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# App extensions

Extend your app’s basic functionality to other parts of the system.

## Topics

### Extension support

Use these Foundation classes to manage interactions between your app extension and its hosting app.

  <doc://com.apple.documentation/documentation/Foundation/NSExtensionRequestHandling>

  <doc://com.apple.documentation/documentation/Foundation/NSExtensionContext>

### Document provider

  <doc://com.apple.documentation/documentation/FileProvider/NSFileProviderExtension>

[`UIDocumentPickerExtensionViewController`](/documentation/UIKit/UIDocumentPickerExtensionViewController)

The principal class for the Document Picker View Controller extension.

### Custom keyboard

[`UITextDocumentProxy`](/documentation/UIKit/UITextDocumentProxy)

An object that provides textual context to a custom keyboard.

[`UIInputViewAudioFeedback`](/documentation/UIKit/UIInputViewAudioFeedback)

A property that enables a custom input or keyboard accessory view to play standard keyboard input clicks.

[`UIInputViewController`](/documentation/UIKit/UIInputViewController)

The primary view controller for a custom keyboard app extension.

[`UILexicon`](/documentation/UIKit/UILexicon)

A read-only array of term pairs, each in a lexicon entry object, for a custom keyboard.

[`UILexiconEntry`](/documentation/UIKit/UILexiconEntry)

A read-only term pair, available within a lexicon object, for a custom keyboard.


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