在Swift中获取UIImage的平均颜色

我最近试图将代码从此处转换为Swift。但是，无论图像如何，我都会保持白色。这是我的代码：

// Playground - noun: a place where people can play

import UIKit

extension UIImage {
    func averageColor() -> UIColor {
        var colorSpace = CGColorSpaceCreateDeviceRGB()
        var rgba: [CGFloat] = [0,0,0,0]
        var context = CGBitmapContextCreate(&rgba, 1, 1, 8, 4, colorSpace, CGBitmapInfo.fromRaw(CGImageAlphaInfo.PremultipliedLast.toRaw())!)
        rgba

        CGContextDrawImage(context, CGRectMake(0, 0, 1, 1), self.CGImage)

        if rgba[3] > 0 {
            var alpha = rgba[3] / 255
            var multiplier = alpha / 255
            return UIColor(red: rgba[0] * multiplier, green: rgba[1] * multiplier, blue: rgba[2] * multiplier, alpha: alpha)
        } else {
            return UIColor(red: rgba[0] / 255, green: rgba[1] / 255, blue: rgba[2] / 255, alpha: rgba[3] / 255)
        }
    }
}

var img = UIImage(data: NSData(contentsOfURL: NSURL(string: "http://upload.wikimedia.org/wikipedia/commons/c/c3/Aurora_as_seen_by_IMAGE.PNG")))

img.averageColor()

提前致谢。