我在Swift中编程。我想使用CALayer和UIImage遮罩图像。我正在以编程方式创建蒙版图像。创建的蒙版图像是UIImage，当我单独查看它时可以正常工作。但是，当我将其用作遮罩时，整个屏幕会变成白色。我怀疑我的问题出在配置CALayer对象。多谢您的协助。谢谢！

class ViewController: UIViewController {

    @IBOutlet weak var imageView: UIImageView!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        var maskImageSize = CGSizeMake(self.imageView.frame.width, self.imageView.frame.height)
        UIGraphicsBeginImageContextWithOptions(maskImageSize, false, 0.0)

        var color = UIColor(white: 1.0, alpha: 1.0)
        color.setFill()
        var rect = CGRectMake(0, 0, self.imageView.frame.width, self.imageView.frame.height)
        UIRectFill(rect)

        color = UIColor(white: 0.0, alpha: 1.0)
        color.setFill()
        rect = CGRectMake((self.imageView.frame.width/2)-100, (self.imageView.frame.height/2)-100, 200, 200)
        UIRectFill(rect)

        var maskImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        var maskLayer = CALayer()
        maskLayer.contents = maskImage
        maskLayer.contentsRect = CGRectMake(0, 0, self.imageView.bounds.width, self.imageView.bounds.height)


        self.imageView.image = UIImage(named: "pictobemasked.png")

        self.imageView.layer.mask = maskLayer;
    }

}