(#) subscribeOn called with the main thread scheduler !!! ERROR: subscribeOn called with the main thread scheduler This is an error. Id : `SubscribeOnMain` Summary : subscribeOn called with the main thread scheduler Severity : Error Category : Correctness Platform : Any Vendor : slack Identifier : slack-lint Contact : https://github.com/slackhq/slack-lints Feedback : https://github.com/slackhq/slack-lints Min : Lint 8.7+ Compiled : Lint 8.7+ Artifact : [com.slack.lint:slack-lint-checks](com_slack_lint_slack-lint-checks.md.html) Since : 0.1.0 Affects : Kotlin and Java files and test sources Editing : This check runs on the fly in the IDE editor Implementation : [Source Code](https://github.com/slackhq/slack-lints/tree/main/slack-lint-checks/src/main/java/slack/lint/rx/RxSubscribeOnMainDetector.kt) Tests : [Source Code](https://github.com/slackhq/slack-lints/tree/main/slack-lint-checks/src/test/java/slack/lint/rx/RxSubscribeOnMainDetectorTest.kt) Copyright Year : 2021 Calling `subscribeOn(AndroidSchedulers.mainThread())` will cause the code ran at subscription time to be executed on the main thread - that is, code above this line. Typically this is not actually desired, and instead you want to use observeOn(AndroidSchedulers.mainThread()) which will cause the code below this line to be run on the main thread (eg the code inside your subscribe() block). !!! Tip This lint check has an associated quickfix available in the IDE. (##) Example Here is an example of lint warnings produced by this check: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text src/com/slack/lint/Foo.java:8:Error: This will make the code for the initial subscription (above this line) run on the main thread. You probably want observeOn(AndroidSchedulers.mainThread()). [SubscribeOnMain] obs.subscribeOn(AndroidSchedulers.mainThread()); ------------------------------------------- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here are the relevant source files: `src/io/reactivex/rxjava3/android/schedulers/AndroidSchedulers.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package io.reactivex.rxjava3.android.schedulers; import io.reactivex.rxjava3.core.Scheduler; public final class AndroidSchedulers { public static Scheduler mainThread() { return null; } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ `src/com/slack/lint/Foo.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package com.slack.lint; import io.reactivex.rxjava3.core.Observable; import io.reactivex.rxjava3.android.schedulers.AndroidSchedulers; public class Foo { public void bar(Observable obs) { obs.subscribeOn(AndroidSchedulers.mainThread()); } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can also visit the [source code](https://github.com/slackhq/slack-lints/tree/main/slack-lint-checks/src/test/java/slack/lint/rx/RxSubscribeOnMainDetectorTest.kt) for the unit tests for this check to see additional scenarios. The above example was automatically extracted from the first unit test found for this lint check, `RxSubscribeOnMainDetector.subscribeOnMain_fullyQualified_fails_java`. To report a problem with this extracted sample, visit https://github.com/slackhq/slack-lints. (##) Including !!! This is not a built-in check. To include it, add the below dependency to your project. This lint check is included in the lint documentation, but the Android team may or may not agree with its recommendations. ``` // build.gradle.kts lintChecks("com.slack.lint:slack-lint-checks:0.9.0") // build.gradle lintChecks 'com.slack.lint:slack-lint-checks:0.9.0' // build.gradle.kts with version catalogs: lintChecks(libs.slack.lint.checks) # libs.versions.toml [versions] slack-lint-checks = "0.9.0" [libraries] # For clarity and text wrapping purposes the following declaration is # shown split up across lines, but in TOML it needs to be on a single # line (see https://github.com/toml-lang/toml/issues/516) so adjust # when pasting into libs.versions.toml: slack-lint-checks = { module = "com.slack.lint:slack-lint-checks", version.ref = "slack-lint-checks" } ``` 0.9.0 is the version this documentation was generated from; there may be newer versions available. [Additional details about com.slack.lint:slack-lint-checks](com_slack_lint_slack-lint-checks.md.html). (##) Suppressing You can suppress false positives using one of the following mechanisms: * Using a suppression annotation like this on the enclosing element: ```kt // Kotlin @Suppress("SubscribeOnMain") fun method() { subscribeOn(...) } ``` or ```java // Java @SuppressWarnings("SubscribeOnMain") void method() { subscribeOn(...); } ``` * Using a suppression comment like this on the line above: ```kt //noinspection SubscribeOnMain problematicStatement() ``` * Using a special `lint.xml` file in the source tree which turns off the check in that folder and any sub folder. A simple file might look like this: ```xml <?xml version="1.0" encoding="UTF-8"?> <lint> <issue id="SubscribeOnMain" severity="ignore" /> </lint> ``` Instead of `ignore` you can also change the severity here, for example from `error` to `warning`. You can find additional documentation on how to filter issues by path, regular expression and so on [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html). * In Gradle projects, using the DSL syntax to configure lint. For example, you can use something like ```gradle lintOptions { disable 'SubscribeOnMain' } ``` In Android projects this should be nested inside an `android { }` block. * For manual invocations of `lint`, using the `--ignore` flag: ``` $ lint --ignore SubscribeOnMain ...` ``` * Last, but not least, using baselines, as discussed [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).