Question

The purpose of my code is to compare the modification dates of a server file and a local file, in case that the server file is newer, it will download it.

My first attempt was to use a synchronous request using the code from http://iphoneincubator.com/blog/server-communication/how-to-download-a-file-only-if-it-has-been-updated

But it didn’t worked.
After that I’ve been struggling to find the solution, tried asynchronous request, tried different codes I found around stackoverflow, google, etc. but nothing works.

If in terminal I do curl -I <url-to-file> I get the header values so I know is not a server problem.

This is the code I’m struggling with right now (It’s written in Appdelegate.m)

- (void)downloadFileIfUpdated {
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL: url
                                                       cachePolicy: NSURLRequestReloadIgnoringLocalCacheData
                                                   timeoutInterval: 10];
[request setHTTPMethod:@"HEAD"];

NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES];
  if(!connection) {
    NSLog(@"connection failed");
  } else {
    NSLog(@"connection succeeded");
  }
}



- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [self downloadFileIfUpdated]
}



#pragma mark NSURLConnection delegate methods
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
NSString *lastModifiedString = nil;
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)response;
  if ([response respondsToSelector:@selector(allHeaderFields)]) {
    lastModifiedString = [[response allHeaderFields] objectForKey:@"Last-Modified"];
  }
  [Here is where the formatting-date-code and downloading would take place]
}

Right now, as it is, it gives me the error No visible @interface for 'NSURLResponse' declares de selector 'allHeaderFields'.

When I use the synchronous approach the error is that NSLog(@"%@",lastModifiedString)returns (null).

PS: If there is a better way I can explain myself or the code, please let me know.

UPDATE

The URL I’m using is of type ftp://and that may be the problem of why I don’t get any HEADERS. But I can’t figure out how to do it then.

Answer 1

Change your code to this… in the ‘if’ conditional, you were checking response instead of httpResponse:

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
NSString *lastModifiedString = nil;
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)response;
  if ([httpResponse respondsToSelector:@selector(allHeaderFields)]) {
    lastModifiedString = [[httpResponse allHeaderFields] objectForKey:@"Last-Modified"];
  }
  // [Here is where the formatting-date-code and downloading would take place]
}

… and once you feel comfortable that the response is going to always be an NSHTTPURLResponse, you could probably just get rid of the conditional statement:

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
  NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)response;
  NSString *lastModifiedString = [[httpResponse allHeaderFields] objectForKey:@"Last-Modified"];
  // [Here is where the formatting-date-code and downloading would take place]
}