---
title: "willApplyChanges(instanceIdentifier:operationIdentifier:)"
framework: spatialpreview
role: symbol
role_heading: Instance Method
platforms: [macOS 27.0+]
path: "spatialpreview/usdpreviewsession/changelistdelegate/willapplychanges(instanceidentifier:operationidentifier:)"
---

# willApplyChanges(instanceIdentifier:operationIdentifier:)

An event emitted to the ChangeListDelegate indicating the start of a USD change.

## Declaration

```swift
@MainActor func willApplyChanges(instanceIdentifier: String, operationIdentifier: UInt)
```

## Parameters

- `instanceIdentifier`: The Spatial Preview receiver instance making the change.
- `operationIdentifier`: Multiple changes with the same operation identifier can be grouped together.