# RememberObserver

Artifact: [androidx.compose.runtime:runtime](https://developer.android.com/jetpack/androidx/releases/compose-runtime) [View Source](https://cs.android.com/search?q=file:androidx/compose/runtime/RememberObserver.kt+class:androidx.compose.runtime.RememberObserver) Added in [1.0.0](https://developer.android.com/jetpack/androidx/releases/compose-runtime#1.0.0) Common/AllAndroid/JVMNative/C/iOSJavaScript
- Cmn

  ```
  interface RememberObserver
  ```
Known direct subclasses [StyleResolver](https://developer.android.com/reference/kotlin/androidx/compose/foundation/style/StyleResolver)

|---|---|
| `https://developer.android.com/reference/kotlin/androidx/compose/foundation/style/StyleResolver` | A resolver that will resolve the properties of a given style lambda. |

*** ** * ** ***

Objects implementing this interface are notified when they are initially used in a composition and when they are no longer being used.

An object is `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onRemembered()` by the composition if it is `https://developer.android.com/reference/kotlin/androidx/compose/runtime/remember.composable#remember(kotlin.Function0)`ed in at least one place in a composition. Compose may implicitly `https://developer.android.com/reference/kotlin/androidx/compose/runtime/remember.composable#remember(kotlin.Function0)` an object if doing so is required to restart the composition later, such as for composable function parameters. An object is `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onForgotten()` when it is no longer `https://developer.android.com/reference/kotlin/androidx/compose/runtime/remember.composable#remember(kotlin.Function0)`ed anywhere in that composition. If a single instance is `https://developer.android.com/reference/kotlin/androidx/compose/runtime/remember.composable#remember(kotlin.Function0)`ed in more than one location in the same composition, its `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onRemembered()` and `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onForgotten()` will be called for each location in the composition.

When objects implementing this interface is remembered and forgotten together, the order of `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onForgotten()` is guaranteed to be called in the opposite order of `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onRemembered()`. For example, if two objects, A and B are `https://developer.android.com/reference/kotlin/androidx/compose/runtime/remember.composable#remember(kotlin.Function0)`ed together, A followed by B, `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onRemembered()` will be called first on A then on B. If they forgotten together then `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onForgotten()` will be called on B first then on A.

Implementations of `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver` should generally not expose those object references outside of their immediate usage sites. A `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver` reference that is propagated to multiple parts of a composition might remain present in the composition for longer than expected if it is remembered (explicitly or implicitly) elsewhere, and a `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver` that appears more once can have its callback methods called multiple times in no meaningful order and on multiple threads.

An object remembered in only one place in only one composition is guaranteed to,

1. have either `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onRemembered()` or `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onAbandoned()` called

2. if `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onRemembered()` is called, `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onForgotten()` will eventually be called

## Summary

| ### Public functions |
|---|---|---|
| `https://kotlinlang.org/api/core/kotlin-stdlib/kotlin/-unit/index.html` | `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onAbandoned()()` Called when this object is returned by the callback to `remember` but is not successfully remembered by a composition. | Cmn |
| `https://kotlinlang.org/api/core/kotlin-stdlib/kotlin/-unit/index.html` | `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onForgotten()()` Called when this object is forgotten by a composition. | Cmn |
| `https://kotlinlang.org/api/core/kotlin-stdlib/kotlin/-unit/index.html` | `https://developer.android.com/reference/kotlin/androidx/compose/runtime/RememberObserver#onRemembered()()` Called when this object is successfully remembered by a composition. | Cmn |

## Public functions

### onAbandoned

Cmn Added in [1.0.0](https://developer.android.com/jetpack/androidx/releases/compose-runtime#1.0.0)

```
fun onAbandoned(): Unit
```

Called when this object is returned by the callback to `remember` but is not successfully remembered by a composition.

### onForgotten

Cmn Added in [1.0.0](https://developer.android.com/jetpack/androidx/releases/compose-runtime#1.0.0)

```
fun onForgotten(): Unit
```

Called when this object is forgotten by a composition. This method is called on the composition's **apply thread.**

### onRemembered

Cmn Added in [1.0.0](https://developer.android.com/jetpack/androidx/releases/compose-runtime#1.0.0)

```
fun onRemembered(): Unit
```

Called when this object is successfully remembered by a composition. This method is called on the composition's **apply thread.**