最常用的設(shè)置圓角的方法：

view.layer.cornerRadius = 5
view.layer.masksToBounds = true

但是masksToBounds這個屬性會造成離屏渲染匾旭，這個屬性才是幀數(shù)下降的罪魁禍?zhǔn)住?/p>

如果想簡便的話可以設(shè)置

view.layer.shouldRasterize=true

這個屬性可以為圓角設(shè)置緩存，但是設(shè)置緩存也是需要時間的凉馆，如果有大量的大小不一的圓形視圖出現(xiàn)，這樣做依然會出現(xiàn)掉幀。

下面提供兩種思路解決當(dāng)頁面上需要設(shè)置的視圖較多時的解決方案：
一澜共、通過繪制方法向叉，將view的背景圖片重新去掉圓角繪制一遍
1.UIView。注:(該方法只返回UIView類型嗦董，其他類型會出問題)

extension UIView {
    func circleView(cornerRadius: CGFloat) -> UIView? {
        // 開始圖形上下文
        UIGraphicsBeginImageContextWithOptions(self.frame.size, false, 0)
        if let context = UIGraphicsGetCurrentContext() {
            let rect = CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)
            var path: UIBezierPath
            if let color = self.backgroundColor {
                CGContextSetLineWidth(context, 1)
                CGContextSetFillColorWithColor(context, color.CGColor)
                CGContextSetStrokeColorWithColor(context, color.CGColor)
            }
            
            if self.frame.size.width == self.frame.size.height {
                if cornerRadius == self.frame.size.width/2 {
                    path = UIBezierPath(arcCenter: self.center, radius: cornerRadius, startAngle: 0, endAngle: 2.0*CGFloat(M_PI), clockwise: true)
                }else {
                    path = UIBezierPath(roundedRect: rect, cornerRadius: cornerRadius)
                }
            }else {
                path = UIBezierPath(roundedRect: rect, cornerRadius: cornerRadius)
            }
            self.drawRect(rect)
            CGContextAddPath(context, path.CGPath)
            CGContextDrawPath(context, .FillStroke)
            CGContextClip(context)
            // 從上下文上獲取剪裁后的照片
            guard let uncompressedImage = UIGraphicsGetImageFromCurrentImageContext() else {
                UIGraphicsEndImageContext()
                return nil
            }
            // 關(guān)閉上下文
            UIGraphicsEndImageContext()
            let view = UIView()
            view.backgroundColor = UIColor.clearColor()
            view.frame = self.frame
            view.addSubview(UIImageView(image: uncompressedImage))
            return view
        }else {
            return nil
        }
    }
}

2母谎、UIImage(常用方法)

extension UIImage {
    func circleImage(cornerRadius: CGFloat, size: CGSize) -> UIImage? {
        let rect = CGRectMake(0, 0, size.width, size.height)
        UIGraphicsBeginImageContextWithOptions(size, false, 0)
        if let context = UIGraphicsGetCurrentContext() {
            var path: UIBezierPath
            if size.height == size.width {
                if cornerRadius == size.width/2 {
                    path = UIBezierPath(arcCenter: CGPointMake(size.width/2, size.height/2), radius: cornerRadius, startAngle: 0, endAngle: 2.0*CGFloat(M_PI), clockwise: true)
                }else {
                    path = UIBezierPath(roundedRect: rect, cornerRadius: cornerRadius)
                }
            }else {
                path = UIBezierPath(roundedRect: rect, cornerRadius: cornerRadius)
            }
            CGContextAddPath(context, path.CGPath)
            CGContextClip(context)
            self.drawInRect(rect)
            // 從上下文上獲取剪裁后的照片
            guard let uncompressedImage = UIGraphicsGetImageFromCurrentImageContext() else {
                UIGraphicsEndImageContext()
                return nil
            }
            // 關(guān)閉上下文
            UIGraphicsEndImageContext()
            return uncompressedImage
        }else {
            return nil
        }
    }
}

二、通過設(shè)置layer京革，可以設(shè)置切割哪個角奇唤，且設(shè)置邊框

let path = UIBezierPath.init(roundedRect: self.myLabel.bounds, byRoundingCorners: [.topRight , .bottomRight] , cornerRadii: self.myLabel.bounds.size);
let layer = CAShapeLayer.init();
layer.path = path.cgPath;
layer.lineWidth = 5;
layer.lineCap = kCALineCapSquare;
layer.strokeColor = UIColor.red.cgColor;
//  注意直接填充layer的顏色，不需要設(shè)置控件view的backgroundColor
layer.fillColor = UIColor.yellow.cgColor;
myLabel.layer.addSublayer(layer);