Swift 渐变 image

// 扩展 UIImage 的 init 方法，获得渐变效果public extension UIImage {    convenience init?(gradientColors:[UIColor], size:CGSize = CGSizeMake(10, 10) )    {        UIGraphicsBeginImageContextWithOptions(size, true, 0)        let context = UIGraphicsGetCurrentContext()        let colorSpace = CGColorSpaceCreateDeviceRGB()        let colors = gradientColors.map {(color: UIColor) -> AnyObject! in return color.CGColor as AnyObject! } as NSArray        let gradient = CGGradientCreateWithColors(colorSpace, colors, nil)        // 第二个参数是起始位置，第三个参数是终止位置        CGContextDrawLinearGradient(context, gradient, CGPoint(x: 0, y: 0), CGPoint(x: size.width, y: 0), CGGradientDrawingOptions(rawValue: 0))        self.init(CGImage:UIGraphicsGetImageFromCurrentImageContext().CGImage!)        UIGraphicsEndImageContext()    }}