AutoDispose

AutoDispose is an RxJava 2+ tool for automatically binding the execution of RxJava streams to a provided scope via disposal/cancellation.

Overview

Often (especially in mobile applications), Rx subscriptions need to stop in response to some event (for instance, when Activity#onStop() executes in an Android app). In order to support this common scenario in RxJava 2, we built AutoDispose.

The idea is simple: construct your chain like any other, and then at subscription you simply drop in the relevant factory call + method for that type as a converter. In everyday use, it usually looks like this:

myObservable
    .doStuff()
    .as(autoDisposable(this))   // The magic
    .subscribe(s -> ...);

By doing this, you will automatically unsubscribe from myObservable as indicated by your scope - this helps prevent many classes of errors when an observable emits an item, but the actions taken in the subscription are no longer valid. For instance, if a network request comes back after a UI has already been torn down, the UI can’t be updated - this pattern prevents this type of bug.

autoDisposable()

The main entry point is via static factory autoDisposable() methods in the AutoDispose class. There are two overloads: Completable and ScopeProvider. They return an AutoDisposeConverter object that implements all the RxJava Converter interfaces for use with the as() operator in RxJava types.

Completable (as a scope)

The Completable semantic is modeled after the takeUntil() operator, which accepts an Observable whose first emission is used as a notification to signal completion. This is logically the behavior of a Single, so we choose to make that explicit. Since the type doesn’t matter, we simplify this further to just be a Completable, where the scope-end emission is just a completion event. All scopes in AutoDispose eventually resolve to a Completable that emits the end-of-scope notification in onComplete. onError will pass through to the underlying subscription.

ScopeProvider

public interface ScopeProvider {
  CompletableSource requestScope() throws Exception;
}

ScopeProvider is an abstraction that allows objects to expose and control and provide their own scopes. This is particularly useful for objects with simple scopes (“stop when I stop”) or very custom state that requires custom handling.

Note that Exceptions can be thrown in this, and will be routed through onError(). If the thrown exception is an instance of OutsideScopeException, it will be routed through any OutsideScopeHandlers (more below) first, and sent through onError() if not handled.

AutoDisposePlugins

Modeled after RxJava’s plugins, this allows you to customize the behavior of AutoDispose.

OutsideScopeHandler

When a scope is bound to outside of its allowable boundary, AutoDispose will send an error event with an OutsideScopeException to downstream consumers. If you want to customize this behavior, you can use AutoDisposePlugins#setOutsideScopeHandler to intercept these exceptions and rethrow something else or nothing at all.

Example

AutoDisposePlugins.setOutsideScopeHandler(t -> {
    // Swallow the exception, or rethrow it, or throw your own!
})

A good use case of this is, say, just silently disposing/logging observers outside of scope exceptions in production but crashing on debug.

The supported mechanism to throw this is in ScopeProvider#requestScope() implementations.

FillInOutsideScopeExceptionStacktraces

If you have your own handling of exceptions in scope boundary events, you can optionally set AutoDisposePlugins#setFillInOutsideScopeExceptionStacktraces to false. This will result in AutoDispose not filling in stacktraces for exceptions, for a potential minor performance boost.

AutoDisposeAndroidPlugins

Similar to AutoDisposePlugins, this allows you to customize the behavior of AutoDispose in Android environments.

MainThreadChecker

This plugin allows for supplying a custom BooleanSupplier that can customize how main thread checks work. The conventional use case of this is Android JUnit tests, where the Looper class is not stubbed in the mock android.jar and fails explosively when touched.

Another potential use of this at runtime to customize checks for more fine-grained main thread checks behavior.

Example

AutoDisposeAndroidPlugins.setOnCheckMainThread(() -> {
    return true; // Use whatever heuristics you prefer.
})

Behavior

Under the hood, AutoDispose decorates RxJava’s real observer with a custom AutoDisposing observer. This custom observer leverages the scope to create a disposable, auto-disposing observer that acts as a lambda observer (pass-through) unless the underlying scope CompletableSource emits onComplete. Both scope emission and upstream termination result in immediate disposable of both the underlying scope subscription and upstream disposable.

These custom AutoDisposing observers are considered public read-only API, and can be found under the observers package. They also support retrieval of the underlying observer via delegateObserver() methods. Read-only API means that the public signatures will follow semantic versioning, but we may add new methods in the future (which would break compilation if you make custom implementations!).

To read this information, you can use RxJava’s onSubscribe hooks in RxJavaPlugins to watch for instances of these observers.

Support/Extensions

Flowable, ParallelFlowable, Observable, Maybe, Single, and Completable are all supported. Implementation is solely based on their Observer types, so conceivably any type that uses those for subscription should work.

Extensions

There are also a number of extension artifacts available, detailed below.

LifecycleScopeProvider

public interface LifecycleScopeProvider<E> extends ScopeProvider {
  Observable<E> lifecycle();

  Function<E, E> correspondingEvents();

  E peekLifecycle();

  // Inherited from ScopeProvider
  CompletableSource requestScope();
}

A common use case for this is objects that have implicit lifecycles, such as Android’s Activity, Fragment, and View classes. Internally at subscription-time, AutoDispose will resolve a CompletableSource representation of the target end event in the lifecycle, and exposes an API to dictate what corresponding events are for the current lifecycle state (e.g. ATTACH -> DETACH). This also allows you to enforce lifecycle boundary requirements, and by default will error if the lifecycle has either not started yet or has already ended.

LifecycleScopeProvider is a special case targeted at binding to things with lifecycles. Its API is as follows: - lifecycle() - returns an Observable of lifecycle events. This should be backed by a BehaviorSubject or something similar (BehaviorRelay, etc). - correspondingEvents() - a mapping of events to corresponding ones, i.e. Attach -> Detach. - peekLifecycle() - returns the current lifecycle state of the object.

In requestScope(), the implementation expects to these pieces to construct a CompletableSource representation of the proper end scope, while also doing precondition checks for lifecycle boundaries. If a lifecycle has not started, it will send you to onError with a LifecycleNotStartedException. If the lifecycle as ended, it is recommended to throw a LifecycleEndedException in your correspondingEvents() mapping, but it is up to the user.

To simplify implementations, there’s an included LifecycleScopes utility class with factories for generating CompletableSource representations from LifecycleScopeProvider instances.

autodispose-lifecycle contains the core LifecycleScopeProvider and LifecycleScopes APIs as well as a convenience test helper.

Android

There are three artifacts with extra support for Android: * autodispose-android has a ViewScopeProvider for use with Android View classes. * autodispose-androidx-lifecycle has a AndroidLifecycleScopeProvider for use with LifecycleOwner and Lifecycle implementations.

Note that the project is compiled against Java 8. If you need support for lower Java versions, you should use D8 (Android Gradle Plugin 3.2+) or desugar as needed (depending on the build system).

Kotlin

Kotlin extensions are bundled with almost every artifact.

For coroutines - there is an autodispose-coroutines-interop artifact for interoperability between CoroutineScope and ScopeProvider/Completable types.

RxLifecycle

As of 0.4.0 there is an RxLifecycle interop module under autodispose-rxlifecycle. This is for interop with RxLifecycle‘s LifecycleProvider interfaces.

Philosophy

Each factory returns a subscribe proxies upon application that just proxy to real subscribe calls under the hood to “AutoDisposing” implementations of the types. These types decorate the actual observer at subscribe-time to achieve autodispose behavior. The types are not exposed directly because autodisposing has ordering requirements; specifically, it has to be done at the end of a chain to properly wrap all the upstream behavior. Lint could catch this too, but we have seen no use cases for disposing upstream (which can cause a lot of unexpected behavior). Thus, we optimize for the common case, and the API is designed to prevent ordering issues while still being a drop-in one-liner.

Motivations

Lifecycle management with RxJava and Android is nothing new, so why yet another tool?

Two common patterns for binding execution in RxJava that we used prior to this were as follows:

• CompositeSubscription field that all subscriptions had to be manually added to.
• RxLifecycle, which works via compose() to resolve the lifecycle end event and ultimately transform the given observable to takeUntil() that event is emitted.

Both implementations are elegant and work well, but came with caveats that we sought to revisit and solve in AutoDispose.

CompositeSubscription requires manual capture of the return value of subscribe calls, and gets tedious to reason about with regards to binding subscription until different events.

RxLifecycle solves the caveats of CompositeSubscription use by working in a dead-simple API and handling resolution of corresponding events. It works great for Observable types, but due to the nature of how takeUntil() works, we found that Single and Completable usage was risky to use (particularly in a large team with varying levels of RxJava experience) considering lifecycle interruption would result in a downstream CancellationException every time. It’s the contract of those types, but induced a lot of ceremony for what would otherwise likely be our most commonly used type (Single). Even with Observable, we were still burned occasionally by the completion event still coming through to an unsuspecting engineer. Another caveat we often ran into (and later aggressively linted against) was that the compose() call had ordering implications, and needed to be as close to the subscribe() call as possible to properly wrap upstream. If binding to views, there were also threading requirements on the observable chain in order to work properly.

At the end of the day, we wanted true disposal/unsubscription-based behavior, but with RxLifecycle-esque semantics around scope resolution. RxJava 2’s Observer interfaces provide the perfect mechanism for this via their onSubscribe() callbacks. The result is de-risked Single/Completable usage, no ordering concerns, no threading concerns (fingers crossed), and true disposal with no further events of any kind upon scope end. We’re quite happy with it, and hope the community finds it useful as well.

Special thanks go to Dan Lew (creator of RxLifecycle), who helped pioneer this area for RxJava in android and humored many of the discussions around lifecycle handling over the past couple years that we’ve learned from. Much of the internal scope resolution mechanics of AutoDispose are inspired by RxLifecycle.

RxJava versions support

RxJava version	AutoDispose version
RxJava 3	AutoDispose 2.x
RxJava 2	AutoDispose 1.x
RxJava 1	Unsupported

• We do not plan to try to backport this to RxJava 1. This pattern is sort of possible in RxJava 1, but only on Subscriber (via onStart()) and CompletableObserver (which matches the API of RxJava 2+)

• 2.x versions of AutoDispose are built for RxJava 3.

Static analysis

Error Prone

There is an optional error-prone checker you can use to enforce use of AutoDispose. Integration steps and more details can be found on the website

Lint Check

AutoDispose ships with a lint check that detects missing AutoDispose scope within defined scoped elements. Integration steps and more details can be found on the website

Download

Java:

implementation 'com.uber.autodispose2:autodispose:x.y.z'

LifecycleScopeProvider:

implementation 'com.uber.autodispose2:autodispose-lifecycle:x.y.z'

Android extensions:

implementation 'com.uber.autodispose2:autodispose-android:x.y.z'

Android Architecture Components extensions:

// AutoDispose 1.x
implementation 'com.uber.autodispose:autodispose-android-archcomponents:x.y.z'

// AutoDispose 2.x
implementation 'com.uber.autodispose2:autodispose-androidx-lifecycle:x.y.z'

Androidx-Lifecycle Test extensions:

// AutoDispose 1.x
implementation 'com.uber.autodispose:autodispose-android-archcomponents-test:x.y.z'

// AutoDispose 2.x
implementation 'com.uber.autodispose2:autodispose-androidx-lifecycle-test:x.y.z'

RxLifecycle interop (AutoDispose 1.x/RxJava 2.x only):

autodispose-rxlifecycle

implementation 'com.uber.autodispose:autodispose-rxlifecycle:x.y.z'

autodispose-rxlifecycle3

implementation 'com.uber.autodispose:autodispose-rxlifecycle3:x.y.z'

Javadocs and KDocs for the most recent release can be found here: https://uber.github.io/AutoDispose/2.x/autodispose/

Snapshots of the development version are available in Sonatype’s snapshots repository.