Grabbing a web file’s last modified date

If you rely on updating files stored on a server, you want a way to find out when a file has been updated on that server, so you can download the latest version.

The easiest way I’ve found is using NSURLConnection’s sendSynchronousRequest:returningResponse:error method, but using a HEAD http method instead of GET. Here’s a simple example:

+ (NSDate*)getFileDate:(NSString*)httpFilePath {
    NSURL *url=[NSURL URLWithString:httpFilePath];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    
    [request setHTTPMethod:@"HEAD"];
    NSHTTPURLResponse *response;
    [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:NULL];
    if (response==nil) return nil;
	
    NSDate *lastModifiedDate=nil;
    NSString *lastModified=[[response allHeaderFields] objectForKey:@"Last-Modified"];
    @try {
        NSDateFormatter *df = [[[NSDateFormatter alloc] init] autorelease];
        df.dateFormat = @"EEE',' dd MMM yyyy HH':'mm':'ss 'GMT'";
        df.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
        lastModifiedDate = [df dateFromString:lastModified];
    }
    @catch (NSException * e) {
        NSLog(@"Error formatting Last-Modified date: %@ (%@)", lastModified, [e description]);
    }
    return lastModifiedDate;
}

For completeness sake, to grab a file, we’d use code like this:

+ (NSData*)getFileData:(NSString*)httpFilePath {
    NSURL *url=[NSURL URLWithString:httpFilePath];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    
    [request setHTTPMethod:@"GET"];
    NSHTTPURLResponse *response;
    NSData *result=[NSURLConnection sendSynchronousRequest:request returningResponse:&response error:NULL];
    if (response==nil || response.statusCode>400) return nil;

    return result;
}

You can do something similar asynchronously, but its a bit more complicated.