I have a list of objects that I want retrieve from a local database (if available), or from a remote server otherwise. I'm using RxJava Observables (SqlBrite for the database and Retrofit for the remote server).
My query code is as follows:
Observable<List<MyObject>> dbObservable = mDatabase
.createQuery(MyObject.TABLE_NAME,MyObject.SELECT_TYPE_A)
.mapToList(MyObject.LOCAL_MAPPER);
Observable<List<MyObject>> remoteObservable = mRetrofitService.getMyObjectApiService().getMyObjects();
return Observable.concat(dbObservable, remoteObservable)
.first(new Func1<List<MyObject>, Boolean>() {
#Override
public Boolean call(List<MyObject> myObjects) {
return !myObjects.isEmpty();
}
});
I see the first observable running and hitting the first method with an empty list, but then the retrofit observable does not run, there is no network request. If I switch the order of the observables, or just return the remote observable, it works as expected, it hits the remote server and returns the list of objects.
Why would the remote observable fail to run in this scenario? The subscriber's onNext, orError and onComplete methods are not called when I concatenate the observables with the db first and retrofit second.
Thanks!
Kaushik Gopal has addressed this in his RxJava-Android-Samples github project.
He recommends using this technique:
getFreshNetworkData()
.publish(network ->
Observable.merge(network,
getCachedDiskData().takeUntil(network)))
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Subscriber<List<MyObject>() {
...
});
In your case, it might look like this:
remoteObservable
.publish(network ->
Observable.merge(network,
dbObservable.takeUntil(network)))
.first(myObjects -> !myObjects.isEmpty());
Edit: It sounds like you just might need this:
dbObservable
.flatMap(localResult -> {
if (localResult.isEmpty()) {
return remoteObservable;
} else {
return Observable.just(localResult);
}
});
I assume you have your observables which can get data from your local and remote like below:
final Observable<Page> localResult = mSearchLocalDataSource.search(query);
final Observable<Page> remoteResult = mSearchRemoteDataSource.search(query)
.doOnNext(new Action1<Page>() {
#Override
public void call(Page page) {
if (page != null) {
mSearchLocalDataSource.save(query, page);
mResultCache.put(query, page);
}
}
});
Then you can map them and get first which means if local available use local if not use remote:
return Observable.concat(localResult, remoteResult)
.first()
.map(new Func1<Page, Page>() {
#Override
public Page call(Page page) {
if (page == null) {
throw new NoSuchElementException("No result found!");
}
return page;
}
});
And subscribe it like below:
mCompositeSubscription.clear();
final Subscription subscription = mSearchRepository.search(this.mQuery)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Observer<Page>() {
#Override
public void onCompleted() {
// Completed
}
#Override
public void onError(Throwable e) {
mView.onDefaultMessage(e.getMessage());
}
#Override
public void onNext(Page page) {
mView.onDefaultMessage(page.getContent());
}
});
mCompositeSubscription.add(subscription);
For more detail or example you can check my github repo:
https://github.com/savepopulation/wikilight
Good luck!
Edit:
You can try a local observable like below. Simply it checks if there's a record and returns an empty observable.
#Override
public Observable<Page> search(#NonNull final String query) {
return Observable.create(new Observable.OnSubscribe<Page>() {
#Override
public void call(Subscriber<? super Page> subscriber) {
final Realm realm = Realm.getInstance(mRealmConfiguration);
final Page page = realm.where(Page.class)
.equalTo("query", query)
.findFirst();
if (page != null && page.isLoaded() && page.isValid()) {
Log.i("data from", "realm");
subscriber.onNext(realm.copyFromRealm(page));
} else {
Observable.empty();
}
subscriber.onCompleted();
realm.close();
}
});
}
Edit 2:
When you return null from local concat and first will not work and your remote will not be called because null means observable returns null but still can observe. When you return observable.empty with concat and first this means observable cannot emit anything from local more and so it can emit from remote.
Related
I'm developing an Android App using Fernando Ceja's clean architecture. One of my Interactors or Use Cases is in charge of getting the User's feed data. In order to get the data, first I have to retrieve the User's Teams from a database table and then I have to get the Feed list from the server-side.
This is how I get the Teams from the database layer:
mTeamCache.getAllTeams().subscribe(new DefaultSubscriber<List<SimpleTeam>>() {
#Override
public void onNext(List<SimpleTeam> simpleTeams) {
super.onNext(simpleTeams);
mTeams = simpleTeams;
}
});
TeamCache is basically just another Interactor that takes care of getting all the teams that I have in the database.
Here's how I get the Feed data from the server-side:
mFeedRepository.getFeed(0, 50).subscribe(new ServerSubscriber<List<ApiFeedResponse>>() {
#Override
protected void onServerSideError(Throwable errorResponse) {
callback.onFeedFetchFailed(...);
}
#Override
protected void onSuccess(List<ApiFeedResponse> responseBody) {
//Do stuff with mTeams
callback.onFeedFetched(...);
}
});
My GetFeedInteractor class has a method called execute, where I pass through the Callback that I'm later using in the UI to handle the response. The issue with all this is that currently I'm chaining the responses like this:
#Override
public void execute(final Callback callback, String userSipId) {
mTeamCache.getAllTeams().subscribe(new DefaultSubscriber<List<SimpleTeam>>() {
#Override
public void onNext(List<SimpleTeam> simpleTeams) {
super.onNext(simpleTeams);
mTeams = simpleTeams;
getFeedFromRepository(callback);
}
});
}
public void getFeedFromRepository(final Callback callback) {
mFeedRepository.getFeedRx(0, 50).subscribe(new ServerSubscriber<List<ApiFeedResponse>>() {
#Override
protected void onServerSideError(Throwable errorResponse) {
callback.onFeedFetchFailed("failed");
}
#Override
protected void onSuccess(List<ApiFeedResponse> responseBody) {
//Do stuff with mTeams
List<BaseFeedItem> responseList = new ArrayList();
for (ApiFeedResponse apiFeedResponse : responseBody) {
responseList.add(FeedDataMapper.transform(apiFeedResponse));
}
callback.onFeedFetched(responseList);
}
});
}
As you can see, once that I get the Team collection from the Cache Interactor I call the method that gets the feed from the very same Subscriber. I don't like this. I want to be able to do something nicer, like using Observable.concat(getTeamsFromCache(), getFeedFromRepository()); chain a call to another rx.Observable inside a Subscriber is not something nice to do. I guess that my question is, how can I chain two rx.Observables that are using different Subscribers?
Update:
ServerSubscriber is a subscriber that I implemted to subscribe to Retrofit services. It simply checks the error codes and some stuff. Here is:
https://gist.github.com/4gus71n/65dc94de4ca01fb221a079b68c0570b5
Default subscriber is an empty default subscriber. Here is:
https://gist.github.com/4gus71n/df501928fc5d24c2c6ed7740a6520330
TeamCache#getAllTeams() returns rx.Observable>
FeedRepository#getFeed(int page, int offset) returns rx.Observable>
Update 2:
This is how the Interactor to get the User's feed looks like now:
#Override
public void execute(final Callback callback, int offset, int pageSize) {
User user = mGetLoggedUser.get();
String userSipid = mUserSipid.get();
mFeedRepository.getFeed(offset, pageSize) //Get items from the server-side
.onErrorResumeNext(mFeedCache.getFeed(userSipid)) //If something goes wrong take it from cache
.mergeWith(mPendingPostCache.getAllPendingPostsAsFeedItems(user)) //Merge the response with the pending posts
.subscribe(new DefaultSubscriber<List<BaseFeedItem>>() {
#Override
public void onNext(List<BaseFeedItem> baseFeedItems) {
callback.onFeedFetched(baseFeedItems);
}
#Override
public void onError(Throwable e) {
if (e instanceof ServerSideException) {
//Handle the http error
} else if (e instanceof DBException) {
//Handle the database cache error
} else {
//Handle generic error
}
}
});
}
I think you're missing the point of RxJava and reactive approach, you should not have different subscribers with OO hierarchy, and callbacks.
You should construct separated Observables that should emit the specific data it's handle, without the Subscriber, then you can chain you're Observable as needed, and at the end, you have the subscriber that react to the final result expected from the chained Observable stream.
something like this (using lambdas to have more thin code):
TeamCache mTeamCache = new TeamCache();
FeedRepository mFeedRepository = new FeedRepository();
Observable.zip(teamsObservable, feedObservable, Pair::new)
.subscribe(resultPair -> {
//Do stuff with mTeams
List<BaseFeedItem> responseList = new ArrayList();
for (ApiFeedResponse apiFeedResponse : resultPair.second) {
responseList.add(FeedDataMapper.transform(apiFeedResponse));
}
}, throwable -> {
//handle errors
}
);
I've use zip and not concat as it's seems you have 2 independent calls here that you want to wait for both to finish ('zip' them together) and then act upon, but ofcourse, as you have separated Observables stream, you can chain them together differently according to your needs.
as for your ServerSubscriber with all the response validation logic, it should be rxify too, so you can compose it along your server Observable stream.
something like this (some logic emitted to simplify, and as I'm not familiar with it...)
Observable<List<SimpleTeam>> teamsObservable = mTeamCache.getAllTeams();
Observable<List<ApiFeedResponse>> feedObservable = mFeedRepository.getFeed(0, 50)
.flatMap(apiFeedsResponse -> {
if (apiFeedsResponse.code() != 200) {
if (apiFeedsResponse.code() == 304) {
List<ApiFeedResponse> body = apiFeedsResponse.body();
return Observable.just(body);
//onNotModified(o.body());
} else {
return Observable.error(new ServerSideErrorException(apiFeedsResponse));
}
} else {
//onServerSideResponse(o.body());
return Observable.just(apiFeedsResponse.body());
}
});
I am using Retrofit 2.2 with RxJava.
The pagination works like this: I get the first batch of data, I have to request the second batch of data with the same params except one which is the lastUpdated date and then if I get empty or the same batch of data it means there are no more items. I have found this great article https://medium.com/#v.danylo/server-polling-and-retrying-failed-operations-with-retrofit-and-rxjava-8bcc7e641a5a#.40aeibaja on how to do it. So my code is:
private Observable<Integer> syncDataPoints(final String baseUrl, final String apiKey,
final long surveyGroupId) {
final List<ApiDataPoint> lastBatch = new ArrayList<>();
Timber.d("start syncDataPoints");
return loadAndSave(baseUrl, apiKey, surveyGroupId, lastBatch)
.repeatWhen(new Func1<Observable<? extends Void>, Observable<?>>() {
#Override
public Observable<?> call(final Observable<? extends Void> observable) {
Timber.d("Calling repeatWhen");
return observable.delay(5, TimeUnit.SECONDS);
}
})
.takeUntil(new Func1<List<ApiDataPoint>, Boolean>() {
#Override
public Boolean call(List<ApiDataPoint> apiDataPoints) {
boolean done = apiDataPoints.isEmpty();
if (done) {
Timber.d("takeUntil : finished");
} else {
Timber.d("takeUntil : will query again");
}
return done;
}
})
.filter(new Func1<List<ApiDataPoint>, Boolean>() {
#Override
public Boolean call(List<ApiDataPoint> apiDataPoints) {
boolean unfiltered = apiDataPoints.isEmpty();
if (unfiltered) {
Timber.d("filtered");
} else {
Timber.d("not filtered");
}
return unfiltered;
}
}).map(new Func1<List<ApiDataPoint>, Integer>() {
#Override
public Integer call(List<ApiDataPoint> apiDataPoints) {
Timber.d("Finished polling server");
return 0;
}
});
}
private Observable<List<ApiDataPoint>> loadAndSave(final String baseUrl, final String apiKey,
final long surveyGroupId, final List<ApiDataPoint> lastBatch) {
return loadNewDataPoints(baseUrl, apiKey, surveyGroupId)
.concatMap(new Func1<ApiLocaleResult, Observable<List<ApiDataPoint>>>() {
#Override
public Observable<List<ApiDataPoint>> call(ApiLocaleResult apiLocaleResult) {
return saveToDataBase(apiLocaleResult, lastBatch);
}
});
}
private Observable<ApiLocaleResult> loadNewDataPoints(final String baseUrl, final String apiKey,
final long surveyGroupId) {
Timber.d("loadNewDataPoints");
return Observable.just(true).concatMap(new Func1<Object, Observable<ApiLocaleResult>>() {
#Override
public Observable<ApiLocaleResult> call(Object o) {
Timber.d("loadNewDataPoints call");
return restApi
.loadNewDataPoints(baseUrl, apiKey, surveyGroupId,
getSyncedTime(surveyGroupId));
}
});
}
As you can see the interesting method is loadNewDataPoints and I want it to be called until there are no more datapoints. As you can see Observable.just(true).concatMap is a hack because if I remove this concat map the restApi.loadNewDataPoints(....) does not get called although in the logs I can see that the api does get called but with the same old params and of course it returns the same results as the first time so syncing stops, saveToDataBase does get called fine. With my hack it works but I want to understand why it does not work the other way and also if there is a better way to do this. Thanks a lot!
So, I've written this kind of APIs (it's called Keyset Pagination) and implemented Rx clients against them.
This is one of the cases where BehaviorSubjects are useful:
S initialState = null;
BehaviorProcessor<T> subject = BehaviorProcessor.createDefault(initialState);
return subject
.flatMap(state -> getNextElements(state).singleOrError().toFlowable(), Pair::of, 1)
.serialize()
.flatMap(stateValuePair -> {
S state = stateValuePair.getLeft();
R retrievedValue = stateValuePair.getRight();
if(isEmpty(retrievedValue)) {
subject.onComplete();
return Flowable.empty();
} else {
subject.onNext(getNextState(state, retrievedValue));
return Flowable.just(retrievedValue);
}
}
.doOnUnsubscribe(subject::onCompleted)
.map(value -> ...)
Here
getNextElement performs the network call based on a state and returns a reactive stream with a single value
isEmpty determines whether the returned value is empty indicating end of elements
getNextState combines the passed-in state with the retrieved value to determine the next state for getNextElement.
It will work correctly if an error occurs (it will be propagated) and if you unsubscribe before the end (queries will get terminated).
Of course, in your specific case these don't need to be separate methods or complex types.
I just start learning rxJava for Android and want to implement the common use case:
request data from cache and show to the user
request data from web
server update data in storage and automatically show it to the user
Traditionally on of the best scenarios was use CursorLoader to get data from cache, run web request in the separate thread and save data to the disk via content provider, content provider automatically notify the listener and CursorLoader autoupdate UI.
In rxJava I can do it by running two different Observers as you can see in code below, but I don't find the way how to combine this two calls into the one to reach my aim. Googling shows this thread but it looks like it just get data from the cache or data from the web server, but don't do both RxJava and Cached Data
Code snippet:
#Override
public Observable<SavingsGoals> getCachedSavingsGoal() {
return observableGoal.getSavingsGoals()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread());
}
#Override
public Observable<SavingsGoals> getRecentSavingsGoal() {
return api.getSavingsGoals()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread());
}
model.getCachedSavingsGoal().subscribe(new Observer<SavingsGoals>() {
#Override
public void onCompleted() {
// no op
}
#Override
public void onError(Throwable e) {
Log.e(App.TAG, "Failed to consume cached data");
view.showError();
}
#Override
public void onNext(SavingsGoals savingsGoals) {
Log.d(App.TAG, "Show the next item");
if (savingsGoals != null && !savingsGoals.getSavingsGoals().isEmpty()) {
view.showData(savingsGoals.getSavingsGoals());
} else {
view.showError();
}
}
});
model.getRecentSavingsGoal().subscribe(new Observer<SavingsGoals>() {
#Override
public void onCompleted() {
// no op
}
#Override
public void onError(Throwable e) {
Log.e(App.TAG, "Failed to consume data from the web", e);
view.showError();
}
#Override
public void onNext(SavingsGoals savingsGoals) {
if (savingsGoals != null && !savingsGoals.getSavingsGoals().isEmpty()) {
view.showData(savingsGoals.getSavingsGoals());
} else {
view.showError();
}
}
});
Also, the one of issues with current approach is cache and web data are not garranted to be run sequently. It is possible when outdated data will come as latest and override recent from web.
To solve this issue I implemented Observer merge with filtration by timestamp: it get data from cache, pass it to the next observer and if cache is outdated fire new call to the web - case for thread competition solved by the filtration with timestamps. However, the issue with this approach I can not return cache data from this Observable - I need to wait when both requests finish their work.
Code snippet.
#Override
public Observable<Timestamped<SavingsGoals>> getSavingGoals() {
return observableGoal
.getTimestampedSavingsGoals()
.subscribeOn(Schedulers.io())
.flatMap(new Func1<Timestamped<SavingsGoals>, Observable<Timestamped<SavingsGoals>>>() {
#Override
public Observable<Timestamped<SavingsGoals>> call(Timestamped<SavingsGoals> cachedData) {
Log.d(App.FLOW, "getTimestampedSavingsGoals");
return getGoalsFromBothSources()
.filter(filterResponse(cachedData));
}
})
.subscribeOn(AndroidSchedulers.mainThread());
}
private Func1<Timestamped<SavingsGoals>, Boolean> filterResponse(Timestamped<SavingsGoals> cachedData) {
return new Func1<Timestamped<SavingsGoals>, Boolean>() {
#Override
public Boolean call(Timestamped<SavingsGoals> savingsGoals) {
return savingsGoals != null
&& cachedData != null
&& cachedData.getTimestampMillis() < savingsGoals.getTimestampMillis()
&& savingsGoals.getValue().getSavingsGoals().size() != 0;
}
};
}
private Observable<Timestamped<SavingsGoals>> getGoalsFromBothSources() {
Log.d(App.FLOW, "getGoalsFromBothSources:explicit");
return Observable.merge(
observableGoal.getTimestampedSavingsGoals().subscribeOn(Schedulers.io()),
api.getSavingsGoals()
.timestamp()
.flatMap(new Func1<Timestamped<SavingsGoals>, Observable<Timestamped<SavingsGoals>>>() {
#Override
public Observable<Timestamped<SavingsGoals>> call(Timestamped<SavingsGoals> savingsGoals) {
Log.d(App.FLOW, "getGoalsFromBothSources:implicit");
return observableGoal.saveAllWithTimestamp(savingsGoals.getTimestampMillis(), savingsGoals.getValue().getSavingsGoals());
}
}))
.subscribeOn(Schedulers.io());
}
Do you know the approach to do this in one Observer?
Potential solution:
#Override
public Observable<SavingsGoals> getSavingGoals() {
return api.getSavingsGoals()
.publish(network ->
Observable.mergeDelayError(
observableGoal.getSavingsGoals().takeUntil(network),
network.flatMap(new Func1<SavingsGoals, Observable<SavingsGoals>>() {
#Override
public Observable<SavingsGoals> call(SavingsGoals savingsGoals) {
return observableGoal.saveAll(savingsGoals.getSavingsGoals());
}
})
)
)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread());
}
Sorry, hot replacement in IDE hide the issue which this approach has: first one in case if network unavailable and cache thread completes first, the error will terminate whole merge (solved by mergeDelayError), second one is in case when cache is empty and returns first data from web request will not be return on subscriber. As you can see my method returns Observable after save and traditional merge as I shown in my code properly handle this case but takeUntil by some reason can not. Question is still open.
For first question : You can save the result from Network Result by using doOnNext Method, It would looks something like this
public Observable<NetworkResponse> getDataFromNetwork(
final Request request) {
return networkCall.doOnNext(networkResponse -> saveToStorage(networkResponse);
}
Now to combine the two results from both Storage and Online, the best way is to combine with publish and merge. I recommend watching this talk.
The code would look something like this
public Observable<Response> getData(final Request request) {
return dataService.getDataFromNetwork(request)
.publish(networkResponse -> Observable.merge(networkResponse, dataService.getDataFromStorage(request).takeUntil(networkResponse)));
}
Why use publish and merge you my ask? publish method makes the response accessible in the callback. takeUntil means that you will take the data from storage but you will stop it IF for some reason, network call is finished before accessing storage data is finished. This way, you can be sure that new data from network is always shown even if it's finished before getting old data from storage.
The last but not least, in your subscriber OnNext just add the items to the list. (list.clear and list.addAll) Or similar functions or in you case view.showData()
EDIT: For The call getting disrupted when there's an error from network, add onErrorResumeNext at the end.
public Observable<Response> getData(final Request request) {
return dataService.getDataFromNetwork(request)
.publish(networkResponse -> Observable.merge(networkResponse, dataService.getDataFromStorage(request).takeUntil(networkResponse)))
.onErrorResumeNext(dataService.getDataFromStorage(request);
}
I'd recommend to "listen" only to local data, and refresh it when API response came.
Let say for getting local data you have something like:
#Nonnull
public Observable<SomeData> getSomeDataObservable() {
return Observable
.defer(new Func0<Observable<SomeData>>() {
#Override
public Observable<SomeData> call() {
return Observable.just(getSomeData());
}
});
}
So you need to add PublishSubject that will emit every time, when local data was updated (refreshSubject):
#Nonnull
public Observable<SomeData> getSomeDataObservableRefreshable() {
return refreshSubject.startWith((Object)null).switchMap(new Func1() {
public Observable<T> call(Object o) {
return getSomeDataObservable();
}
}
}
Now you need to subscribe only to getSomeDataObservableRefreshable(), and each time when data came from API, you update it and make refreshSubject .onNext(new Object())
Also i'd recommend to take a look to rx-java-extensions lib, it has alot of "cool tools" for RxAndroid. For example solution for your problem would be:
#Nonnull
public Observable<SomeData> getSomeDataObservable() {
return Observable
.defer(new Func0<Observable<SomeData>>() {
#Override
public Observable<SomeData> call() {
return Observable.just(getSomeData());
}
})
.compose(MoreOperators.<SomeData>refresh(refreshSubject));
}
I'm trying to wrap my head around RxJava currently, but I'm having a little trouble with handling service call exceptions in an elegant manner.
Basically, I have a (Retrofit) service that returns an Observable<ServiceResponse>. ServiceResponse is defined like so:
public class ServiceResponse {
private int status;
private String message;
private JsonElement data;
public JsonElement getData() {
return data;
}
public int getStatus() {
return status;
}
public String getMessage() {
return message;
}
}
Now what I want is to map that generic response to a List<Account> contained within the data JsonElement field (I assume you don't care what the Account object looks like, so I won't pollute the post with it). The following code works really well for the success case, but I can't find a nice way to handle my API exceptions:
service.getAccounts()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.map(new Func1<ServiceResponse, AccountData>() {
#Override
public AccountData call(ServiceResponse serviceResponse) {
// TODO: ick. fix this. there must be a better way...
ResponseTypes responseType = ResponseTypes.from(serviceResponse.getStatus());
switch (responseType) {
case SUCCESS:
Gson gson = new GsonBuilder().create();
return gson.fromJson(serviceResponse.getData(), AccountData.class);
case HOST_UNAVAILABLE:
throw new HostUnavailableException(serviceResponse.getMessage());
case SUSPENDED_USER:
throw new SuspendedUserException(serviceResponse.getMessage());
case SYSTEM_ERROR:
case UNKNOWN:
default:
throw new SystemErrorException(serviceResponse.getMessage());
}
}
})
.map(new Func1<AccountData, List<Account>>() {
#Override
public List<Account> call(AccountData accountData) {
Gson gson = new GsonBuilder().create();
List<Account> res = new ArrayList<Account>();
for (JsonElement account : accountData.getAccounts()) {
res.add(gson.fromJson(account, Account.class));
}
return res;
}
})
.subscribe(accountsRequest);
Is there a better way to do this? This does work, onError will fire to my observer, and I will receive the error that I threw, but it definitely does not seem like I'm doing this right.
Thanks in advance!
Edit:
Let me clarify exactly what I want to achieve:
I want to have a class that can be called from the UI (e.g. an Activity, or Fragment, or whatever). That class would take an Observer<List<Account>> as a parameter like so:
public Subscription loadAccounts(Observer<List<Account>> observer, boolean forceRefresh) {
...
}
That method would return a subscription that can be unsubscribed when the UI is detached/destroyed/etc.
The parameterized observer would handle onNext for the successful responses passing in a list of Accounts. OnError would handle any exceptions, but would also get passed any API exceptions (e.g. if the response status != 200 we would create a Throwable and pass it to onError). Ideally I don't want to just "throw" the Exception, I want to pass it directly to the Observer. That's what all the examples I see do.
The complication is that my Retrofit service returns a ServiceResponse object, so my observer cannot subscribe to that. The best I've come up with is to create an Observer wrapper around my Observer, like so:
#Singleton
public class AccountsDatabase {
private AccountsService service;
private List<Account> accountsCache = null;
private PublishSubject<ServiceResponse> accountsRequest = null;
#Inject
public AccountsDatabase(AccountsService service) {
this.service = service;
}
public Subscription loadAccounts(Observer<List<Account>> observer, boolean forceRefresh) {
ObserverWrapper observerWrapper = new ObserverWrapper(observer);
if (accountsCache != null) {
// We have a cached value. Emit it immediately.
observer.onNext(accountsCache);
}
if (accountsRequest != null) {
// There's an in-flight network request for this section already. Join it.
return accountsRequest.subscribe(observerWrapper);
}
if (accountsCache != null && !forceRefresh) {
// We had a cached value and don't want to force a refresh on the data. Just
// return an empty subscription
observer.onCompleted();
return Subscriptions.empty();
}
accountsRequest = PublishSubject.create();
accountsRequest.subscribe(new ObserverWrapper(new EndObserver<List<Account>>() {
#Override
public void onNext(List<Account> accounts) {
accountsCache = accounts;
}
#Override
public void onEnd() {
accountsRequest = null;
}
}));
Subscription subscription = accountsRequest.subscribe(observerWrapper);
service.getAccounts()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(accountsRequest);
return subscription;
}
static class ObserverWrapper implements Observer<ServiceResponse> {
private Observer<List<Account>> observer;
public ObserverWrapper(Observer<List<Account>> observer) {
this.observer = observer;
}
#Override
public void onCompleted() {
observer.onCompleted();
}
#Override
public void onError(Throwable e) {
observer.onError(e);
}
#Override
public void onNext(ServiceResponse serviceResponse) {
ResponseTypes responseType = ResponseTypes.from(serviceResponse.getStatus());
switch (responseType) {
case SUCCESS:
Gson gson = new GsonBuilder().create();
AccountData accountData = gson.fromJson(serviceResponse.getData(), AccountData.class);
List<Account> res = new ArrayList<>();
for (JsonElement account : accountData.getAccounts()) {
res.add(gson.fromJson(account, Account.class));
}
observer.onNext(res);
observer.onCompleted();
break;
default:
observer.onError(new ApiException(serviceResponse.getMessage(), responseType));
break;
}
}
}
}
I still feel like I am not using this correctly though. I definitely haven't seen anyone else using an ObserverWrapper before. Perhaps I shouldn't be using RxJava, though the guys at SoundCloud and Netflix really sold me on it in their presentations and I'm pretty eager to learn it.
Please read below I've added an edit.
It's perfectly correct to throw within an Action/Func/Observer with RxJava. The exception will be propagate by the framework right down to your Observer.
If you limit yourself to calling onError only then you'll be twisting yourself to make that happen.
With that being said a suggestion would be to simply remove this wrapper and add a simple validation
Action within the service.getAccount... chain of Observables.
I'd use the doOnNext(new ValidateServiceResponseOrThrow) chained with a map(new MapValidResponseToAccountList). Those are simple classes which implements the necessary code to keep the Observable chain a bit more readable.
Here's your loadAccount method simplified using what I suggested.
public Subscription loadAccounts(Observer<List<Account>> observer, boolean forceRefresh) {
if (accountsCache != null) {
// We have a cached value. Emit it immediately.
observer.onNext(accountsCache);
}
if (accountsRequest != null) {
// There's an in-flight network request for this section already. Join it.
return accountsRequest.subscribe(observer);
}
if (accountsCache != null && !forceRefresh) {
// We had a cached value and don't want to force a refresh on the data. Just
// return an empty subscription
observer.onCompleted();
return Subscriptions.empty();
}
accountsRequest = PublishSubject.create();
accountsRequest.subscribe(new EndObserver<List<Account>>() {
#Override
public void onNext(List<Account> accounts) {
accountsCache = accounts;
}
#Override
public void onEnd() {
accountsRequest = null;
}
});
Subscription subscription = accountsRequest.subscribe(observer);
service.getAccounts()
.doOnNext(new ValidateServiceResponseOrThrow())
.map(new MapValidResponseToAccountList())
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(accountsRequest);
return subscription;
}
private static class ValidateResponseOrThrow implements Action1<ServiceResponse> {
#Override
public void call(ServiceResponse response) {
ResponseTypes responseType = ResponseTypes.from(serviceResponse.getStatus());
if (responseType != SUCCESS)
throw new ApiException(serviceResponse.getMessage(), responseType));
}
}
private static class MapValidResponseToAccountList implements Func1<ServiceResponse, List<Account>> {
#Override
public Message call(ServiceResponse response) {
// add code here to map the ServiceResponse into the List<Accounts> as you've provided already
}
}
Edit:
Unless someone says otherwise I think it's best practice to return errors using flatMap.
I've thrown Exceptions from Action in the past but I don't believe it's the recommended way.
You'll have a cleaner Exception stack if you use flatMap. If you throw from inside an Action the Exception stack
will actually contain rx.exceptions.OnErrorThrowable$OnNextValue Exception which isn't ideal.
Let me demonstrate the example above using the flatMap instead.
private static class ValidateServiceResponse implements rx.functions.Func1<ServiceResponse, Observable<ServiceResponse>> {
#Override
public Observable<ServiceResponse> call(ServiceResponse response) {
ResponseTypes responseType = ResponseTypes.from(serviceResponse.getStatus());
if (responseType != SUCCESS)
return Observable.error(new ApiException(serviceResponse.getMessage(), responseType));
return Observable.just(response);
}
}
service.getAccounts()
.flatMap(new ValidateServiceResponse())
.map(new MapValidResponseToAccountList())
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(accountsRequest);
As you can see the the difference is subtle. The ValidateServiceResponse now implements the Func1 instead of Action1 and we're no longer using the throw keyword. We use Observable.error(new Throwable) instead. I believe this fits better with the expected Rx contract.
You could read this good article about error handling http://blog.danlew.net/2015/12/08/error-handling-in-rxjava/
Is it acceptable to create Rx Observables in custom Application subclass. Reason for doing is, I can create BehaviorSubject inside the Application and will ask for changes every 10 minutes from Server, every Activity or Fragment which subscribes to this Observable will get only last state of changes.
Question is whether this architecture could be considered safe in terms of application lifecycle handling and easy to use?
class CustomApplication extends Application {
...
BehaviorSubject<Friends> mFriends = new BehaviorSubject<Friends>;
public void createObservables() {
Observable.create(new Observable.OnSubscribe<Friends>() {
public void call(Subscriber<?> s) {
while(true) {
mFriends.onNext("randomFriendN");
sleep(10sec);
}
}
})
.subscribeOn(Schedulers.newThread())
.subscribe(new Observer<List<NewsCategory>>() {
public void onNext(Friends f) { //empty }
});
}
public BehaviorSubject<Friends> getFriends() {
return mFriends;
}
}
UPDATE:
Everytime when new activity created and it wants to get data it can get it ApplicationContext's BehaviorSubject then subscribe to it, and Subject will emit last emitted value;
Why I want to do like this? E.g. Lets say you have news items, you fetched news feed and you want to start background task which fetches news item full content, in that case I can start fetching data while you are scrolling news list, and when you click detailed activity, we can show it from already fetched, or just download it.
I think this is perfectly safe as long as createObservables() is only called once during application initialization. A few suggested changes...
I wouldn't expose the BehaviorSubject part of mFriends in the returned value from getFriends(). That way callers of getFriends() will not be tempted to call onNext(). Change it to:
public Observable<Friends> getFriends() {
return mFriends;
}
If you want to be super safe use .asObservable() and callers will not even be able to cast the return value back to a BehaviorSubject.
public Observable<Friends> getFriends() {
return mFriends.asObservable();
}
I would also update your createObservable() method to call the BehaviorSubject onNext() from the subscribe callback. Here is your code slightly modified to use NewsItems.
BehaviorSubject<List<NewsItem>> mNewsItemSubject = BehaviorSubject.create();
void createObservables() {
Observable
.timer(10, 10, TimeUnit.SECONDS, Schedulers.newThread())
.flatMap(new Func1<Long, Observable<List<NewsItem>>>() {
#Override
public Observable<List<NewsItem>> call(Long aLong) {
// Normally you would create a network API that returns Observable<NewsItem>.
// For now just pretend this returned Observable makes an Observable
// network request.
return Observable.just(
Arrays.asList(
new NewsItem("fakeNewsItem"),
new NewsItem("fakeNewsItem1")
)
);
}
})
.subscribe(new Action1<List<NewsItem>>() {
#Override
public void call(List<NewsItem> newsItems) {
mNewsItemSubject.onNext(newsItems);
}
});
}
public Observable<List<NewsItem>> observeNewsItems() {
return mNewsItemSubject;
}
Your Android Activities can then call ((CustomApplication)getApplication()).observeNewsItems() to get the latest news items and any updates while the Activity is visible.
final Observable<List<NewsItem>> newsItemsObservable =
((CustomApplication) getApplication()).observeNewsItems();
newsItemsObservable
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Observer<List<NewsItem>>() {
#Override
public void onCompleted() {
// All done.
}
#Override
public void onError(Throwable e) {
// Notify user of error (maybe)
}
#Override
public void onNext(List<NewsItem> newsItems) {
// Update the UI with newsItems.
}
});