iOS 通过URL获取图片尺寸

在开发过程中，我们经常需要通过网络请求加载图片。有时，我们需要在创建UIImageView时就知道请求图片的尺寸，根据图片尺寸来设置控件的Frame。如微信朋友圈，新浪微博，在图片未加载出来之前，控件的Frame就已经设置好。（图片宽高不一致100✘200或者 400 ✘ 150）。

• 通过服务器返回图片尺寸设置
  这种方法是体验最好的方法。在后台返回url的同时，返回图片宽高，再设置控件frame；
• 通过获取image文件头方法

//类方法 传入图片url  返回图片size
+(CGSize)downloadImageSizeWithURL:(id)imageURL{
    NSURL* URL = nil;
    //判断url类型
    if([imageURL isKindOfClass:[NSURL class]]){
        URL = imageURL;
    }
    if([imageURL isKindOfClass:[NSString class]]){
        URL = [NSURL URLWithString:imageURL];
    }
    if(URL == nil)
    return CGSizeZero;
    NSString* absoluteString = URL.absoluteString;
    //通过SDWebimage 查看图片是否已有缓存，如果有直接获取，如果没有，请求文件头
    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;
        }
    }    
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:URL];
    NSString* pathExtendsion = [URL.pathExtension lowercaseString];
    CGSize size = CGSizeZero;
    //区分加载图片的类型
    if([pathExtendsion isEqualToString:@"png"]){
        size =  [self downloadPNGImageSizeWithRequest:request];
    }else if([pathExtendsion isEqual:@"gif"]){
        size =  [self downloadGIFImageSizeWithRequest:request];
    }else{
        size = [self downloadJPGImageSizeWithRequest:request];
    }
    if(CGSizeEqualToSize(CGSizeZero, size)){
        NSData* data = [NSURLConnection sendSynchronousRequest:[NSURLRequest requestWithURL:URL] returningResponse:nil error:nil];
        UIImage* image = [UIImage imageWithData:data];
        if(image){
            [[SDImageCache sharedImageCache] storeImage:image recalculateFromImage:YES imageData:data forKey:URL.absoluteString toDisk:YES];
            size = image.size;
        }
    }
    return size;
}

//PNG格式的图片
+(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;
}

//GIF格式
+(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;
}

//JPG格式
+(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;
        }
    }
}

该方法通过请求少量字节的网络请求，获取到图片的文件头中的尺寸信息，效果还是比较理想的。
但是，如果在网络环境较差的情况下，这种方法又会有一个致命的弱点--同步请求。若自定义cell中用此方法获取图片尺寸， 较差的网络环境，TableView的不断滚动，可以明显感受到主线程阻塞带来的卡顿。

总结

如果在自定义cell中想预先获取加载图片的尺寸，最理想的方法还是与后台协商返回吧！
当然，还可以通过SDWebImage获取

[imageView sd_setImageWithURL: imageUrl completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
            //图片的宽
            CGFloat width = image.size.width;
            //图片的高
            CGFloat height = image.size.height;
        }];

异步加载完毕后，重新布局cell，设置高度，reload当前cell。