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How to make UIViewPropertyAnimator scrub with a custom curve: scrubsLinearly

Swift version: 5.1

Paul Hudson    @twostraws   

UIViewPropertyAnimator is an incredibly easy way to build custom animations on iOS, making it trivial to support scrubbable, reversible animations. However, by default scrubbing behaves differently from your regular animation: scrubbing always happens linearly, even if your animation was performed using a curve such as ease-in-ease-out.

This is actually a feature rather than a bug, and it’s incredibly intelligent – iOS automatically maps your curve onto a linear animation at the same point, and does the same in reverse, because it can be confusing for users if they try to scrub an animation and it doesn’t follow their finger.

On the flip side, of you’re scrubbing an animation programmatically this behavior isn’t always desirable. Fortunately, iOS 11 introduced a scrubsLinearly property for UIViewPropertyAnimator: set this to false to make scrubbing retain your custom animation curve.

The default setting is true, which makes UIViewPropertyAnimator continue working like it did on iOS 10. To change it, just add this code:

animator.scrubsLinearly = false
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