NSKeyedArchiver to serialize to GPX XML

Question

I have a Location class that contains a lat/lon as well as an elevation and timestamp:

@interface Location : NSObject <NSCoding> {
 double lon; 
 double lat; 
 double ele;
 NSDate *time;
 NSDateFormatter *gpxDateFormatter;
}

I want to be able to serialize an NSMutableArray containing an arbitrary number of these Location objects into GPX XML format:

<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
<gpx xmlns="http://www.topografix.com/GPX/1/1" creator="" version="1.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd"&gt;
 <trk>
  <name>ACTIVE LOG173327</name>
  <trkseg>
   <trkpt lat="52.347223" lon="4.893477">
    <ele>-1.514</ele>
    <time>2008-02-04T16:33:26Z</time>
   </trkpt>
   [........]
  </trkseg>
 </trk>
</gpx>

I'd like to do this using an NSKeyedArchiver, but I don't know if that would be possible and if it would, how to do it. Any ideas?

EDIT For posterity and if it indeed proves too complicated: here's my quick and dirty solution. I extended the NSMutableArray class with this method:

- (NSString *)locationsAsGPX {
    NSMutableString *tempGPXString = [NSMutableString stringWithFormat:@"<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\" ?>\n<gpx xmlns=\"http://www.topografix.com/GPX/1/1\" creator=\"\" version=\"1.1\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xsi:schemaLocation=\"http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd\"&gt;\n"];
    [tempGPXString appendString:@"<trk>\n<name>%@</name>\n<trkseg>\n"];
    for (Location *l in self) {
        [tempGPXString appendFormat:@"%@",[l asGPXTrackpoint]];
    }
    [tempGPXString appendString:@"<trkseg>\n<trk>\n<gpx>"];
    return tempGPXString;
}

and the asGPXTrackpoint method in the Location class looks like this:

-(NSString *)asGPXTrackpoint {
    if (self.lat==0.0 || self.lon==0.0) {
        return [NSString string];
    }
    NSMutableString *tempTrackpointString = [NSMutableString stringWithFormat:@"<trkpt lat=\""];
    [tempTrackpointString appendFormat:@"%f\" lon=\"%f\">",self.lat,self.lon];
     if(self.ele!=0.0) {
         [tempTrackpointString appendString:@"\n\t<ele>%f</ele>"];
     }
    if (self.time!=nil) {
        NSString *timeString = [self.gpxDateFormatter stringFromDate:time];
        [tempTrackpointString appendFormat:@"\n\t<time>%@</time>",timeString];
    }
    [tempTrackpointString appendString:@"\n</trkpt>\n"];
    return tempTrackpointString;
}

So what's dirty about this? A few things that I can see:

• I assume that the NSMutableArray contains only Location objects
• I do not use a proper XML serializer
• I assume 0.0 is an illegal value which really it isn't
• I haven't tested this with huge numbers of Locations