(#) `remember` calls must not return `Unit` !!! ERROR: `remember` calls must not return `Unit` This is an error. Id : `RememberReturnType` Summary : `remember` calls must not return `Unit` Severity : Error Category : Correctness Platform : Any Vendor : Jetpack Compose Identifier : androidx.compose.runtime Feedback : https://issuetracker.google.com/issues/new?component=612128 Min : Lint 8.0 and 8.1 Compiled : Lint 8.0 and 8.1 Artifact : [androidx.compose.runtime:runtime-android](androidx_compose_runtime_runtime-android.md.html) Affects : Kotlin and Java files and test sources Editing : This check runs on the fly in the IDE editor Implementation : [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/main/java/androidx/compose/runtime/lint/RememberDetector.kt) Tests : [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/test/java/androidx/compose/runtime/lint/RememberDetectorTest.kt) Copyright Year : 2020 A call to `remember` that returns `Unit` is always an error. This typically happens when using `remember` to mutate variables on an object. `remember` is executed during the composition, which means that if the composition fails or is happening on a separate thread, the mutated variables may not reflect the true state of the composition. Instead, use `SideEffect` to make deferred changes once the composition succeeds, or mutate `MutableState` backed variables directly, as these will handle composition failure for you. (##) Example Here is an example of lint warnings produced by this check: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text src/androidx/compose/runtime/foo/FooState.kt:14:Error: remember calls must not return Unit [RememberReturnType] remember { -------- src/androidx/compose/runtime/foo/FooState.kt:17:Error: remember calls must not return Unit [RememberReturnType] val unit = remember { -------- src/androidx/compose/runtime/foo/FooState.kt:21:Error: remember calls must not return Unit [RememberReturnType] remember(unitLambda) -------- src/androidx/compose/runtime/foo/FooState.kt:22:Error: remember calls must not return Unit [RememberReturnType] val unit2 = remember(unitLambda) -------- src/androidx/compose/runtime/foo/FooState.kt:28:Error: remember calls must not return Unit [RememberReturnType] remember(number) { -------- src/androidx/compose/runtime/foo/FooState.kt:31:Error: remember calls must not return Unit [RememberReturnType] val unit = remember(number) { -------- src/androidx/compose/runtime/foo/FooState.kt:35:Error: remember calls must not return Unit [RememberReturnType] remember(number, unitLambda) -------- src/androidx/compose/runtime/foo/FooState.kt:36:Error: remember calls must not return Unit [RememberReturnType] val unit2 = remember(number, unitLambda) -------- src/androidx/compose/runtime/foo/FooState.kt:42:Error: remember calls must not return Unit [RememberReturnType] remember(number1, number2) { -------- src/androidx/compose/runtime/foo/FooState.kt:46:Error: remember calls must not return Unit [RememberReturnType] val unit = remember(number1, number2) { -------- src/androidx/compose/runtime/foo/FooState.kt:51:Error: remember calls must not return Unit [RememberReturnType] remember(number1, number2, unitLambda) -------- src/androidx/compose/runtime/foo/FooState.kt:52:Error: remember calls must not return Unit [RememberReturnType] val unit2 = remember(number1, number2, unitLambda) -------- src/androidx/compose/runtime/foo/FooState.kt:58:Error: remember calls must not return Unit [RememberReturnType] remember(number1, number2, number3) { -------- src/androidx/compose/runtime/foo/FooState.kt:63:Error: remember calls must not return Unit [RememberReturnType] val unit = remember(number1, number2, number3) { -------- src/androidx/compose/runtime/foo/FooState.kt:69:Error: remember calls must not return Unit [RememberReturnType] remember(number1, number2, number3, unitLambda) -------- src/androidx/compose/runtime/foo/FooState.kt:70:Error: remember calls must not return Unit [RememberReturnType] val unit2 = remember(number1, number2, number3, unitLambda) -------- src/androidx/compose/runtime/foo/FooState.kt:76:Error: remember calls must not return Unit [RememberReturnType] remember(number1, number2, number3, flag) { -------- src/androidx/compose/runtime/foo/FooState.kt:81:Error: remember calls must not return Unit [RememberReturnType] val unit = remember(number1, number2, number3, flag) { -------- src/androidx/compose/runtime/foo/FooState.kt:87:Error: remember calls must not return Unit [RememberReturnType] remember(number1, number2, number3, flag, calculation = unitLambda) -------- src/androidx/compose/runtime/foo/FooState.kt:88:Error: remember calls must not return Unit [RememberReturnType] val unit2 = remember(number1, number2, number3, flag, calculation = unitLambda) -------- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is the source file referenced above: `src/androidx/compose/runtime/foo/FooState.kt`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers package androidx.compose.runtime.foo import androidx.compose.runtime.Composable import androidx.compose.runtime.remember class FooState { fun update(new: Int) {} } @Composable fun Test() { val state = remember { FooState() } remember { state.update(5) } val unit = remember { state.update(5) } val unitLambda: () -> Unit = {} remember(unitLambda) val unit2 = remember(unitLambda) } @Composable fun Test(number: Int) { val state = remember { FooState() } remember(number) { state.update(number) } val unit = remember(number) { state.update(number) } val unitLambda: () -> Unit = {} remember(number, unitLambda) val unit2 = remember(number, unitLambda) } @Composable fun Test(number1: Int, number2: Int) { val state = remember { FooState() } remember(number1, number2) { state.update(number1) state.update(number2) } val unit = remember(number1, number2) { state.update(number1) state.update(number2) } val unitLambda: () -> Unit = {} remember(number1, number2, unitLambda) val unit2 = remember(number1, number2, unitLambda) } @Composable fun Test(number1: Int, number2: Int, number3: Int) { val state = remember { FooState() } remember(number1, number2, number3) { state.update(number1) state.update(number2) state.update(number3) } val unit = remember(number1, number2, number3) { state.update(number1) state.update(number2) state.update(number3) } val unitLambda: () -> Unit = {} remember(number1, number2, number3, unitLambda) val unit2 = remember(number1, number2, number3, unitLambda) } @Composable fun Test(number1: Int, number2: Int, number3: Int, flag: Boolean) { val state = remember { FooState() } remember(number1, number2, number3, flag) { state.update(number1) state.update(number2) state.update(number3) } val unit = remember(number1, number2, number3, flag) { state.update(number1) state.update(number2) state.update(number3) } val unitLambda: () -> Unit = {} remember(number1, number2, number3, flag, calculation = unitLambda) val unit2 = remember(number1, number2, number3, flag, calculation = unitLambda) } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can also visit the [source code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/test/java/androidx/compose/runtime/lint/RememberDetectorTest.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, `RememberDetector.returnsUnit`. To report a problem with this extracted sample, visit https://issuetracker.google.com/issues/new?component=612128. (##) Including !!! This is not a built-in check. To include it, add the below dependency to your project. ``` // build.gradle.kts implementation("androidx.compose.runtime:runtime-android:1.8.0-alpha06") // build.gradle implementation 'androidx.compose.runtime:runtime-android:1.8.0-alpha06' // build.gradle.kts with version catalogs: implementation(libs.runtime.android) # libs.versions.toml [versions] runtime-android = "1.8.0-alpha06" [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: runtime-android = { module = "androidx.compose.runtime:runtime-android", version.ref = "runtime-android" } ``` 1.8.0-alpha06 is the version this documentation was generated from; there may be newer versions available. NOTE: These lint checks are **also** made available separate from the main library. You can also use `androidx.compose.runtime:runtime-lint:1.8.0-alpha06`. [Additional details about androidx.compose.runtime:runtime-android](androidx_compose_runtime_runtime-android.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("RememberReturnType") fun method() { remember(...) } ``` or ```java // Java @SuppressWarnings("RememberReturnType") void method() { remember(...); } ``` * Using a suppression comment like this on the line above: ```kt //noinspection RememberReturnType 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="RememberReturnType" 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 'RememberReturnType' } ``` In Android projects this should be nested inside an `android { }` block. * For manual invocations of `lint`, using the `--ignore` flag: ``` $ lint --ignore RememberReturnType ...` ``` * Last, but not least, using baselines, as discussed [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).