SDWebImage 处理url链接中图片更新问题

1. [imageview sd_setImageWithURL:[NSURL URLWithString:imageUrlString] placeholderImage:[UIImage imageWithContentsOfFile:[[NSBundle mainBundle]pathForResource:kDefaultAvatarIcon
ofType:kPngName]] options:SDWebImageRefreshCached completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {

        

    }];

将options
设置为SDWebImageRefreshCached

2.appdelegate中didfinishlunch
中添加如下方法

-(void)sdWEbDownloaderRegister{

    SDWebImageDownloader *imgDownloader = SDWebImageManager.sharedManager.imageDownloader;

    imgDownloader.headersFilter  = ^NSDictionary *(NSURL *url, NSDictionary *headers) {

        

        NSFileManager *fm = [[NSFileManager alloc] init];

        NSString *imgKey = [SDWebImageManager.sharedManager cacheKeyForURL:url];

        NSString *imgPath = [SDWebImageManager.sharedManager.imageCache defaultCachePathForKey:imgKey];

        NSDictionary *fileAttr = [fm attributesOfItemAtPath:imgPath error:nil];

        

        NSMutableDictionary *mutableHeaders = [headers mutableCopy];

        

        NSDate *lastModifiedDate = nil;

        

        if (fileAttr.count > 0) {

            if (fileAttr.count > 0) {

                lastModifiedDate = (NSDate *)fileAttr[NSFileModificationDate];

            }

            

        }

        NSDateFormatter *formatter = [[NSDateFormatter alloc] init];

        formatter.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];

        formatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];

        formatter.dateFormat = @"EEE, dd MMM yyyy HH:mm:ss z";

        

        NSString *lastModifiedStr = [formatter stringFromDate:lastModifiedDate];

        lastModifiedStr = lastModifiedStr.length > 0 ? lastModifiedStr : @"";

 
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      [mutableHeaders setValue:lastModifiedStr forKey:@"If-Modified-Since"];

        

        return mutableHeaders;

    };

}

加上以上代码，sdwebimage在下载图片前会将上次获取的Etag、Last-Modified和本次需要下载的图片的Etag、Last-Modified比对，若一致不会下载，若不一致说明有更新

Etag由服务器端生成，客户端通过If-Match或者说If-None-Match这个条件判断请求来验证资源是否修改。常见的是使用If-None-Match.请求一个文件的流程可能如下：

====第一次请求===

1.客户端发起 HTTP GET
请求一个文件；

2.服务器处理请求，返回文件内容和一堆Header，当然包括Etag(例如"2e681a-6-5d044840")(假设服务器支持Etag生成和已经开启了Etag).状态码200

====第二次请求===

1.客户端发起 HTTP GET
请求一个文件，注意这个时候客户端同时发送一个If-None-Match头，这个头的内容就是第一次请求时服务器返回的Etag：2e681a-6-5d044840

2.服务器判断发送过来的Etag和计算出来的Etag匹配，因此If-None-Match为False，不返回200，返回304，客户端继续使用

sdwebimage有一段代码如下：

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {

    

    //'304 Not Modified' is an exceptional one

    if (![response respondsToSelector:@selector(statusCode)] || ([((NSHTTPURLResponse *)response) statusCode] < 400 && [((NSHTTPURLResponse *)response) statusCode] != 304))
{

        NSInteger expected = response.expectedContentLength > 0 ? (NSInteger)response.expectedContentLength : 0;

        self.expectedSize = expected;

        if (self.progressBlock) {

            self.progressBlock(0, expected);

        }

        self.imageData = [[NSMutableData alloc] initWithCapacity:expected];

        self.response = response;

        dispatch_async(dispatch_get_main_queue(), ^{

            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadReceiveResponseNotification object:self];

        });

    }

    else {

        NSUInteger code = [((NSHTTPURLResponse *)response) statusCode];

        

        //This is the case when server returns '304 Not Modified'. It means that remote image is not changed.

        //In case of 304 we need just cancel the operation and return cached image from the cache.

        if (code == 304) {

            [self cancelInternal];

        } else {

            [self.connection cancel];

        }

        dispatch_async(dispatch_get_main_queue(), ^{

            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];

        });

        if (self.completedBlock) {

            self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:[((NSHTTPURLResponse *)response) statusCode] userInfo:nil], YES);

        }

        CFRunLoopStop(CFRunLoopGetCurrent());

        [self done];

    }

}