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NSSortDescriptor

Written by Mattt Thompson on Jul 24th, 2012

Sorting: the old mainstay of the exams of an intro CS class and whiteboards of an entry-level programming interview. When was the last time you really needed to know how to implement Quicksort anyway?

When making apps, sorting is just something you can assume to be fast, and utility is measured in how easy it is to do what you need. In this respect, Foundation's NSSortDescriptor is perhaps the most useful and elegant implementations you'll find.

NSSortDescriptor is constructed with either 2 or 3 parameters:

• key: for a given collection, the key for the corresponding value to be sorted on for each object in the collection.
• ascending: a boolean specifying whether the collection should be sorted in ascending (YES) or descending (NO) order.

There is an optional third parameter that relates to how the sorted values are compared to one another. By default, this is a simple equality check, but this behavior can be changed by passing either a selector (SEL) or comparator (NSComparator). Any time you're sorting string values, be sure to pass the selector localizedStandardCompare:, which will sort according to the language rules of the current locale (locales may differ on ordering of case, diacritics, and so forth).

Collection classes like NSArray and NSSet have methods to return sorted arrays of the objects that take a sortDescriptors parameter. Sort descriptors are applied in order, so that any subsequent descriptors are used to break ties in the previous results so far.

Let's say that we have a Person object that has properties for firstName & lastName of typeNSString *, and age, which is an NSUInteger. Given the following dataset:

index0123firstNameAliceBobCharlieQuentinlastNameSmithJonesSmithAlbertsage24273331

@interface Person : NSObject@property NSString *firstName;@property NSString *lastName;@property NSNumber *age;@end@implementation Person- (NSString *)description {    return [NSString stringWithFormat:@"%@ %@", self.firstName, self.lastName];}@end#pragma mark -NSArray *firstNames = @[ @"Alice", @"Bob", @"Charlie", @"Quentin" ];NSArray *lastNames = @[ @"Smith", @"Jones", @"Smith", @"Alberts" ];NSArray *ages = @[ @24, @27, @33, @31 ];NSMutableArray *people = [NSMutableArray array];[firstNames enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {    Person *person = [[Person alloc] init];    person.firstName = [firstNames objectAtIndex:idx];    person.lastName = [lastNames objectAtIndex:idx];    person.age = [ages objectAtIndex:idx];    [people addObject:person];}];NSSortDescriptor *firstNameSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"firstName"   ascending:YES   selector:@selector(localizedStandardCompare:)];NSSortDescriptor *lastNameSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"lastName"   ascending:YES   selector:@selector(localizedStandardCompare:)];NSSortDescriptor *ageSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"age"   ascending:NO];NSLog(@"By age: %@", [people sortedArrayUsingDescriptors:@[ ageSortDescriptor ]]);// "Charlie Smith", "Quentin Alberts", "Bob Jones", "Alice Smith"NSLog(@"By first name: %@", [people sortedArrayUsingDescriptors:@[ firstNameSortDescriptor ]]);// "Alice Smith", "Bob Jones", "Charlie Smith", "Quentin Alberts"NSLog(@"By last name, first name: %@", [people sortedArrayUsingDescriptors:@[ lastNameSortDescriptor, firstNameSortDescriptor ]]);// "Quentin Alberts", "Bob Jones", "Alice Smith", "Charlie Smith"

Sort descriptors can be found throughout Foundation as well as most other frameworks--including a prominent role in Core Data. Anytime your own classes need to define sort ordering, follow the convention of specifying a sortDescriptors parameter as appropriate.

Because, truth is, sorting should be treated in terms of business logic, not mathematical formulas and map-reduce functions. NSSortDescriptor is a slam dunk, and will have you pining for it anytime you venture out of Objective-C.