<!--
{
  "availability" : [
    "macOS: -"
  ],
  "documentType" : "symbol",
  "framework" : "AppKit",
  "identifier" : "/documentation/AppKit/NSAnimation/start(when:reachesProgress:)",
  "metadataVersion" : "0.1.0",
  "role" : "Instance Method",
  "symbol" : {
    "kind" : "Instance Method",
    "modules" : [
      "AppKit"
    ],
    "preciseIdentifier" : "c:objc(cs)NSAnimation(im)startWhenAnimation:reachesProgress:"
  },
  "title" : "start(when:reachesProgress:)"
}
-->

# start(when:reachesProgress:)

Starts running the animation represented by the receiver when another animation reaches a specific progress mark.

```
func start(when animation: NSAnimation, reachesProgress startProgress: NSAnimation.Progress)
```

## Parameters

`animation`

The other `NSAnimation` object with which the receiver is linked.

`startProgress`

A `float` value (typed as NSAnimationProgress) that specifies a progress mark of the other animation.

## Discussion

This method links the running of two animations together. You can set only one `NSAnimation` object as a start animation and one as a stop animation at any one time. Setting a new start animation removes any animation previously set.

## See Also

[`start()`](/documentation/AppKit/NSAnimation/start())

Starts the animation represented by the receiver.


---

Copyright &copy; 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)