将NSArrayController中项的属性的聚合设置为从属键

在引用数组控制器的控制器类中,我尝试按如下方式执行此操作:

+ (NSSet *)keyPathsForValuesAffectingTotalPercentProfit {
    return [NSSet setWithObjects:
            @"myArrayController.arrangedObjects.@sum.netProfit", 
            @"myArrayController.arrangedObjects.@sum.netCost", 
            nil];
}

- (NSDecimalNumber *)totalPercentProfit {
    NSDecimalNumber *totalProfit = [self valueForKeyPath:@"myArrayController.arrangedObjects.@sum.netProfit"];
    NSDecimalNumber *totalCost = [self valueForKeyPath:@"myArrayController.arrangedObjects.@sum.netCost"];
    if (!([totalCost compare:[NSDecimalNumber zero]] == NSOrderedSame)) {
        return [totalProfit decimalNumberByDividingBy:totalCost];
    } else {
        return nil;
    }
}

但是,当相关密钥路径更改时,绑定到此totalPercentProfit属性的标签不会刷新。

有人能给我指出解决这个问题的正确方向吗?