Generates an image with radial gradient.

UIImage+RadialGradient.swift

private extension UIImage {
    static func radialGradientImageWithSize(size: CGSize, outerColor: UIColor, innerColor: UIColor) -> UIImage {
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        let gradient = CGGradientCreateWithColors(colorSpace, [outerColor.CGColor, innerColor.CGColor], [1.0, 0.0])
        let center = CGPoint(x: size.width / 2.0, y: size.height / 2.0)

        UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.mainScreen().scale)
        let imageContext = UIGraphicsGetCurrentContext()

        CGContextDrawRadialGradient(imageContext, gradient, center, 0.0, center, size.width / 2.0, CGGradientDrawingOptions.DrawsAfterEndLocation)

        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
    }
}