NSProgresses树

Question

我尝试理解创建树NSProgresses的逻辑。但对我来说并非一切顺利。我尝试实现全局NSProgress管理器，它将观察应用程序中的所有任务和操作，并要求在任务之一未完成时请求静默。在我的想象中，它应该是根NSProgress，子进程将附加。当我们完全应用时，我们需要询问fractionCompleted。这意味着根NSProgress将递归遍历所有子NSProgress并计算总进度。但在实践中却没有。 我制作了用作测试台的代码（http://pastebin.com/CbXiFwCi）。请帮助我理解NSProgress逻辑，因为网络纯文档。谢谢。

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    self.progress = [[NSProgress alloc] initWithParent:nil
                                              userInfo:nil];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{

        [self.progress becomeCurrentWithPendingUnitCount:0]; ///what I should set as pendingUnitCount?

        NSProgress * progressA = [NSProgress progressWithTotalUnitCount:INT64_MAX-1];

        progressA.cancellable = NO;/// set cancellable to NO

        for(NSUInteger i = 0; i < INT64_MAX; i++)
        {
            //do work
            progressA.completedUnitCount ++;
        }

        [self.progress resignCurrent];
    });

    dispatch_async(dispatch_get_main_queue(), ^{

        [self.progress becomeCurrentWithPendingUnitCount:0];///what I should set as pendingUnitCount?

        NSProgress * progressB = [NSProgress progressWithTotalUnitCount:150];

        for(NSUInteger i = 0; i < 150; i++)
        {
            //do work
            progressB.completedUnitCount ++;
        }

        [self.progress resignCurrent];
    });
}

- (void)action:(id)sender
{
    double fractionCompleted = self.progress.fractionCompleted;///why fractionCompleted -> NAN?

    BOOL cancellable = self.progress.cancellable; ///why cancellable is YES if progressA.cancellable -> NO
}