Rx-error-prone: Catch common Rx-Java errors at compile time.

It is a set of checks for RxJava code. The code below will emit the value on computation and not the main thread like one might think.

public class SampleVerifier {
  public static void main(String[] args) {
    Observable observable = Observable.just(1).observeOn(Schedulers.io()).delay(1, TimeUnit.SECONDS);
  }
}

$ ./gradlew clean :sample:build
> Task :sample:compileJava FAILED
/Users/harshitbangar/Desktop/RIBs/rx/sample/src/main/java/SampleVerifier.java:7: error: [DefaultSchedulerCheck] Using a default scheduler
    Observable observable = Observable.just(1).observeOn(Schedulers.io()).delay(1, TimeUnit.SECONDS);
                                                                               ^
    (see https://bitbucket.org/littlerobots/rxlint/)
1 error

How to use

Follow the setup instruction in the gradle error-prone repo and just add to your build.gradle

apt "com.github.bangarharshit:rxbase:0.0.3"

By default all the checks are enabled. To disable check for code/unit-tests use the snippet below.

tasks.withType(JavaCompile) {
    if (!name.toLowerCase().contains("test")) {
        // For actual code.
        options.compilerArgs += ["-Xep:DefaultSchedulerCheck:OFF"]
    } else {
        // For unit tests.
        options.compilerArgs += [ '-Xep:DefaultSchedulerCheck:OFF', '-Xep:DanglingSubscriptionCheck:OFF']
    }
}

The list of checks

1. DefaultSchedulerCheck
2. DanglingSubscriptionCheck
3. OnCreateCheck (for Rx1 create methods without backpressure)
4. CacheCheck
5. SubscribeOnErrorMissingCheck
6. SubscriptionInConstructorCheck

SubscribeOnErrorMissingCheck

Check if the subscriber is handling the onError() callback.

Every observable can report errors. Not implementing onError will throw an exception at runtime, which can be hard to debug when the error is thrown on a Scheduler that is not the invoking thread.

If a subscriber doesn't handle the error and the Observable emits an error, it is wrapped into an OnErrorNotImplementedException and routed to the RxJavaPlugins.onError handler.

Another advantage of implementing onError is a better stack-trace. You can include tags to enrich the trace.

DefaultSchedulerCheck

Operators like delay or interval run on Schedulers.computation() by default. It can be extremely confusing for the scenarios like - someObservable.observeOn(AndroidSchedulers.MainThread()).interval(2, TimeUnit.Seconds) which will emit the value on computation and not the main thread like one might think.

An overloaded version is available where a custom scheduler can be provided and should be used.

DanglingSubscriptionCheck

Check that all the subscriptions are assigned to disposables. Any unassigned subscription means the subscription can't be disposed() or unSubscribe() which can lead to memory leaks and crashes in runtime.

CacheCheck

cache() is a hard to understand operator. Use replay() instead. cache() is same as replay().autoConnect(). replay() is much more flexible, less confusing and can be combined with different multi-casting operations. Check Dan Lew's talk on the same.

OnCreateCheck (for Rx1)

<T>Observable.create(rx.Observable.OnSubscribe<T>) and unSafeCreate doesn't handle backpressure. These operators construct an Observable in an unsafe manner, that is, unsubscription and backpressure handling is the responsibility of the OnSubscribe implementation.

SubscriptionInConstructorCheck

Subscription in constructor leads to unsafe object creation and is similar to starting a thread in the constructor. Check the safe construction technique article for more details.

Use any of the other overloaded create methods or just/fromCallable any other generator.

Rxlint

Rxlint is a similar tool and can be used to detect these errors.

Contributing

Feel free to help with issues, and PR. Go through the issues with tag help wanted and just send a PR. Comment on the issue that you are interested to avoid duplication.

Tutorials:

1. How to write a new check

Google code style is automatically applied as a part of git hook so no explicit steps are required.

Acknowledgement

Uber/Nullaway for steps related to publishing.

License

MIT License

Copyright (c) 2017 bangarharshit

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