GlossyView.swift

import UIKit


class ViewController: UIViewController {
    
    @IBOutlet weak var glossyView: GlossyView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    override func viewDidAppear(_ animated: Bool) {
        self.glossyView.layer.cornerRadius = 16.0
        self.glossyView.layer.masksToBounds = true
    }

}


class GlossyView: UIView {
    
    
    class MotionDetector: UIInterpolatingMotionEffect {
        
        var manipulatedView: GlossyView? = nil
        
        override func keyPathsAndRelativeValues(forViewerOffset viewerOffset: UIOffset) -> [String : Any]? {
            
            guard let glossyView = self.manipulatedView else {
                return super.keyPathsAndRelativeValues(forViewerOffset: viewerOffset)
            }
            
            let x = viewerOffset.horizontal
            let y = viewerOffset.vertical
            
            let xPoint = CGPoint(x: x, y: y)
            let yPoint = CGPoint(x: 0.5, y: 1.0)
            
            glossyView.configureGloss(from: xPoint, to: yPoint)
            
            return super.keyPathsAndRelativeValues(forViewerOffset: viewerOffset)
        }
        
    }
    
    
    var colors: [CGColor]? = [UIColor.cyan.cgColor, UIColor.blue.cgColor]
    
    
    override func awakeFromNib() {
        
        super.awakeFromNib()
        
        let detector = MotionDetector()
        detector.manipulatedView = self
        self.addMotionEffect(detector)
        
    }
    
    
    
    func configureGloss(from startPoint: CGPoint, to endPoint: CGPoint) {
        
        let identifier = "glossy-layer"
        
        func createGradient() {
            let gradient = CAGradientLayer()
            gradient.name = identifier
            gradient.frame = self.bounds
            gradient.startPoint = startPoint
            gradient.endPoint = endPoint
            gradient.colors = self.colors
            self.layer.addSublayer(gradient)
        }
        
        guard let sublayers = self.layer.sublayers else {
            createGradient()
            return
        }
        
        if let glossyIndex = sublayers.index(where: { $0.name == identifier }) {
            if let glossyLayer = sublayers[glossyIndex] as? CAGradientLayer {
                glossyLayer.startPoint = startPoint
                glossyLayer.endPoint = endPoint
            }
        } else {
            createGradient()
        }

    }
    
    
}