Realm: Creating Observable from asynchronous Transaction

Question

In my Android app, I am using Realm and RxJava, and I want to update an Object by passing a new value. I use realm.executeTransactionAsync() for that. But I would like to subscribe to this event to do error handling in the class where I use this method. What I have done so far is this:

public Observable updateDogAge(final String dogId, final int age) {
    return Observable.create(new Observable.OnSubscribe<Dog>() {
        @Override
        public void call(final Subscriber<? super Dog> subscriber) {
            realm.executeTransactionAsync(realm1 -> realm1.where(Dog.class)
                            .equalTo(DOG_ID_FIELD, dogId)
                            .findFirst()
                            .setAge(age),
                    subscriber::onCompleted,
                    subscriber::onError);
        }
    });
}

Now I would like to get a boolean flag of whether or not updating this object was successful. Right now I feel like just calling subscriber::onCompleted is not the right solution. I am also not sure about the use of Observable.onCreate().

Can you tell me what is the right way to get an Observable (or Single) from an asynchronous Realm transaction?

Answer 1

I'd recommend creating your own "asynchronous" transaction for RxJava support.

public Observable updateDogAge(final String dogId, final int age) {
    return Observable.defer(() -> {
        try(Realm r = Realm.getDefaultInstance()) {
            r.executeTransaction((realm) -> realm.where(Dog.class)
                        .equalTo(DOG_ID_FIELD, dogId)
                        .findFirst()
                        .setAge(age));
            return Observable.just(true);
        } catch(Throwable e) {
            return Observable.error(e);
        }
    });
}

Note: my RxJava is a bit hazy, I don't use it regularly.

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And of course, you need to subscribe this on a scheduler that runs on background threads.

updateDogAge(id, age).subscribeOn(Schedulers.io())./*...*/

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In fact, I think you can use Observable.fromCallable() instead:

public Observable updateDogAge(final String dogId, final int age) {
    return Observable.fromCallable(() -> {
        try(Realm r = Realm.getDefaultInstance()) {
            r.executeTransaction((realm) -> realm.where(Dog.class)
                        .equalTo(DOG_ID_FIELD, dogId)
                        .findFirst()
                        .setAge(age));
            return true;
        }
    });
}