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# Action Identifiers

These constants are the predefined action identifiers used by [`action(forKey:)`](/documentation/QuartzCore/CALayer/action(forKey:)), [`add(_:forKey:)`](/documentation/QuartzCore/CALayer/add(_:forKey:)), [`defaultAction(forKey:)`](/documentation/QuartzCore/CALayer/defaultAction(forKey:)), [`removeAnimation(forKey:)`](/documentation/QuartzCore/CALayer/removeAnimation(forKey:)), Layer Filters, and the [`CAAction`](/documentation/QuartzCore/CAAction) protocol method [`run(forKey:object:arguments:)`](/documentation/QuartzCore/CAAction/run(forKey:object:arguments:)).

## Topics

### Constants

[`let kCAOnOrderIn: String`](/documentation/QuartzCore/kCAOnOrderIn)

The identifier that represents the action taken when a layer becomes visible, either as a result being inserted into the visible layer hierarchy or the layer is no longer set as hidden.

[`let kCAOnOrderOut: String`](/documentation/QuartzCore/kCAOnOrderOut)

The identifier that represents the action taken when the layer is removed from the layer hierarchy or is hidden.

[`let kCATransition: String`](/documentation/QuartzCore/kCATransition)

The identifier that represents a transition animation.


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