iOS 绘制圆角的注意事项

1、设置简单，性能差别不明显，简单圆角场景下推荐使用。

       // 设置 layer 的 cornerRadius
      view.layer.masksToBounds = YES;
      view.layer.cornerRadius = imgSize.width / 2;

苹果在iOS9后优化了 cornerRadius 的绘图方式，此种方式不再需要离屏渲染。

2、在位图尺寸很大，数量很多的情况下，但要注意内存警告，最好配合缓存机制使用，避免因内存溢出而崩溃。

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                UIImage *image = view.image;
                image = [image drawCircleImage];
                dispatch_async(dispatch_get_main_queue(), ^{
                    view.image = image;
                });
            });
////////////////////////
@implementation UIImage (RoundedCorner)

- (UIImage *)drawCircleImage {
    CGFloat side = MIN(self.size.width, self.size.height);
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(side, side), false, [UIScreen mainScreen].scale);
    CGContextAddPath(UIGraphicsGetCurrentContext(),
                     [UIBezierPath bezierPathWithOvalInRect:CGRectMake(0, 0, side, side)].CGPath);
    CGContextClip(UIGraphicsGetCurrentContext());
    CGFloat marginX = -(self.size.width - side) / 2.f;
    CGFloat marginY = -(self.size.height - side) / 2.f;
    [self drawInRect:CGRectMake(marginX, marginY, self.size.width, self.size.height)];
    CGContextDrawPath(UIGraphicsGetCurrentContext(), kCGPathFillStroke);
    UIImage *output = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return output;
}