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# animation(_:valueForProgress:)

Requests a custom curve value for the current progress value.

```
nonisolated optional func animation(_ animation: NSAnimation, valueForProgress progress: NSAnimation.Progress) -> Float
```

## Parameters

`animation`

An `NSAnimation` object that is running.

`progress`

A `float` value (typed as `NSAnimationProgress`) that indicates a progress mark of `animation`. This value is always between 0.0 and 1.0.

## Return Value

A `float` value representing a custom curve.

## Discussion

The delegate can compute and return a custom curve value for the given progress value.  If the delegate does not implement this method, `NSAnimation` computes the current curve value.

The animation:valueForProgress: message is sent to the delegate when an `NSAnimation` object receives a [`currentValue`](/documentation/AppKit/NSAnimation/currentValue) message. The value the delegate returns is used as the value of [`currentValue`](/documentation/AppKit/NSAnimation/currentValue); if there is no delegate, or it doesn’t implement animation:valueForProgress:, `NSAnimation` computes and returns the current value. `NSAnimation` does not invoke [`currentValue`](/documentation/AppKit/NSAnimation/currentValue)itself, but subclasses might.

See the description of [`currentValue`](/documentation/AppKit/NSAnimation/currentValue) for more information.

## See Also

[`var currentValue: Float`](/documentation/AppKit/NSAnimation/currentValue)

The current value of the animation effect, based on the current progress


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