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？

+ (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;
        }
    }
}

getBytes

Data

range

short

12 条回复 • 2016-04-29 19:16:11 +08:00

sutra

2016-04-28 18:20:05 +08:00

方法 1. 在获得图片 URL 时的 API 请求中就告知客户端图片的 meta 信息。
方法 2. 使用 HTTP Head 请求：
$ curl -I 'https://www.baidu.com/img/bd_logo1.png'
HTTP/1.1 200 OK
Server: bfe/1.0.8.14
Date: Thu, 28 Apr 2016 10:21:46 GMT
Content-Type: image/png
Content-Length: 7877
Connection: keep-alive
P3P: CP=" OTI DSP COR IVA OUR IND COM "
Set-Cookie: BAIDUID=0D422F47D99BEF3218B73FC146CF8608:FG=1; expires=Fri, 28-Apr-17 10:21:46 GMT; max-age=31536000; path=/; domain=.baidu.com; version=1
Last-Modified: Wed, 03 Sep 2014 10:00:27 GMT
ETag: "1ec5-502264e2ae4c0"
Accept-Ranges: bytes
Cache-Control: max-age=315360000
Expires: Sun, 26 Apr 2026 10:21:46 GMT