Setting and maintaining the first SortDescriptor for an NSTableView

Question

I have an NSTableView with columns bound to an NSArrayController.

The table view shows a list of email messages:

• Flag if unread
• Subject
• Attachment size

The user can click on the Attachment Size column to sort the list, but I would like the table to always be sorted by the "unread" flag first so that the unread messages always remain at the top of the list.

I did not bind the Array Controller's sort descriptors to anything, yet table sorting works magically by clicking on the table columns (why?). Is there some way I can intercept setting the Array Controller's sort descriptors and insert the "Unread" sort descriptor first?

Example of a table sorted by attachment size:

UNREAD▼ SUBJECT ATTACHMENT SIZE▼
------  ------- ------------------
  yes   Hello.. 110kb
  yes   Test...  90kb

  no    Foobar  200kb
  no    Hey     100kb
  no    Test2    10kb

Answer 1

Well, the reason it "just works" is because the table columns call setSortDescriptors: on their bound NSArrayController.

Assuming you want the table to remain sortable, but you always want to sort by "unread", this is how I would go about it:

First, subclass NSArrayController and override arrangeObjects:

- (NSArray *)arrangeObjects:(NSArray *)objects {

    NSMutableArray *oldSorted = [[super arrangeObjects:objects] mutableCopy];
    NSMutableArray *newSorted = [NSMutableArray arrayWithCapacity:[oldSorted count]];

    for (id anObject in oldSorted)
        if ([[anObject valueForKey:@"isUnread"] boolValue])
            [newSorted addObject:anObject];

    [oldSorted removeObjectsInArray:newSorted];
    [newSorted addObjectsFromArray:oldSorted];
    [oldSorted release];

    return newSorted;
}

This puts unread messages at the "top" (beginning of array). I'm not sure this is the most efficient sorting algorithm, but I believe it's the correct way to go about it.