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; } } }