems with "connecting" them. I just thought about such a task: An Activity A starts an Activity B. Activity B has a button with which you can create a Date containing the current date. Another button emits this Date and finishes Activity B. Activity A subscribes to the emitted Date-Observable and displays the current date. (I know this is normally done by using Activity results). I have the following problem:

I can create a Date-Observable within Activity B by this code(The instance mDate is of type Date and is created somewhere else in Activity B):

Observable<Date> dateObservable =
            Observable.create(sub -> {
        sub.onNext(mDate);
        sub.onCompleted();
    });

But in Activity A I have to subscribe to it. But I have no reference to the Observable in Activity B. I thought about creating the Observable statically in my Application class but there I don't know the value of mDate yet. Can I somehow create an Observable of a certain type without implementing the "call()"-Method (where onNext() and onCompleted() are called)? So then I would already have an Observable instance I could subscribe to in my Aativity.And then later I could implement the "call()"-Method in Activity B? Ore are there other RxJava features I can use to achieve my described goals?

This thing might help: youtube.com/watch?v=QdmkXL7XikQ

EpicPandaForce
– EpicPandaForce

2016-04-12 12:36:18 +00:00
Commented Apr 12, 2016 at 12:36 — EpicPandaForce
– EpicPandaForce, Commented Apr 12, 2016 at 12:36

satorikomeiji · Accepted Answer · 2016-04-12 12:15:10Z

2

You need a thing called ReplaySubject.

In your custom MyApplication class:

ReplaySubject<Date> dateSubject = ReplaySubject.create();

In activity B:

ReplaySubject<Date> subject = ((MyApplication)getApplication()).getSubject();
subject.onNext(mDate);
subject.onCompleted();

In activity A:

ReplaySubject<Date> subject = ((MyApplication)getApplication()).getSubject();
subject.subscribe(...);

answered Apr 12, 2016 at 12:15

satorikomeiji

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5 Comments

unlimited101 Over a year ago

Thanks, that helped. But now I am still a bit unsatisfied. Because I have to use the Application class as a "passing interface" and I would like to avoid that. As I heard RxJava can be used to send events on an EventBus. How to achieve this?

satorikomeiji Over a year ago

@unlimited101, in most implementations event bus is basically a simple observable. You could put it in a static variable instead of an Application and make a wrapper around it. There's no other way to share an object across activities as far as I'm aware.

unlimited101 Over a year ago

By eventbus (github.com/greenrobot/EventBus) you can send data between Android components like Services, Activities, etc. But I would like to realize this in RxJava.

satorikomeiji Over a year ago

@unlimited101 if you look at the code, for example EventBus.java, you'll notice that it's implemented as a singleton and its instance is stored in a static variable. You could make something similar by making a wrapper around your Observable which would be instantiated as a static variable.

njzk2 Over a year ago

in most case your event bus will be either a field of the application, a static variable somewhere, or an injected singleton (I'd go with the latter, obv., but others are fine if you don't use dependency injection in your project)

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