Gradient.swift

import UIKit
import Foundation

class GradientView: UIView, Gradientable {}

protocol Gradientable: class {}

extension Gradientable where Self: UIView {
    private func getCGPoints (for position: GradientPosition, start at: StartAt) -> (CGPoint, CGPoint)? {
        switch position{
        case .diagonal:
            if at == .lowerRight {
                return (CGPoint(x: 0, y: 0), CGPoint(x: 1, y: 1))
            } else if at == .lowerLeft {
                return (CGPoint(x: 1, y: 0), CGPoint(x: 0, y: 1))
            } else if at == .upperRight {
                return (CGPoint(x: 0, y: 1), CGPoint(x: 1, y: 0))
            } else if at == .upperLeft {
                return (CGPoint(x: 1, y: 1), CGPoint(x: 0, y: 0))
            } else {
                print("Unaccepted diagonal position")
                return nil
            }
        case .horizontal:
            if at == .right {
                return (CGPoint(x: 0, y: 0.5), CGPoint(x: 1, y: 0.5))
            } else if at == .left {
                return (CGPoint(x: 1, y: 0.5), CGPoint(x: 0, y: 0.5))
            } else {
                print("Unaccepted horizontal position")
                return nil
            }
        case .vertical:
            if at == .top {
                return (CGPoint(x: 0.5, y: 1), CGPoint(x: 0.5, y: 0))
            } else if at == .bottom {
                return (CGPoint(x: 0.5, y: 0), CGPoint(x: 0.5, y: 1))
            } else {
                print("Unaccepted vertical position")
                return nil
            }
        }
    }
    
    func applyGradient (position: GradientPosition, startAt: StartAt) {
        let gradientLayer = CAGradientLayer()
        let cgPoints = getCGPoints(for: position, start: startAt)!
        gradientLayer.startPoint = cgPoints.0
        gradientLayer.endPoint = cgPoints.1
        gradientLayer.frame = self.bounds
        gradientLayer.locations = [0.0]
        gradientLayer.colors = [Colors.blue, Colors.lightBlue, Colors.lighterBlue,].map { $0.cgColor as CGColor }
        self.layer.insertSublayer(gradientLayer, at: 0)
    }
    
    func applyGradient (colors: [UIColor], position: GradientPosition, startAt: StartAt) {
        let gradientLayer = CAGradientLayer()
        let cgPoints = getCGPoints(for: position, start: startAt)!
        gradientLayer.startPoint = cgPoints.0
        gradientLayer.endPoint = cgPoints.1
        gradientLayer.frame = self.bounds
        gradientLayer.locations = [0.0]
        gradientLayer.colors = colors.map { $0.cgColor as CGColor }
        self.layer.insertSublayer(gradientLayer, at: 0)
    }
}

enum GradientPosition {
    case vertical
    case horizontal
    case diagonal
}

enum StartAt {
    // Vertical
    case top
    case bottom
    // Horizontal
    case left
    case right
    // Diagonal
    case upperRight
    case upperLeft
    case lowerRight
    case lowerLeft
}