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# scheme

The scheme URL component, or nil if not present.

```
var scheme: String? { get set }
```

## Discussion

For example, in the URL `http://www.example.com/index.html`, the scheme is `http`.

If you attempt to set the scheme to an invalid scheme string, this class throws an exception.

> Note:
> The term “protocol” is also sometimes used when talking about network-based URL schemes. However, not all URL schemes are networking protocols—`data://` URLs, for example.

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