how to use sortUsingFunction:context - iphone

Question

appdata.items is NSMutableArray.
I connot compile This code.
Error code is "prop.173 has an incomplete type".
NSInteger compareInfo(id aInfo1, id aInfo2, void *context){
NSDate* info1 = [aInfo1 objectAtIndex:2];
NSDate* info2 = [aInfo2 objectAtIndex:2];
return [info1 compare:info2];
}
-(void)saveData{
NSData* data = [[NSMutableData alloc] init];
appdata.items = [appdata.items sortUsingFunction:compareInfo context:NULL];
NSKeyedArchiver* archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
[archiver encodeObject:appdata forKey:DATAKEY];
[archiver finishEncoding];
[data writeToFile:[self dataFilePath] atomically:YES];
[archiver release];
[data release];
}

Answer 1

Here is some code that might help you, (I spend pretty much time making it working, doc is not really helpfull)
It computes the distance between POI object and a currentLocation and order POI Object from the closest to the fartest from this location
NSInteger compareDistance(id num1, id num2, void *context)
{
int retour;
// fist we need to cast all the parameters
CLLocation* location = context;
POI* param1 = num1;
POI* param2 = num2;
// then we can use them as standard ObjC objects
CLLocation* locationCoordinates = [[CLLocation alloc] initWithLatitude:location.coordinate.latitude longitude:location.coordinate.longitude];
CLLocation* locationPOI1 = [[CLLocation alloc] initWithLatitude:param1.coords.latitude longitude:param1.coords.longitude];
CLLocation* locationPOI2 = [[CLLocation alloc] initWithLatitude:param2.coords.latitude longitude:param2.coords.longitude];
CLLocationDistance distance1 = [locationPOI1 distanceFromLocation:locationCoordinates];
CLLocationDistance distance2 = [locationPOI2 distanceFromLocation:locationCoordinates];
//make the comparaison
if (distance1 < distance2)
retour = NSOrderedAscending;
else if (distance1 > distance2)
retour = NSOrderedDescending;
else
retour = NSOrderedSame;
[locationCoordinates release];
[locationPOI1 release];
[locationPOI2 release];
return retour;
}
-(void) orderByProximityFromLocation:(CLLocationCoordinate2D) coordinates
{
CLLocation* currentLocation = [[CLLocation alloc] initWithLatitude:coordinates.latitude longitude:coordinates.longitude];
[listOfPOI sortUsingFunction:compareDistance context:currentLocation];
[currentLocation release];
}

Answer 2

The method sortUsingFunction:context: is for NSMutableArray, not NSArray, which sorts the contents of the mutable array. The method you want is sortedArrayUsingFunction:context: which will return a sorted array which you can assign, as you're currently trying to do.
Unless, of course, items is an NSMutableArray, in which case you can call sortUsingFunction:context: but as it doesn't return anything, so you don't assign it to items.