// 性能不好，适合圆角图形数量比较少的情况

    UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(, , , )];

   //设置圆角

    imageView.layer.cornerRadius = imageView.frame.size.width / ;

   //将多余的部分切掉

    imageView.layer.masksToBounds = YES;

// 设置圆形图片(放到分类中使用)

- (UIImage *)circularImage {

    UIGraphicsBeginImageContextWithOptions(self.size, NO, 0.0);

    // 获取上下文

    CGContextRef ctr = UIGraphicsGetCurrentContext();

    // 设置圆形

    CGRect rect = CGRectMake(, , self.size.width, self.size.height);

    CGContextAddEllipseInRect(ctr, rect);

    // 裁剪

    CGContextClip(ctr);

    // 将图片画上去

    [self drawInRect:rect];

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return image;

}

// 圆角图片

- (UIImage *)cornerRoundedImage {

    //进行画图

    UIGraphicsBeginImageContextWithOptions(self.size, NO, [UIScreen mainScreen].scale);

    //使用贝塞尔曲线画出一个圆角图，圆角半径为12

    CGRect rect = CGRectMake(, , self.size.width, self.size.height);

    [[UIBezierPath bezierPathWithRoundedRect:rect

                                cornerRadius:] addClip];

    [self drawInRect:rect];

    UIImage* image = UIGraphicsGetImageFromCurrentImageContext();

    //结束画图

    UIGraphicsEndImageContext();

    return image;

}

//通过图片Data数据第一个字节 来获取图片扩展名

- (NSString *)contentTypeForImageData:(NSData *)data {

    uint8_t c;

    [data getBytes:&c length:];

    switch (c) {

        case 0xFF:

            return @"jpeg";

        case 0x89:

            return @"png";

        case 0x47:

            return @"gif";

        case 0x49:

        case 0x4D:

            return @"tiff";

        case 0x52:

            if ([data length] < ) {

                return nil;

            }

            NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(, )] encoding:NSASCIIStringEncoding];

            if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {

                return @"webp";

            }

            return nil;

    }

    return nil;

}

NSString *path = @"http://xxx.xxx.xxx/xxx.png";

NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:path]];

//调用

NSString *string = [self contentTypeForImageData:data];

//输出结果为 png

NSLog(@"%@",string);