Michał Michał - 2 months ago 79
Android Question

Correct flow in RxJava with Retrofit and Realm

I'm implementing network API with the combination of RxJava and Retrofit, and I use Realm as my database. I got it pretty much working but I'm wondering if it is the correct approach and flow of events. So, here is the

RetrofitApiManager
.

public class RetrofitApiManager {

private static final String BASE_URL = "***";

private final ShopApi shopApi;

public RetrofitApiManager(OkHttpClient okHttpClient) {

// GSON INITIALIZATION

Retrofit retrofit = new Retrofit.Builder()
.client(okHttpClient)
.addCallAdapterFactory(RxJavaCallAdapterFactory.create())
.addConverterFactory(GsonConverterFactory.create(gson))
.baseUrl(BASE_URL)
.build();

shopApi = retrofit.create(ShopApi.class);
}

public Observable<RealmResults<Shop>> getShops() {
return shopApi.getShops()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.doOnNext(response -> {
Realm realm = Realm.getDefaultInstance();
realm.executeTransaction(realm1 ->
realm1.copyToRealmOrUpdate(response.shops));
realm.close();
})
.flatMap(response -> {
Realm realm = Realm.getDefaultInstance();
Observable<RealmResults<Shop>> results = realm.where(Shop.class)
.findAllAsync()
.asObservable()
.filter(RealmResults::isLoaded);
realm.close();
return results;
});
}
}


And here is the call to get
RealmResults<Shop>
inside a
Fragment
.

realm.where(Shop.class)
.findAllAsync()
.asObservable()
.filter(RealmResults::isLoaded)
.first()
.flatMap(shops ->
shops.isEmpty() ? retrofitApiManager.getShops() : Observable.just(shops))
.subscribe(
shops -> initRecyclerView(),
throwable -> processError(throwable));


Here are my questions:


  1. Is it a correct approach to chain events like in the example above or should I manage them in a different way?

  2. Is it OK to use
    Realm
    instance in
    getShops()
    method and close i there or would it be better to pass it as an argument and then manage it somehow? Although, this idea seems to be a bit problematic with threads and calling
    Realm.close()
    always at the right time.


Answer

1) I would try to do as much as possible on the background thread, right now you are doing a lot of the work on the UI thread.

2)

  public Observable<RealmResults<Shop>> getShops() {
        return shopApi.getShops()
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .doOnNext(response -> {
                    try(Realm realm = Realm.getDefaultInstance()) {
                        realm.executeTransaction(realm1 -> 
                            realm1.insertOrUpdate(response.shops));
                    } // auto-close
                })
                .flatMap(response -> {
                    try(Realm realm = Realm.getDefaultInstance()) {
                        Observable<RealmResults<Shop>> results = realm.where(Shop.class)
                            .findAllAsync()
                            .asObservable()
                            .filter(RealmResults::isLoaded);
                    } // auto-close
                    return results;
                });
    }

All Realm data is lazy-loaded, so it is only available while the Realm instance is open, so closing it after retrieving it has a high chance of not working. In your case though you are flat-mapping on the main thread, so most likely there is already an open instance there.

If you want you can use copyFromRealm() to get unmanaged data out that can be moved across threads and are not connected to Realm anymore, but they will also loose their live update features and take up more memory.

It would probably do this instead:

  public Observable<RealmResults<Shop>> getShops() {
        return shopApi.getShops()
                .subscribeOn(Schedulers.io())
                .doOnNext(response -> {
                    try(Realm realm = Realm.getDefaultInstance()) {
                        realm.executeTransaction(realm1 -> 
                            realm1.copyToRealmOrUpdate(response.shops));
                    } // auto-close
                })
                .observeOn(AndroidSchedulers.mainThread())
                .flatMap(response -> {
                    Observable<RealmResults<Shop>> results = realm.where(Shop.class)
                            .findAllAsync()
                            .asObservable()
                            .filter(RealmResults::isLoaded);
                    return results;
                });

Alternatively you can treat the network request as a side-effect and just depend on Realm notifying you when there is changes (better approach IMO as you separate network from DB access which is e.g. what the Repository pattern is about)

public Observable<RealmResults<Shop>> getShops() {
    // Realm will automatically notify this observable whenever data is saved from the network
    return realm.where(Shop.class).findAllAsync().asObservable()
            .filter(RealmResults::isLoaded)
            .doOnNext(results -> {
                if (results.size() == 0) {
                    loadShopsFromNetwork();
                }
            }); 
}

private void loadShopsFromNetwork() {
    shopApi.getShops()
            .subscribeOn(Schedulers.io())
            .subscribe(response -> {
                try(Realm realm = Realm.getDefaultInstance()) {
                    realm.executeTransaction(r -> r.insertOrUpdate(response.shops));
                } // auto-close
            });
}
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