Ios 透明UIImage在navBar中不可传输

我已经创建了一个白色图像，0.2上有一个白色alpha。但是，当我添加它时，它在导航栏中似乎并不透明。它是完全白色的。我对tabBar做了同样的事情，效果很好。我做错了什么。这在viewControllers ViewDidLoad中：

self.navigationController?.navigationBar.setBackgroundImage(UIImage(named: "Navbar"), forBarMetrics: UIBarMetrics.Default)

let newNavBar = UIImage(named: "navBar")!.alpha(0.2)

self.navigationController?.navigationBar.setBackgroundImage(UIImage(named: "newNavBar"), forBarMetrics: UIBarMetrics.Default)

---

当不使用UIImageView时，控制UIImage alpha值的一种简单方法是将图像的初始alpha值设置为1，然后添加此UIImage扩展：

extension UIImage{

func alpha(value:CGFloat)->UIImage
{
    UIGraphicsBeginImageContextWithOptions(self.size, false, 0.0)

    let ctx = UIGraphicsGetCurrentContext();
    let area = CGRect(x: 0, y: 0, width: self.size.width, height: self.size.height);

    CGContextScaleCTM(ctx, 1, -1);
    CGContextTranslateCTM(ctx, 0, -area.size.height);
    CGContextSetAlpha(ctx, value);
    CGContextDrawImage(ctx, area, self.CGImage);

    let newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return newImage;
}
}

…然后在ViewDidLoad中：

self.navigationController?.navigationBar.setBackgroundImage(UIImage(named: "Navbar"), forBarMetrics: UIBarMetrics.Default)

let newNavBar = UIImage(named: "navBar")!.alpha(0.2)

self.navigationController?.navigationBar.setBackgroundImage(UIImage(named: "newNavBar"), forBarMetrics: UIBarMetrics.Default)