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# preparePreviewOfFile(at:completionHandler:)

Prepares the preview of a file at the specified file’s URL.

```
optional func preparePreviewOfFile(at url: URL, completionHandler handler: @escaping @Sendable ((any Error)?) -> Void)
```

## Parameters

`url`

The URL of the file to preview.

`handler`

A completion handler that notifies the platform that a preview is available. The operating system shows a loading spinner, so call the handler as soon as possible. You can call the completion handler asynchronously after returning from the callback.

## Discussion

The operating system calls this method only once from the main thread before it presents the previewing controller. To avoid blocking the main thread, don’t perform long-running or resource-intensive work.

When preparing and displaying the view controller, avoid holding open a file descriptors while the user is previewing the file.

---

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