各位大神们，有没有什么好的方法提前获取到 JPG 格式的网络图片的大小？

以下这段代码似乎没有什么作用,各位大神有没有好的解决方案？

+ (CGSize)downloadJPGImageSizeWithRequest:(NSMutableURLRequest*)request {
    [request setValue:@"bytes=0-209" forHTTPHeaderField:@"Range"];
    NSData* data = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];
    
    if ([data length] <= 0x58) {
        return CGSizeZero;
    }
    
    if ([data length] < 210) {// 肯定只有一个 DQT 字段
        short w1 = 0, w2 = 0;
        [data getBytes:&w1 range:NSMakeRange(0x60, 0x1)];
        [data getBytes:&w2 range:NSMakeRange(0x61, 0x1)];
        short w = (w1 << 8) + w2;
        short h1 = 0, h2 = 0;
        [data getBytes:&h1 range:NSMakeRange(0x5e, 0x1)];
        [data getBytes:&h2 range:NSMakeRange(0x5f, 0x1)];
        short h = (h1 << 8) + h2;
        return CGSizeMake(w, h);
    } else {
        short word = 0x0;
        [data getBytes:&word range:NSMakeRange(0x15, 0x1)];
        if (word == 0xdb) {
            [data getBytes:&word range:NSMakeRange(0x5a, 0x1)];
            if (word == 0xdb) {// 两个 DQT 字段
                short w1 = 0, w2 = 0;
                [data getBytes:&w1 range:NSMakeRange(0xa5, 0x1)];
                [data getBytes:&w2 range:NSMakeRange(0xa6, 0x1)];
                short w = (w1 << 8) + w2;
                short h1 = 0, h2 = 0;
                [data getBytes:&h1 range:NSMakeRange(0xa3, 0x1)];
                [data getBytes:&h2 range:NSMakeRange(0xa4, 0x1)];
                short h = (h1 << 8) + h2;
                return CGSizeMake(w, h);
            } else {// 一个 DQT 字段
                short w1 = 0, w2 = 0;
                [data getBytes:&w1 range:NSMakeRange(0x60, 0x1)];
                [data getBytes:&w2 range:NSMakeRange(0x61, 0x1)];
                short w = (w1 << 8) + w2;
                short h1 = 0, h2 = 0;
                [data getBytes:&h1 range:NSMakeRange(0x5e, 0x1)];
                [data getBytes:&h2 range:NSMakeRange(0x5f, 0x1)];
                short h = (h1 << 8) + h2;
                return CGSizeMake(w, h);
            }
        } else {
            return CGSizeZero;
        }
    }
}