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# USDPreviewSession.ChangeListDelegate

A protocol to provide shared undo/redo tracking in a USDPreviewSession.

```
@MainActor protocol ChangeListDelegate : AnyObject
```

## Topics

### Responding to changes

[`func willApplyChanges(instanceIdentifier: String, operationIdentifier: UInt)`](/documentation/SpatialPreview/USDPreviewSession/ChangeListDelegate/willApplyChanges(instanceIdentifier:operationIdentifier:))

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

[`func didApplyChanges(instanceIdentifier: String, operationIdentifier: UInt)`](/documentation/SpatialPreview/USDPreviewSession/ChangeListDelegate/didApplyChanges(instanceIdentifier:operationIdentifier:))

An event emitted to the ChangelistDelegate indicating the end of a USD change.

### Handling undo and redo

[`func onUndoRequest()`](/documentation/SpatialPreview/USDPreviewSession/ChangeListDelegate/onUndoRequest())

An undo has been requested in the session

[`func onRedoRequest()`](/documentation/SpatialPreview/USDPreviewSession/ChangeListDelegate/onRedoRequest())

An redo has been requested in the session

### Deprecated

[`func didApplyChanges(USDPreviewSession.Event)`](/documentation/SpatialPreview/USDPreviewSession/ChangeListDelegate/didApplyChanges(_:))

[`func willApplyChanges(USDPreviewSession.Event)`](/documentation/SpatialPreview/USDPreviewSession/ChangeListDelegate/willApplyChanges(_:))


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