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];

Solution 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).

Solution 2

Did you try by specifying the NSDate comparator?

Eg:

NSSortDescriptor *dateDescriptor = [NSSortDescriptor
   sortDescriptorWithKey:@"startDateTime"
   ascending:YES
   selector:@selector(compare:)];

This should enforce the usage of the correct comparator of the NSDate class.

Solution 3

Ok, I know this is a little late, but this is how I would do it:

NSArray *sortedEventArray = [events sortedArrayUsingComparator:^NSComparisonResult(Event *event1, Event *event2) {
    return [event1.startDateTime compare:event2.startDateTime];
}];

Obviously, replace Event with the class of your custom object. The advantage of this approach is that you are protected against future refactoring. If you were to refactor and rename the startDateTime property to something else, Xcode probably would not change the string you're passing into the sort descriptor. Suddenly, your sorting code would break/do nothing.