cocoa - NSTreeController - 選択したノードの取得

Question

bookController (NSCreeController) に Book オブジェクトを追加しました。行を選択したときに、保存された Book オブジェクトを取得したいと思います。

- (IBAction)addClicked:(id)sender {
    NSTimeInterval timeStamp = [[NSDate date] timeIntervalSince1970];
    // NSTimeInterval is defined as double

    NSUInteger indexArr[] = {0,0};
    Book *obj = [[Book alloc] init];
    NSString *dateString = [NSDateFormatter localizedStringFromDate:[NSDate date]                                                          dateStyle:NSDateFormatterNoStyle                                                          timeStyle:NSDateFormatterLongStyle];
    obj.title = [NSString stringWithFormat:@"New %@",dateString];
    obj.filename = [NSString stringWithFormat:@"%d",arc4random()%100000];
    [self.booksController insertObject:obj atArrangedObjectIndexPath:[NSIndexPath indexPathWithIndexes:indexArr length:2]];
}