使用Retrofit和Realm在RxJava中纠正流程

Question

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

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 eg 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
            });
}

Answer 2

What Christian Melchior mentioned in his answer, makes perfect sense, and should solve the problem you are having at your hand, but down the line this approach may introduce other issue(s).

In a good architecture, all the major modules(or libraries) should be isolated from rest of the code. Since Realm, RealmObject or RealmResult can not be passed across threads it is even more important to make Realm & Realm related operations isolated from rest of the code.

For each of your jsonModel class, you should have a realmModel class and a DAO (Data Access Object). Idea here is that other than DAO class none of the class must know or access realmModel or Realm. DAO class takes jsonModel, converts to realmModel, performs read/write/edit/remove operations, for read operations DAO converts realmModel to jsonModel and returns with it.

This way it is easy to maintain Realm, avoid all Thread related issues, easy to test and debug.

Here is an article about Realm best practices with a good architechture https://medium.com/@Viraj.Tank/realm-integration-in-android-best-practices-449919d25f2f

Also a sample project demonstrating Integration of Realm on Android with MVP(Model View Presenter), RxJava, Retrofit, Dagger, Annotations & Testing. https://github.com/viraj49/Realm_android-injection-rx-test

Answer 3

In my case, I seem to have defined a query for the RealmRecyclerViewAdapter like this:

    recyclerView.setAdapter(new CatAdapter(getContext(),
            realm.where(Cat.class).findAllSortedAsync(CatFields.RANK, Sort.ASCENDING)));

And otherwise defined a condition for Retrofit with RxJava to download more stuff when the condition is met:

    Subscription downloadCats = Observable.create(new RecyclerViewScrollBottomOnSubscribe(recyclerView))
            .filter(isScrollEvent -> isScrollEvent || realm.where(Cat.class).count() <= 0)
            .switchMap(isScrollEvent -> catService.getCats().subscribeOn(Schedulers.io()))  // RETROFIT
            .retry()
            .subscribe(catsBO -> {
                try(Realm outRealm = Realm.getDefaultInstance()) {
                    outRealm.executeTransaction((realm) -> {
                        Cat defaultCat = new Cat();
                        long rank;
                        if(realm.where(Cat.class).count() > 0) {
                            rank = realm.where(Cat.class).max(Cat.Fields.RANK.getField()).longValue();
                        } else {
                            rank = 0;
                        }
                        for(CatBO catBO : catsBO.getCats()) {
                            defaultCat.setId(catBO.getId());
                            defaultCat.setRank(++rank);
                            defaultCat.setSourceUrl(catBO.getSourceUrl());
                            defaultCat.setUrl(catBO.getUrl());
                            realm.insertOrUpdate(defaultCat);
                        }
                    });
                }
            }, throwable -> {
                Log.e(TAG, "An error occurred", throwable);
            });

And this is for example a search based on an edit text's input:

    Subscription filterDogs = RxTextView.textChanges(editText)
                     .switchMap((charSequence) -> 
                           realm.where(Dog.class)
                                .contains(DogFields.NAME, charSequence.toString())
                                .findAllAsyncSorted(DogFields.NAME, Sort.ASCENDING)
                                .asObservable())
                     .filter(RealmResults::isLoaded) 
                     .subscribe(dogs -> realmRecyclerAdapter.updateData(dogs));