GradientView.swift

//
//  GradientView.swift
//  Notion
//
//  Created by Cody Robertson on 3/16/16.
//  Copyright © 2016 Lift, Inc. All rights reserved.
//

import Foundation
import UIKit


class GradientView: UIView {
    
    // MARK: -
    
    enum GradientType {
        case Linear
        case Radial
    }
    
    // MARK: -
    // MARK: Public Properties
    
    var type: GradientType {
        didSet {
            setNeedsDisplay()
            layoutIfNeeded()
        }
    }
    
    var colors: [UIColor] {
        didSet {
            setNeedsDisplay()
            layoutIfNeeded()
        }
    }
    
    // MARK: -
    // MARK: Private Properties
    
    private var locations: [CGFloat] {
        return colors
            .enumerate()
            .map { (index, element) in
                return (1.0 / CGFloat(colors.count - 1)) * CGFloat(index)
        }
    }
    
    private var components: [CGFloat] {
        return colors
            .map {
                let components = CGColorGetComponents($0.CGColor)
                let colorComponents = Array(count: 4, repeatedValue: 0.0)
                
                return colorComponents
                    .enumerate()
                    .map { (index, element) in components[index] }
            }
            .reduce([], combine: +)
    }
    
    // MARK: -
    // MARK: Initialization
    
    init(type: GradientType, colors: [UIColor]) {
        self.type = type
        self.colors = colors
        super.init(frame: CGRectZero)
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("Use init(colors:) instead.")
    }
    
    // MARK: -
    // MARK: Methods
    
    override func drawRect(rect: CGRect) {
        guard colors.count > 0 else { return }
        
        let context = UIGraphicsGetCurrentContext()
        let colorspace = CGColorSpaceCreateDeviceRGB()
        let gradient = CGGradientCreateWithColorComponents(colorspace, components, locations, locations.count)
        
        switch type {
        case .Linear:
            let start = CGPoint(x: rect.midX, y: 0)
            let end = CGPoint(x: rect.midX, y: rect.maxY)
            
            CGContextDrawLinearGradient(context, gradient, start, end, .DrawsAfterEndLocation)
        case .Radial:
            let center = CGPoint(x: rect.midX, y: rect.midY)
            let radius = max(rect.midX, rect.midY) * 0.75
            
            CGContextDrawRadialGradient(context, gradient, center, 0, center, radius, .DrawsAfterEndLocation)
        }
    }
    
}