Just a snippet of code for detecting a possible match. This seems inefficient for just single-character matches..

NSTextViewDelegate_textchange.m

- (void)textDidChange:(NSNotification *)aNotification
{
    NSTextView *textView      = self.detailTextView;
    NSString   *contents      = textView.textStorage.string;
    NSRange    selectionRange = textView.selectedRange;
    
    if (selectionRange.length == 0 && contents.length > 0) {
        NSString *possibleMatch = [contents substringWithRange:NSMakeRange(selectionRange.location - 1, 1)];
        if ([possibleMatch isEqualToString:@"{"]) {
            [textView complete:nil];
        }
    }
}