---
title: "CompositionRegistrationObserver"
description: "Observe when new compositions are added to a recomposer. This, combined with,
[CompositionObserver], allows observing when any composition is being performed.

This observer is registered with a [Recomposer] by calling [Recomposer.observe]."
type: "interface"
---

<div class='type'>Interface</div>


<a id='references'></a>

<div class='sourceset sourceset-common'>Common</div>



```kotlin
@ExperimentalComposeRuntimeApi
public interface CompositionRegistrationObserver
```


Observe when new compositions are added to a recomposer. This, combined with,
`CompositionObserver`, allows observing when any composition is being performed.

This observer is registered with a `Recomposer` by calling `Recomposer.observe`.


## Functions

```kotlin
public fun onCompositionRegistered(composition: ObservableComposition)
```


Called whenever a `Composition` is registered with a `Recomposer` for which this is an
observer. A Composition is registered with its Recomposer when it begins its initial
composition, before any content is added. When a `CompositionRegistrationObserver` is
registered, this method will be called for all the `Recomposer`'s currently known
composition.

This method is called on the same thread that the `Composition` being registered is being
composed on. During the initial dispatch, it is invoked on the same thread that the callback
is being registered on. Implementations of this method should be thread safe as they might be
called on an arbitrary thread.

#### Parameters

| | |
| --- | --- |
| composition | The `Composition` instance that is being registered with the recomposer. |



```kotlin
public fun onCompositionUnregistered(composition: ObservableComposition)
```


Called whenever a `Composition` is unregistered with a `Recomposer` for which this is an
observer. A Composition is unregistered from its Recomposer when the composition is
`disposed`. This method is called on the same thread that the
`Composition` being unregistered was composed on. Implementations of this method should be
thread safe as they might be called on an arbitrary thread.

#### Parameters

| | |
| --- | --- |
| composition | The `Composition` instance that is being unregistered with the recomposer. |