iOS - 仅下载修改后的文件（NSURL 和 NSData）

我正在从服务器下载一堆图像文件，并且我想确保仅在更新时才下载它们。 目前，此方法可以很好地下载图像。但是，我不想在用户每次登录应用程序时浪费时间或精力重新下载图像。相反，我只想下载 A) 不存在 B) 服务器上比设备上更新的任何文件

这是我下载图像的方式： *图像 URL 与其关联的视频一起保存在 Core Data 中。 url是使用我构建的简单转换方法生成的（生成缩略图URL）

-(void)saveThumbnails{
    NSManagedObjectContext *context = [self managedObjectContextThumbnails];
    NSError *error;
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
                                   entityForName:@"Videos" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];
    NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
    NSLog(@"Videos: %i",fetchedObjects.count);
    if (fetchedObjects.count!=0) {
        for(Videos *currentVideo in fetchedObjects){
            // Get an image from the URL below
            NSURL *thumbnailURL = [self generateThumbnailURL:[currentVideo.videoID intValue]];

            UIImage *image = [[UIImage alloc] initWithData:[NSData dataWithContentsOfURL:thumbnailURL]];

            // Let's save the file into Document folder.
            // You can also change this to your desktop for testing. (e.g. /Users/kiichi/Desktop/)
            NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);//Find Application's Document Directory
            NSString *documentsDirectory = [paths objectAtIndex:0];
            NSString *dataPath = [documentsDirectory stringByAppendingPathComponent:@"DownloadedThumbnails"];
            //        NSString *dataPath = @"/Users/macminidemo/Desktop/gt";//DEBUG SAVING IMAGE BY SAVING TO DESKTOP FOLDER

            //Check if Sub-directory exists, if not, try to create it
            if (![[NSFileManager defaultManager] fileExistsAtPath:dataPath]){
                NSError* error;
                if([[NSFileManager defaultManager] createDirectoryAtPath:dataPath withIntermediateDirectories:NO attributes:nil error:&error]){
                    NSLog(@"New Folder Created!");
                }
                else
                {
                    NSLog(@"[%@] ERROR: attempting to write create new directory", [self class]);
                    NSAssert( FALSE, @"Failed to create directory maybe out of disk space?");
                }
            }
            NSArray *splitFilename = [[self generateThumbnailFilename:[currentVideo.videoID intValue]] componentsSeparatedByString:@"."];//Break Filename Extension Off (not always PNGs)
            NSString *subString = [splitFilename objectAtIndex:0];
            NSString *formattedFilename = [NSString stringWithFormat:@"%@~ipad.png",subString];
            NSString *localFilePath = [dataPath stringByAppendingPathComponent:formattedFilename];
            NSData *imageData = [NSData dataWithData:UIImagePNGRepresentation(image)];
            [imageData writeToFile:localFilePath atomically:YES];
            NSLog(@"Image: %@ Saved!",formattedFilename);
        }
    }
}

我最终使用此方法来检测文件的修改日期： *发现于HERE

-(bool)isThumbnailModified:(NSURL *)thumbnailURL forFile:(NSString *)thumbnailFilePath{
    // create a HTTP request to get the file information from the web server
    NSMutableURLRequest* request = [NSMutableURLRequest requestWithURL:thumbnailURL];
    [request setHTTPMethod:@"HEAD"];

    NSHTTPURLResponse* response;
    [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:nil];

    // get the last modified info from the HTTP header
    NSString* httpLastModified = nil;
    if ([response respondsToSelector:@selector(allHeaderFields)])
    {
        httpLastModified = [[response allHeaderFields]
                            objectForKey:@"Last-Modified"];
    }

    // setup a date formatter to query the server file's modified date
    // don't ask me about this part of the code ... it works, that's all I know :)
    NSDateFormatter* df = [[NSDateFormatter alloc] init];
    df.dateFormat = @"EEE',' dd MMM yyyy HH':'mm':'ss 'GMT'";
    df.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];
    df.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];

    // get the file attributes to retrieve the local file's modified date
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSDictionary* fileAttributes = [fileManager attributesOfItemAtPath:thumbnailFilePath error:nil];

    // test if the server file's date is later than the local file's date
    NSDate* serverFileDate = [df dateFromString:httpLastModified];
    NSDate* localFileDate = [fileAttributes fileModificationDate];

    NSLog(@"Local File Date: %@ Server File Date: %@",localFileDate,serverFileDate);
    //If file doesn't exist, download it
    if(localFileDate==nil){
        return YES;
    }
    return ([localFileDate laterDate:serverFileDate] == serverFileDate);
}