UIImageView添加圆角
最直接的方法就是使用如下属性设置：

imgView.layer.cornerRadius = 10;// 这一行代码是很消耗性//
imgView.clipsToBounds = YES;//这是离屏渲染（off-screen-rendering），消耗性能的使用到了CPU//

给UIImage添加生成圆角图片的扩展API：这是on-screen-rendering

-(UIImage *)imageWithCornerRadius:(CGFloat)radius {
CGRect rect = (CGRect){0.f, 0.f, self.size};
​
UIGraphicsBeginImageContextWithOptions(self.size, NO, UIScreen.mainScreen.scale);
CGContextAddPath(UIGraphicsGetCurrentContext(),
[UIBezierPath bezierPathWithRoundedRect:rect cornerRadius:radius].CGPath);
CGContextClip(UIGraphicsGetCurrentContext());
​
[self drawInRect:rect];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
​
UIGraphicsEndImageContext();
​
return image;
}