After seeing the animation when toggling Dark Theme in Slack for iOS, I thought I'd try my hand at replicating the animation. After that I wondered how I could spice up the animation by doing something different.

This effect is achieved by leveraging a few techniques:

• Create a snapshot of the current window's view using snapshotView(afterScreenUpdates:)
• Add the snapshotView to the window's rootViewController.view
• Create a layer mask shape using CAShapeLayer
• Add the layer mask to the snapshotView
• Use CABasicAnimation to animate the mask's path
• Finally, remove the snapshotView when the animation finishes

By default CABasicAnimation returns to the initial state once the animation finishes. You can prevent this behaviour by setting:

animation.fillMode = .forwards
animation.isRemovedOnCompletion = false

Now the animation will stay at it's end state.

Additionally we use a thin closure wrapper around CAAnimationDelegate's animationDidStop(_:finished:) method, to create a more modern completion block:

animation.delegate = AnimationDelegate { finished in
    snapshotView.removeFromSuperview()
    animation.delegate = nil
    completion?(finished)
}

Linear	Circular