Update BWWalkthroughViewController.swift

[relisiuol]

Added walkthroughDidScroll method to BWWalkthroughViewControllerDelegate.

[relisiuol]

I use this delegate method to animate background color.

Returned percent variable, isn't a real percentage.

50 : current page visible only
Up to 50 : scroll to left (show previous page)
51 to 100 : scroll to right (show next page)

Assume you have defined walkthrough as BWWalkthroughViewController and walkthroughPagesBackgroundColor as [UIColor].

extension MyCustomViewController: BWWalkthroughViewControllerDelegate {
    func walkthroughDidScroll(percent: CGFloat) {
        guard let walkthrough = walkthrough,
            walkthrough.numberOfPages == walkthroughPagesBackgroundColor.count else {
            return
        }
        if percent == 50 {
            walkthrough.view.backgroundColor = walkthroughPagesBackgroundColor[walkthrough.currentPage]
        } else if percent > 50,
            (walkthrough.currentPage + 1) < walkthroughPages.count {
            let fromColor = walkthroughPagesBackgroundColor[walkthrough.currentPage]
            let toColor = walkthroughPagesBackgroundColor[(walkthrough.currentPage + 1)]
            let percentage = (percent - 50) * 2
            walkthrough.view.backgroundColor = fromColor.toColor(toColor, percentage: percentage).withAlphaComponent(0.8)
        } else if percent < 50,
            (walkthrough.currentPage - 1) >= 0 {
            let fromColor = walkthroughPagesBackgroundColor[walkthrough.currentPage]
            let toColor = walkthroughPagesBackgroundColor[(walkthrough.currentPage - 1)]
            let percentage = (50 - percent) * 2
            walkthrough.view.backgroundColor = fromColor.toColor(toColor, percentage: percentage)
        }
    }
}
extension UIColor {
    func toColor(_ color: UIColor, percentage: CGFloat) -> UIColor {
        let percentage = max(min(percentage, 100), 0) / 100
        switch percentage {
            case 0: return self
            case 1: return color
            default:
                var (r1, g1, b1, a1): (CGFloat, CGFloat, CGFloat, CGFloat) = (0, 0, 0, 0)
                var (r2, g2, b2, a2): (CGFloat, CGFloat, CGFloat, CGFloat) = (0, 0, 0, 0)
                guard self.getRed(&r1, green: &g1, blue: &b1, alpha: &a1) else { return self }
                guard color.getRed(&r2, green: &g2, blue: &b2, alpha: &a2) else { return self }
                return UIColor(red: CGFloat(r1 + (r2 - r1) * percentage),
                               green: CGFloat(g1 + (g2 - g1) * percentage),
                               blue: CGFloat(b1 + (b2 - b1) * percentage),
                               alpha: CGFloat(a1 + (a2 - a1) * percentage))
        }
    }
}