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# parser(_:didStartMappingPrefix:toURI:)

Sent by a parser object to its delegate the first time it encounters a given namespace prefix, which is mapped to a URI.

```
optional func parser(_ parser: XMLParser, didStartMappingPrefix prefix: String, toURI namespaceURI: String)
```

## Parameters

`parser`

A parser object.

`prefix`

A string that is a namespace prefix.

`namespaceURI`

A string that specifies a namespace URI.

## Discussion

The parser object sends this message only when namespace-prefix reporting is turned on through the [`shouldReportNamespacePrefixes`](/documentation/Foundation/XMLParser/shouldReportNamespacePrefixes) method.

---

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