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# VNRequestCompletionHandler

A type alias to encapsulate the syntax for the completion handler the system calls after the request finishes processing.

```
typealias VNRequestCompletionHandler = (VNRequest, (any Error)?) -> Void
```

## Discussion

Vision executes the completion handler on the same queue that it executes the request; however, this queue differs from the one where you called [`perform(_:)`](/documentation/Vision/VNImageRequestHandler/perform(_:)).

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