AutoDispose

项目地址:uber/AutoDispose
简介:Automatic binding+disposal of RxJava 2 streams.
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AutoDispose is an RxJava 2 tool for automatically binding the execution of RxJava 2 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 Scoper for that type as a converter. In everyday use, it usually looks like this:

myObservable
    .doStuff()
    .to(new ObservableScoper<SomeType>(this))   // The scope
    .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helps prevent many classes of errors when an observable emits and item, but the actions taken in the subscription are no longer valid.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.

Scoper

There are relevant Scoper classes, one per RxJava type (ObservableScoper, SingleScoper, etc). These are implementations of a converter Function, intended for use with the to() operator on RxJava types. Each Scoper accepts three overloads: Maybe, ScopeProvider, and LifecycleScopeProvider.

Maybe

The Maybe semantic is modeled after the takeUntil() operator, which accepts an Observable whose first emission is used as a notification to signal completion. This is is logically the behavior of a Maybe, so we choose to make that explicit. All scopes eventually resolve to a single Maybe that emits the end-of-scope notification.

Providers

The provider options allow you to pass in an interface of something can provide a resolvable scope. 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 Maybe 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
public interface LifecycleScopeProvider<E> {
  Observable<E> lifecycle();

  Function<E, E> correspondingEvents();

  E peekLifecycle();
}

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.

AutoDispose uses these pieces to construct a Maybe 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.

ScopeProvider
public interface ScopeProvider {
  Maybe<?> requestScope();
}

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.

Plugins

When a lifecycle has not started or has already ended, AutoDispose will send an error event with an OutsideLifecycleException to downstream consumers. If you want to customize this behavior, you can use AutoDisposePlugins to intercept these exceptions and rethrow something else or nothing at all.

Behavior

The created observer encapsulates the parameters of around to create a disposable, auto-disposing observer that acts as a lambda observer (pass-through) unless the underlying scope Maybe emits. Both scope end and upstream termination result in immediate disposable of both the underlying scope subscription and upstream disposable.

Support

Flowable, 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.

There is a separate Android artifact with extra APIs for Android components, such as support for View lifecycle binding.

Philosophy

Each scoper returns a subscribe proxy 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 1

This pattern is sort of possible in RxJava 1, but only on Subscriber (via onStart()) and CompletableObserver (which matches RxJava 2's API). We are aggressively migrating our internal code to RxJava 2, and do not plan to try to backport this to RxJava 1.

Download

Java:

Maven Central

compile 'com.uber.autodispose:autodispose:x.y.z'

Android extensions:

Maven Central

compile 'com.uber.autodispose:autodispose-android:x.y.z'

Kotlin extensions:

Maven Central

compile 'com.uber.autodispose:autodispose-kotlin:x.y.z'

Snapshots of the development version will eventually be available in Sonatype's snapshots repository.

License

Copyright (C) 2017 Uber Technologies

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
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