Update.swift

func update(progress: CGFloat) {
    let shiftedProgress = (progress - AnimationState.start.rawValue) / (AnimationState.full.rawValue - AnimationState.start.rawValue)
    let height: CGFloat = foregroundButton.bounds.height * shiftedProgress
    let rect = CGRect(x: 0, y: foregroundButton.bounds.height - height, width: foregroundButton.bounds.width, height: height)
    let path = UIBezierPath(rect: rect).cgPath
    shapeLayer.path = path
}