UIView animateWithDuration doesn't animate cornerRadius variation

tl;dr: Corner radius is not animatable in animateWithDuration:animations:.

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What the documentation says about view animations.

As the section on Animations in the "View Programming Guide for iOS" says

Both UIKit and Core Animation provide support for animations, but the level of support provided by each technology varies. In UIKit, animations are performed using UIView objects

The full list of properties that you can animate using either the older

[UIView beginAnimations:context:];
[UIView setAnimationDuration:];
// Change properties here...
[UIView commitAnimations];

or the newer

[UIView animateWithDuration:animations:];

(that you are using) are:

• frame
• bounds
• center
• transform (CGAffineTransform, not the CATransform3D)
• alpha
• backgroundColor
• contentStretch

As you can see, cornerRadius is not in the list.

Some confusion

UIView animations is really only meant for animating view properties. What confuses people is that you can also animate the same properties on the layer inside the UIView animation block, i.e. the frame, bounds, position, opacity, backgroundColor. So people see layer animations inside animateWithDuration and believe that they can animate any view property in there.

The same section goes on to say:

In places where you want to perform more sophisticated animations, or animations not supported by the UIView class, you can use Core Animation and the view’s underlying layer to create the animation. Because view and layer objects are intricately linked together, changes to a view’s layer affect the view itself.

A few lines down you can read the list of Core Animation animatable properties where you see this one:

• The layer’s border (including whether the layer’s corners are rounded)

So to animate the cornerRadius you need to use Core Animation as you've already said in your updated question (and answer). I just added tried to explain why its so.

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Some extra clarification

When people read the documentations that says that animateWithDuration is the recommended way of animating it is easy to believe that it is trying to replace CABasicAnimation, CAAnimationGroup, CAKeyframeAnimation, etc. but its really not. Its replacing the beginAnimations:context: and commitAnimations that you seen above.

I use this extension to animate change of corner radius:

extension UIView
{
    func animateCornerRadius(from: CGFloat, to: CGFloat, duration: CFTimeInterval)
    {
        CATransaction.begin()
        let animation = CABasicAnimation(keyPath: "cornerRadius")
        animation.timingFunction = CAMediaTimingFunction(name: CAMediaTimingFunctionName.linear)
        animation.fromValue = from
        animation.toValue = to
        animation.duration = duration
        CATransaction.setCompletionBlock { [weak self] in
            self?.layer.cornerRadius = to
        }
        layer.add(animation, forKey: "cornerRadius")
        CATransaction.commit()
    }
}

A corner radius variation can be animated but the only way to do so is to use a CABasicAnimation. Hope this helps somebody out there.