import UIKit

open class GradientView: UIView {
    
    fileprivate struct AnimationKey {
        static let colors = "colors"
        static let colorChange = "ColorChange"
    }
    
    public enum Point {
        case left, right
        case top, bottom
        
        /// 上部分邊角
        case upperLeft, upperRight
        
        /// 下部分邊角
        case lowerLeft, lowerRight
        
        case custom(position: CGPoint)
        
        
        var point: CGPoint {
            switch self {
            case .left:                         return CGPoint(x: 0, y: 0.5)
            case .right:                        return CGPoint(x: 1, y: 0.5)
                
            case .top:                          return CGPoint(x: 0.5, y: 0)
            case .bottom:                       return CGPoint(x: 0.5, y: 1)
                
            case .upperLeft:                    return CGPoint(x: 0, y: 0)
            case .upperRight:                   return CGPoint(x: 1, y: 0)
                
            case .lowerLeft:                    return CGPoint(x: 0, y: 1)
            case .lowerRight:                   return CGPoint(x: 1, y: 1)
                
            case .custom(position: let point):  return point
            }
        }
    }
    
    
    open var startPoint: Point = .upperRight
    open var endPoint: Point = .lowerLeft
    
    open var animationDuration: CFTimeInterval = 3.0
    
    
    fileprivate let gradientLayer = CAGradientLayer()
    fileprivate var currentGradient = 0
    fileprivate var colors = [#colorLiteral(red: 0.6117647059, green: 0.1529411765, blue: 0.6901960784, alpha: 1), #colorLiteral(red: 1, green: 0.2509803922, blue: 0.5058823529, alpha: 1), #colorLiteral(red: 0.4823529412, green: 0.1215686275, blue: 0.6352941176, alpha: 1), #colorLiteral(red: 0.1254901961, green: 0.2980392157, blue: 1, alpha: 1), #colorLiteral(red: 0.1254901961, green: 0.6196078431, blue: 1, alpha: 1), #colorLiteral(red: 0.3529411765, green: 0.4705882353, blue: 0.4980392157, alpha: 1), #colorLiteral(red: 0.2274509804, green: 1, blue: 0.8509803922, alpha: 1)]
    
    
    open override func removeFromSuperview() {
        super.removeFromSuperview()
        gradientLayer.removeAllAnimations()
        gradientLayer.removeFromSuperlayer()
    }
    
}


extension GradientView {
    
    open func startAnimation() {
        gradientLayer.removeAllAnimations()
        setupGradientLayer()
        setupAnimation()
    }
    
    open func set(colors: [UIColor]) {
        guard colors.count > 0 else { return }
        self.colors = colors
    }
    
    open func add(color: UIColor) {
        colors.append(color)
    }
    
}


extension GradientView {
    
    fileprivate func setupGradientLayer() {
        gradientLayer.frame = self.bounds
        gradientLayer.colors = convertToCGColor()
        gradientLayer.startPoint = startPoint.point
        gradientLayer.endPoint = endPoint.point
        gradientLayer.drawsAsynchronously = true
        
        self.layer.insertSublayer(gradientLayer, at: 0)
    }
    
    fileprivate func convertToCGColor() -> [CGColor]? {
        guard colors.count > 0 else { return nil }
        
        return [colors[currentGradient % colors.count].cgColor,
                colors[(currentGradient + 1) % colors.count].cgColor]
    }
    
    fileprivate func setupAnimation() {
        currentGradient += 1
        let animation = CABasicAnimation(keyPath: AnimationKey.colors)
        animation.duration = animationDuration
        animation.toValue = convertToCGColor()
        animation.fillMode = kCAFillModeForwards
        animation.isRemovedOnCompletion = false
        animation.delegate = self
        
        gradientLayer.add(animation, forKey: AnimationKey.colorChange)
    }
    
}


extension GradientView: CAAnimationDelegate {
    
    public func animationDidStop(_ anim: CAAnimation, finished flag: Bool) {
        if flag {
            gradientLayer.colors = convertToCGColor()
            setupAnimation()
        }
    }
    
}