---
title: "SnapshotStateObserver"
description: "Helper class to efficiently observe snapshot state reads. See [observeReads] for more details.

NOTE: This class is not thread-safe, so implementations should not reuse observer between
different threads to avoid race conditions."
type: "class"
---

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


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

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


```kotlin
public class SnapshotStateObserver(private val onChangedExecutor: (callback: () -> Unit) -> Unit)
```


Helper class to efficiently observe snapshot state reads. See `observeReads` for more details.

NOTE: This class is not thread-safe, so implementations should not reuse observer between
different threads to avoid race conditions.


## Functions

```kotlin
public fun <T : Any> observeReads(
        scope: T,
        onValueChangedForScope: (T) -> Unit,
        block: () -> Unit,
    )
```


Executes `block`, observing state object reads during its execution.

The `scope` and `onValueChangedForScope` are associated with any values that are read so that
when those values change, `onValueChangedForScope` will be called with the `scope` parameter.

Observation can be paused with `Snapshot.withoutReadObservation`.

#### Parameters

| | |
| --- | --- |
| scope | value associated with the observed scope. |
| onValueChangedForScope | is called with the `scope` when value read within `block` has been changed. For repeated observations, it is more performant to pass the same instance of the callback, as `observedScopeMaps` grows with each new callback instance. |
| block | to observe reads within. |



```kotlin
public fun withNoObservations(block: () -> Unit)
```


Stops observing state object reads while executing `block`. State object reads may be
restarted by calling `observeReads` inside `block`.


```kotlin
public fun clear(scope: Any)
```


Clears all state read observations for a given `scope`. This clears values for all
`onValueChangedForScope` callbacks passed in `observeReads`.


```kotlin
public fun clearIf(predicate: (scope: Any) -> Boolean)
```


Remove observations using `predicate` to identify scopes to be removed. This is used when a
scope is no longer in the hierarchy and should not receive any callbacks.


```kotlin
public fun start()
```


Starts watching for state commits.


```kotlin
public fun stop()
```


Stops watching for state commits.


```kotlin
@TestOnly
    public fun notifyChanges(changes: Set<Any>, snapshot: Snapshot)
```


This method is only used for testing. It notifies that `changes` have been made on
`snapshot`.


```kotlin
public fun clear()
```


Remove all observations.