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# previewInteraction(_:didUpdatePreviewTransition:ended:)

Informs the delegate of the progress through the preview phase of the preview interaction.

```
func previewInteraction(_ previewInteraction: UIPreviewInteraction, didUpdatePreviewTransition transitionProgress: CGFloat, ended: Bool)
```

## Parameters

`previewInteraction`

The preview interaction associated with the current user input.

`transitionProgress`

The progress through the preview phase of the transition. A <doc://com.apple.documentation/documentation/CoreFoundation/CGFloat-swift.struct> with a value from `0` to `1`.

`ended`

A Boolean whose value indicates whether the preview phase of the transition is complete.

## Discussion

This method is called repeatedly during the preview phase of the preview interaction. Use the supplied `transitionProgress` parameter to update the UI to reflect the progress of the interaction. For example, the *peek* effect in view controller preview transitions progressively blurs everything except the appropriate view.

The `ended` parameter is <doc://com.apple.documentation/documentation/Swift/false> throughout the preview phase and becomes <doc://com.apple.documentation/documentation/Swift/true> as the phase is completed. The preview interaction then transitions to the commit phase, so you should use this point to update the UI as required.

---

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