Question

I have the following sort descriptors that sort an array of my business objects, ready to be displayed in a table, I’m starting off with some sample sorting code from a previous SO question

NSSortDescriptor *sortDescriptor1 = [[NSSortDescriptor alloc] initWithKey:@"awardedOn" ascending:NO];
NSSortDescriptor *sortDescriptor2 = [[NSSortDescriptor alloc] initWithKey:@"title" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor1, sortDescriptor2, nil];
NSArray *sortedArray = [returnable sortedArrayUsingDescriptors:sortDescriptors];

The objects that I’m displaying all will have a title. Only some of them will have an “awardedOn” set, which is an NSDate.

What I want to do:

1. Sort entire array so all the objects with an “awardedOn” set are
   displayed at the top
2. Within the two “sets”, order them alphabetically
3. I don’t care about the actual value of the date, I’m more interested
   if it exists or not

Something like this (Titles, the bold ones have a value for awardedOn)

• Awesome
• Better
• Cool
• Another
• Another One
• One more
• Yet Another

Answer 1

You should be able to do that using two descriptors like you first said, first by awardedOn, then by title. However, you need to provide a custom NSSortDescriptor for the awardedOn sort that looks someting like this:

#define NULL_OBJECT(a) ((a) == nil || [(a) isEqual:[NSNull null]]) 
@interface AwardedOnSortDescriptor : NSSortDescriptor {} 
@end 
@implementation AwardedOnSortDescriptor 
- (id)copyWithZone:(NSZone*)zone 
{ 
    return [[[self class] alloc] initWithKey:[self key] ascending:[self ascending] selector:[self selector]]; 
} 
- (NSComparisonResult)compareObject:(id)object1 toObject:(id)object2 
{ 
    if (NULL_OBJECT([object1 valueForKeyPath:[self key]])) { 
        if (NULL_OBJECT([object2 valueForKeyPath:[self key]]))  
            return NSOrderedSame; // If both objects have no awardedOn field, they are in the same "set"
        return NSOrderedDescending; // If the first one has no awardedOn, it is sorted after         
    } 
    if (NULL_OBJECT([object2 valueForKeyPath:[self key]])) { 
        return NSOrderedAscending; // If the second one has no awardedOn, it is sorted after
    } 
    return NSOrderedSame; // If they both have an awardedOn field, they are in the same "set"
} 
@end 

This will allow you to have to separate sets: Awesome/Better/Cool and Another/Another One/One More/Yet another, in your example. After that, you should be good with:

NSSortDescriptor *sortDescriptor1 = [[AwardedOnSortDescriptor alloc] initWithKey:@"awardedOn" ascending:YES];
NSSortDescriptor *sortDescriptor2 = [[NSSortDescriptor alloc] initWithKey:@"title" ascending:YES];

On a final note, you might need a litte more work depending on what your “empty” awardedOn fields look like (I assumed, in the code above, that the field was set to null). You can take a look here: https://stackoverflow.com/a/3145789