The result of subscribe is not used
AndroidAndroid StudioRx Java2LintAndroid Studio-3.1Android Problem Overview
I've upgraded to Android Studio 3.1 today, which seems to have added a few more lint checks. One of these lint checks is for one-shot RxJava2 subscribe()
calls that are not stored in a variable. For example, getting a list of all players from my Room database:
Single.just(db)
.subscribeOn(Schedulers.io())
.subscribe(db -> db.playerDao().getAll());
Results in a big yellow block and this tooltip:
> The result of subscribe
is not used
What is the best practice for one-shot Rx calls like this? Should I keep hold of the Disposable
and dispose()
on complete? Or should I just @SuppressLint
and move on?
This only seems to affect RxJava2 (io.reactivex
), RxJava (rx
) does not have this lint.
Android Solutions
Solution 1 - Android
The IDE does not know what potential effects your subscription can have when it's not disposed, so it treats it as potentially unsafe. For example, your Single
may contain a network call, which could cause a memory leak if your Activity
is abandoned during its execution.
A convenient way to manage a large amount of Disposable
s is to use a CompositeDisposable; just create a new CompositeDisposable
instance variable in your enclosing class, then add all your Disposables to the CompositeDisposable (with RxKotlin you can just append addTo(compositeDisposable)
to all of your Disposables). Finally, when you're done with your instance, call compositeDisposable.dispose()
.
This will get rid of the lint warnings, and ensure your Disposables
are managed properly.
In this case, the code would look like:
CompositeDisposable compositeDisposable = new CompositeDisposable();
Disposable disposable = Single.just(db)
.subscribeOn(Schedulers.io())
.subscribe(db -> db.get(1)));
compositeDisposable.add(disposable); //IDE is satisfied that the Disposable is being managed.
disposable.addTo(compositeDisposable); //Alternatively, use this RxKotlin extension function.
compositeDisposable.dispose(); //Placed wherever we'd like to dispose our Disposables (i.e. in onDestroy()).
Solution 2 - Android
The moment the Activity will be destroyed, the list of Disposables gets cleared and we’re good.
io.reactivex.disposables.CompositeDisposable mDisposable;
mDisposable = new CompositeDisposable();
mDisposable.add(
Single.just(db)
.subscribeOn(Schedulers.io())
.subscribe(db -> db.get(1)));
mDisposable.dispose(); // dispose wherever is required
Solution 3 - Android
You can subscribe with DisposableSingleObserver:
Single.just(db)
.subscribeOn(Schedulers.io())
.subscribe(new DisposableSingleObserver<Object>() {
@Override
public void onSuccess(Object obj) {
// work with the resulting todos...
dispose();
}
@Override
public void onError(Throwable e) {
// handle the error case...
dispose();
}});
In case you need to directly dispose Single
object (e.g. before it emits) you can implement method onSubscribe(Disposable d)
to get and use the Disposable
reference.
You can also realize SingleObserver
interface by your own or use other child classes.
Solution 4 - Android
As was suggested you may use some global CompositeDisposable
to add the result of the subscribe operation there.
The RxJava2Extensions library contains useful methods to automatically remove created disposable from the CompositeDisposable
when it completes. See subscribeAutoDispose section.
In your case it may look like this
SingleConsumers.subscribeAutoDispose(
Single.just(db)
.subscribeOn(Schedulers.io()),
composite,
db -> db.playerDao().getAll())
Solution 5 - Android
You can use Uber AutoDispose and rxjava .as
Single.just(db)
.subscribeOn(Schedulers.io())
.as(AutoDispose.autoDisposable(AndroidLifecycleScopeProvider.from(this)))
.subscribe(db -> db.playerDao().getAll());
Make sure that you understand when you unsubscribe based on the ScopeProvider.
Solution 6 - Android
Again and again I find myself coming back to the question of how to correctly dispose of subscriptions, and to this posting in particular. Several blogs and talks claim that failing to call dispose
necessarily leads to a memory leak, which I think is a too general statement. In my understanding, the lint warning about not storing the result of subscribe
is a non-issue in some cases, because:
- Not all observables run in the context of an Android activity
- The observable can be synchronous
- Dispose is called implicitly, provided the observable completes
Since I don't want to suppress lint warnings I recently started to use the following pattern for cases with a synchronous observable:
var disposable: Disposable? = null
disposable = Observable
.just(/* Whatever */)
.anyOperator()
.anyOtherOperator()
.subscribe(
{ /* onSuccess */ },
{ /* onError */ },
{
// onComplete
// Make lint happy. It's already disposed because the stream completed.
disposable?.dispose()
}
)
I'd be interested in any comments on this, regardless of whether it's a confirmation of correctness or the discovery of a loophole.
Solution 7 - Android
There's another way available, which is avoiding to use Disposables manually (add and remove subscriptions).
You can define an Observable and that observable is going to receive the content from a SubjectBehaviour (in case you use RxJava). And by passing that observable to your LiveData, that should work. Check out the next example based on the initial question:
private val playerSubject: Subject<Player> = BehaviorSubject.create()
private fun getPlayer(idPlayer: String) {
playerSubject.onNext(idPlayer)
}
private val playerSuccessful: Observable<DataResult<Player>> = playerSubject
.flatMap { playerId ->
playerRepository.getPlayer(playerId).toObservable()
}
.share()
val playerFound: LiveData<Player>
get() = playerSuccessful
.filterAndMapDataSuccess()
.toLiveData()
val playerNotFound: LiveData<Unit>
get() = playerSuccessful.filterAndMapDataFailure()
.map { Unit }
.toLiveData()
// These are a couple of helpful extensions
fun <T> Observable<DataResult<T>>.filterAndMapDataSuccess(): Observable<T> =
filter { it is DataResult.Success }.map { (it as DataResult.Success).data }
fun <T> Observable<DataResult<T>>.filterAndMapDataFailure(): Observable<DataResult.Failure<T>> =
filter { it is DataResult.Failure }.map { it as DataResult.Failure<T> }
Solution 8 - Android
If you are sure that disposable handled correctly, for example using doOnSubscribe() operator, you may add this to Gradle:
android {
lintOptions {
disable 'CheckResult'
}}