Sort NSArray of custom objects by their NSDate properties

Question

I am attempting to sort an NSArray that is populated with custom objects. Each object has a property startDateTime that is of type NSDate.

The following code results in an array, sortedEventArray, populated but not sorted. Am I going about this the completely wrong way or am I just missing something small?

NSSortDescriptor *dateDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"startDateTime" 
                                                                 ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObject:dateDescriptor];
NSArray *sortedEventArray = [nodeEventArray sortedArrayUsingDescriptors:sortDescriptors];

Answer 1

Are you sure that the startDateTime instance variables of the node events are non-nil?

If you don't have one already, you might add a (custom) -description method to your node event objects that does something like this:

- (NSString *)description {
   return [NSString stringWithFormat:@"%@ - %@",
                   [super description], startDateTime]];
}

Then in your sorting code log the array before and after:

NSLog(@"nodeEventArray == %@", nodeEventArray);
NSSortDescriptor *dateDescriptor = [NSSortDescriptor
                                     sortDescriptorWithKey:@"startDateTime" 
                                                 ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObject:dateDescriptor];
NSArray *sortedEventArray = [nodeEventArray
         sortedArrayUsingDescriptors:sortDescriptors];
NSLog(@"sortedEventArray == %@", sortedEventArray);

If the startDateTime's are all nil, then the before and after arrays will have the same order (since the sorting operation will equate to sending all the -compare: messages to nil, which basically does nothing).