Providing a Random Ordering for a Core Data Fetch

Question

I have a mac app with a scrollview that displays a list of items. The data for the items is provided from Core Data. I need to provide a 'shuffle' button that with randomize the order of the items within the scroll view.

Answer 1

If you are using bindings, you have only two choices: (1), bind to a custom object which has a key that returns the sorted array or (2) use a sort descriptor that randomizes instead of ordering.

The second option is easier in some circumstance. To produce a randomizing sort descriptor you can (1) create a NSSortDescriptor subclass and override compareObject:toObject:

- (NSComparisonResult)compareObject:(id)object1 toObject:(id)object2{
  NSUInteger ran=(arc4random() % 3);
  switch (ran) {
    case 0:
      return NSOrderedSame;
      break;
    case 1:
      return NSOrderedDescending;
    default:
      return NSOrderedAscending;
      break;
  }  
}

or (2) provide a selector or block that provides a random ordering in the same way:

  NSSortDescriptor *rs=[NSSortDescriptor sortDescriptorWithKey:@"intValue" 
                                                    ascending:YES
                                                   comparator:^(id obj1, id obj2){
                                                     NSUInteger ran=(arc4random() % 3);
                                                     switch (ran) {
                                                       case 0:
                                                         return NSOrderedSame;
                                                         break;
                                                       case 1:
                                                         return NSOrderedDescending;
                                                       default:
                                                         return NSOrderedAscending;
                                                         break;
                                                     }
                                                   }];

The disadvantage of (2) is that you must provide a key that the object being sorted to understands e.g. in the above, I sorted an array of NSNumber objects so I have to provide a key intValue that NSNumber responds to. The subclass method does not really care about keys.

Using the sorts, however, returns only a weakly randomized array because the randomization is just a messed up sort i.e. most of the elements do not move very far in the array. To create a more randomized effect, stack the randomizing sorts into an array so that they make multiple passes over the array e.g.:

[ aMutableArray sortUsingDescriptors:[NSArray arrayWithObjects:rs,rs,rs, nil]];

If you use bindings, just use the randomizing sort descriptors where you use a normal sort descriptor and you should get something that is usefully randomized.