Autocomplete with twitter usernames in text field

I was looking at NSTokenField, NSTextField and NSTextView with no luck to do the following:

I am writing a Twtitter client and when you want to twitter a new Tweet then you begin to write in a text field for example:

Going to make coffee, @pe

When you begin to write a @ then I would like to help the user to autocomplete the username for example @peter. I have a NSArray with the usernames like:

NSArray *usernames = [NSArray arrayWithObjects:@"@andreas", @"@clara", @"@jeena", @"@peter"]

What should I do to enable a simple autocompletation? I'd be happy if you would have to press F5 or something for starters too. The problem I am having is that with NSTokenField I don't know how I should tokenize the string, with NSTextField it only works when I write the @username at the beginning of the tweet and NSTextView seems really complicated and too much for such a simple thing.

The most basic implementation involves overriding this method... Definitely not optimal, but you should get the idea:

- (NSArray *) completionsForPartialWordRange:(NSRange)charRange indexOfSelectedItem:(NSInteger *)index {
    // this would be defined somewhere else, but just for example.. 
    NSArray *usernames = [NSArray arrayWithObjects:@"@andreas", @"@clara", @"@jeena", @"@peter"];

    NSMutableArray *matchedNames = [NSMutableArray array];
    NSString *toMatch = [[self string] substringWithRange:charRange];
    for(NSString *username in usernames) {
        [matchedNames addObject:username];
    }

    return matchedNames; // that's it. 
}

Once you start having a lot of data, you'll need to employ strategies to pre-do your searches by storing words into hashes with the partial pieces of text in them (like, "Hello" would be put into 4 different arrays stuff into NSDictionary keys for "H", "He", "Hel", "Hell" .. Repeat with every word in your Lexicon. Much quicker that way.

If you want to support auto-complete, just call the 'complete:' method when you detect text is changing in your control.