实现思路
1、先生成一个矩形的UIBezierPath对象，设置该对象的一些属性等。用作外切的图片的边框。
2、再生成一个矩形的UIBezierPath对象，用作裁剪图片。
3、最后生成图片即可。
代码实例
+ (nonnull UIImage *)circleImageWithOriginImage:(UIImage *)image
borderColor:(UIColor *)borderColor
borderWidth:(CGFloat)borderWidth
corner:(CGFloat)corner
finalSize:(CGSize)finalSize
{
CGFloat scale = [UIScreen mainScreen].scale;
UIGraphicsBeginImageContextWithOptions(finalSize, NO, scale);
UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:CGRectMake(0, 0, finalSize.width, finalSize.height) cornerRadius:corner];
path.lineWidth = borderWidth;
[borderColor set];
[path addClip];
[path fill];
UIBezierPath *clicPath = [UIBezierPath bezierPathWithRoundedRect:CGRectMake(borderWidth, borderWidth, finalSize.width - 2 * borderWidth, finalSize.height - 2 * borderWidth) cornerRadius:corner];
[clicPath addClip];
[image drawInRect:CGRectMake(borderWidth, borderWidth, finalSize.width - 2 * borderWidth, finalSize.height - 2 * borderWidth)];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}
