Sorting array of wrapper objects in iOS using NSSortDescriptor

Question

I am trying to sort an array of UserWrapper objects. The wrapper object contains the object User, and the User object contains the property UserName (that I want to sort by).

It us easy enough to sort an array of Users ( source ), but the added layer of UserWrapper complicates things for me. Help please!

Here is my code, which worked for a simple User array:

NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"UserName"
                                                               ascending:YES
                                                                selector:@selector(localizedCaseInsensitiveCompare:)] ; 

NSArray *descriptors = [NSArray arrayWithObject:nameDescriptor];
NSMutableArray *contactsStartingWithKey = [nameIndexesDictionary objectForKey:aKey];
[contactsStartingWithKey sortUsingDescriptors:descriptors]; // Exception thrown here because UserName is not a property of UserWrapper, but of UserWrapper.User

Answer 1

The key argument of the sort descriptor can also by a key path , in your case:

[[NSSortDescriptor alloc] initWithKey:@"User.UserName"
                            ascending:YES
                             selector:@selector(localizedCaseInsensitiveCompare:)] ; 

Answer 2

Although the API and documentation of NSSortDescriptor are a bit inconsistent, the "key" parameter is actually a key path. So, you can just specify @"User.UserName" as the key and your code should work.