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# CAAction

An interface that allows instances to respond to actions triggered by a Core Animation layer change.

```
protocol CAAction
```

## Overview

When queried with an action identifier (a key path, an external action name, or a predefined action identifier) a layer returns the appropriate action object–which must implement the [`CAAction`](/documentation/QuartzCore/CAAction) protocol–and sends it a [`run(forKey:object:arguments:)`](/documentation/QuartzCore/CAAction/run(forKey:object:arguments:)) message.

## Topics

### Responding to an action

[`run(forKey:object:arguments:)`](/documentation/QuartzCore/CAAction/run(forKey:object:arguments:))

Called to trigger the action specified by the identifier.


---

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