在 UIView 外部绘制阴影

背景

我有一个UIView具有以下属性：

• 阿尔法 = 1
• 背景颜色 = 白色，不透明度 0.35
• 圆角
• 阴影

Code

这就是我创建阴影的方法:(UIView扩大）

self.layer.masksToBounds = false
self.layer.shadowColor = UIColor.darkGrayColor().CGColor
self.layer.shadowOffset = CGSizeMake(0, 5)
self.layer.shadowOpacity = 0.35
self.layer.shadowPath = UIBezierPath(roundedRect: self.bounds, cornerRadius: self.layer.cornerRadius).CGPath

Results

结果如下：

...虽然我不想看到这样的视图下面的阴影：

Question

如何仅在视图外部绘制阴影，使其在其下方不可见？

提前致谢！

这可能比需要的稍微复杂一些，但这里有一个解决方案。

Extend UIView用以下方法：

extension UIView {
    // Note: the method needs the view from which the context is taken as an argument.
    func dropShadow(superview: UIView) {
        // Get context from superview
        UIGraphicsBeginImageContext(self.bounds.size)
        superview.drawViewHierarchyInRect(CGRect(x: -self.frame.minX, y: -self.frame.minY, width: superview.bounds.width, height: superview.bounds.height), afterScreenUpdates: true)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        // Add a UIImageView with the image from the context as a subview
        let imageView = UIImageView(frame: self.bounds)
        imageView.image = image
        imageView.layer.cornerRadius = self.layer.cornerRadius
        imageView.clipsToBounds = true
        self.addSubview(imageView)

        // Bring the background color to the front, alternatively set it as UIColor(white: 1, alpha: 0.2)
        let brighter = UIView(frame: self.bounds)
        brighter.backgroundColor = self.backgroundColor ?? UIColor(white: 1, alpha: 0.2)
        brighter.layer.cornerRadius = self.layer.cornerRadius
        brighter.clipsToBounds = true
        self.addSubview(brighter)

        // Set the shadow
        self.layer.masksToBounds = false
        self.layer.shadowColor = UIColor.darkGrayColor().CGColor
        self.layer.shadowOffset = CGSizeMake(0, 5)
        self.layer.shadowOpacity = 0.35
        self.layer.shadowPath = UIBezierPath(roundedRect: self.bounds, cornerRadius: self.layer.cornerRadius).CGPath
    }
}

用法，考虑到背景视图的命名view:

let shadowView = UIView(frame: CGRect(x: 100, y: 100, width: 300, height: 200))
shadowView.layer.cornerRadius = 15.0
shadowView.dropShadow(view)

view.addSubview(shadowView)

结果是这样的视图：

Note: the dropShadow方法可以not被称为来自viewDidLoad因为这会导致图形上下文出现问题。所以，使用这个方法viewWillAppear最早得到上述结果。

这是背景视图的代码，以防万一有人想在游乐场进行测试：

let view = UIView(frame: CGRect(x: 0, y: 0, width: 500, height: 400))
view.backgroundColor = UIColor.clearColor()

let color1 = UIColor(hue: 0.39, saturation: 0.7, brightness: 1.0, alpha: 1.0).CGColor
let color2 = UIColor(hue: 0.51, saturation: 0.9, brightness: 0.6, alpha: 1.0).CGColor

let gradient = CAGradientLayer()
gradient.frame = view.frame
gradient.colors = [color1, color2]
gradient.startPoint = CGPoint(x: 0, y: 0)
gradient.endPoint = CGPoint(x: 1, y: 1)
view.layer.insertSublayer(gradient, atIndex: 0)