---
title: "setObserver"
description: "Observe the composition. Calling this twice on the same composition will implicitly dispose the
previous observer. the [CompositionObserver] will be called for this composition and all
sub-composition, transitively, for which this composition is a context. If [setObserver] is
called on a sub-composition, it will override the parent composition observer for itself and all
its sub-compositions."
type: "function"
---

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


<a id='references'></a>
<div class='sourceset sourceset-common'>Common</div>


```kotlin
@ExperimentalComposeRuntimeApi
public fun Composition.setObserver(observer: CompositionObserver): CompositionObserverHandle?
```


Observe the composition. Calling this twice on the same composition will implicitly dispose the
previous observer. the `CompositionObserver` will be called for this composition and all
sub-composition, transitively, for which this composition is a context. If `setObserver` is
called on a sub-composition, it will override the parent composition observer for itself and all
its sub-compositions.

#### Parameters

| | |
| --- | --- |
| observer | the observer that will be informed of composition events for this composition and all sub-compositions for which this composition is the composition context. Observing a composition will prevent the parent composition's observer from receiving composition events about this composition. |


#### Returns

| | |
| --- | --- |
|  | a handle that allows the observer to be disposed and detached from the composition. Disposing an observer for a composition with a parent observer will begin sending the events to the parent composition's observer. A `null` indicates the composition does not support being observed. |