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 : @"";
[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];
}
}