iOS - Download file only if modified (NSURL & NSData)

Question

I am downloading a bunch of image files from a server, and I want to ensure that they are downloaded only if they are newer. This method currently downloads the images just

Answer 1

If your server supports HTTP caching you can specify you want cached content with NSURLRequestReloadRevalidatingCacheData:

NSURLRequest* request = [NSURLRequest requestWithURL:thumbnailURL cachePolicy:NSURLRequestReloadRevalidatingCacheData timeoutInterval:20];
NSURLResponse* response;
NSError* error;
NSData* data = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
UIImage* image = [UIImage imageWithData:data];

For more info read the NSURLRequest documentation.

Answer 2

I ended up using this method to detect the modified date on the file: *Found on 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);
}