获取网络图片的尺寸

有时，需要在创建UIImageView或UIButton来显示图片之前需要提前知道图片的尺寸，根据图片尺寸创建对应大小的控件。但是对于网络图片来说，要想通过最优的方法获得尺寸就略微有点困难，大体思路就是下面这种：

如果有使用SDWebImage，则首先检查是否缓存过该图片，如果没有，先通过文件头获取图片大小(针对格式为png、gif、jpg文件获取其尺寸大小)，如果获取失败，则下载完整的图片data，然后计算大小，如果有使用SDWebImage，则使用SDWebImage缓存该图片。

如果有使用SDWebImage，则首先检查是否缓存过该图片，如果没有，先通过文件头获取图片大小(针对格式为png、gif、jpg文件获取其尺寸大小)，如果获取失败，则下载完整的图片data，然后计算大小，如果有使用SDWebImage，则使用SDWebImage缓存该图片。

+(CGSize)downloadImageSizeWithURL:(id)imageURL

{

    NSURL* URL = nil;

    if([imageURL isKindOfClass:[NSURL class]]){

        URL = imageURL;

    }

    if([imageURL isKindOfClass:[NSString class]]){

        URL = [NSURL URLWithString:imageURL];

    }

    if(URL == nil)

        return CGSizeZero;

#ifdef dispatch_main_sync_safe

    if([[SDImageCache sharedImageCache] diskImageExistsWithKey:absoluteString])

    {

        UIImage* image = [[SDImageCache sharedImageCache] imageFromMemoryCacheForKey:absoluteString];

        if(!image)

        {

            NSData* data = [[SDImageCache sharedImageCache] performSelector:@selector(diskImageDataBySearchingAllPathsForKey:) withObject:URL.absoluteString];

            image = [UIImage imageWithData:data];

        }

        if(image)

        {

            return image.size;

        }

    }

#endif

    NSMutableURLRequest *request = [[NSMutableURLRequest alloc]initWithURL:URL cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:1];

    

    NSString* pathExtendsion = [URL.pathExtension lowercaseString];

    CGSize size = CGSizeZero;

    if ([pathExtendsion rangeOfString:@"png"].location != NSNotFound) {

        size =  [self downloadPNGImageSizeWithRequest:request];

    }

    else if([pathExtendsion rangeOfString:@"gif"].location != NSNotFound)

    {

        size =  [self downloadGIFImageSizeWithRequest:request];

    }

    else{

        size = [self downloadJPGImageSizeWithRequest:request];

    }

    if(CGSizeEqualToSize(CGSizeZero, size))

    {

        NSData* data = [NSData dataWithContentsOfURL:URL];

        UIImage* image = [UIImage imageWithData:data];

        if(image)

        {

            //如果未使用SDWebImage，则忽略；缓存该图片

#ifdef dispatch_main_sync_safe

            [[SDImageCache sharedImageCache] storeImage:image recalculateFromImage:YES imageData:data forKey:URL.absoluteString toDisk:YES];

#endif

            size = image.size;

        }

    }

    //过滤掉不符合大小的图片，大图太大浪费流量，用户体验不好

    if (size.height > 2048 || size.height <= 0 || size.width > 2048 || size.width <= 0 ) {

        return CGSizeZero;

    }

    else

    {

        return size;

    }

}

+(CGSize)downloadPNGImageSizeWithRequest:(NSMutableURLRequest*)request

{

    [request setValue:@"bytes=16-23" forHTTPHeaderField:@"Range"];

    NSData* data = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];

    if(data.length == 8)

    {

        int w1 = 0, w2 = 0, w3 = 0, w4 = 0;

        [data getBytes:&w1 range:NSMakeRange(0, 1)];

        [data getBytes:&w2 range:NSMakeRange(1, 1)];

        [data getBytes:&w3 range:NSMakeRange(2, 1)];

        [data getBytes:&w4 range:NSMakeRange(3, 1)];

        int w = (w1 << 24) + (w2 << 16) + (w3 << 8) + w4;

        int h1 = 0, h2 = 0, h3 = 0, h4 = 0;

        [data getBytes:&h1 range:NSMakeRange(4, 1)];

        [data getBytes:&h2 range:NSMakeRange(5, 1)];

        [data getBytes:&h3 range:NSMakeRange(6, 1)];

        [data getBytes:&h4 range:NSMakeRange(7, 1)];

        int h = (h1 << 24) + (h2 << 16) + (h3 << 8) + h4;

        return CGSizeMake(w, h);

    }

    return CGSizeZero;

} 

+(CGSize)downloadGIFImageSizeWithRequest:(NSMutableURLRequest*)request

{

    [request setValue:@"bytes=6-9" forHTTPHeaderField:@"Range"];

    NSData* data = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];

    if(data.length == 4)

    {

        short w1 = 0, w2 = 0;

        [data getBytes:&w1 range:NSMakeRange(0, 1)];

        [data getBytes:&w2 range:NSMakeRange(1, 1)];

        short w = w1 + (w2 << 8);

        short h1 = 0, h2 = 0;

        [data getBytes:&h1 range:NSMakeRange(2, 1)];

        [data getBytes:&h2 range:NSMakeRange(3, 1)];

        short h = h1 + (h2 << 8);

        return CGSizeMake(w, h);

    }

    return CGSizeZero;

} 

+(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;

        }

    }

}