iOS申请延长进入后台代码片段

void runTaskInBackground(void (^taskBlock)(void), void (^timeOutBlock)(void), BOOL aSync)
{
    __block UIBackgroundTaskIdentifier taskId = 0;
    taskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^
    {
        if (NULL != timeOutBlock)
        {
            timeOutBlock();
        }
        [[UIApplication sharedApplication] endBackgroundTask:taskId];
        taskId = UIBackgroundTaskInvalid;
        ATGLog(@"Timeout, background task has be ended");
    }];
    
    if (UIBackgroundTaskInvalid != taskId)
    {
        if (aSync && [UIDevice currentDevice].multitaskingSupported)
        {
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^
            {
                if (NULL != taskBlock)
                {
                    ATGLog(@"starting run background task in multi-thread:%@, remain time:%f", [NSThread currentThread],
                           [UIApplication sharedApplication].backgroundTimeRemaining);
                    taskBlock();
                    [[UIApplication sharedApplication] endBackgroundTask:taskId];
                    taskId = UIBackgroundTaskInvalid;
                    ATGLog(@"finished background task in multi-thread:%@, remain time:%f", [NSThread currentThread],
                           [UIApplication sharedApplication].backgroundTimeRemaining);
                }
            });
        }
        else
        {
            if (NULL != taskBlock)
            {
                ATGLog(@"starting run background task in single-thread:%@, remain time:%f", [NSThread currentThread],
                       [UIApplication sharedApplication].backgroundTimeRemaining);
                taskBlock();
                [[UIApplication sharedApplication] endBackgroundTask:taskId];
                taskId = UIBackgroundTaskInvalid;
                ATGLog(@"finished background task in single-thread:%@, remain time:%f", [NSThread currentThread],
                       [UIApplication sharedApplication].backgroundTimeRemaining);
            }
        }
    }
}

通过测试，发现无论在iOS6和iOS7上出现与SDK描述不符的现象，情况如下：

1、调用beginBackgroundTaskWithExpirationHandler之后，就算超时(iOS6 600S iOS7 180S)或者调用endBackgroundTask之后，app其它线程依然正常运行没有被杀死(注：该线程有日志打印和文件操作)；

2、调用beginBackgroundTaskWithExpirationHandler之后，超时(iOS6 600S iOS7 180S)之后，如果beginBackgroundTaskWithExpirationHandler之后采用异步调用该线程将暂停等到程序进入前台接着运行，如果是直接在主线程调用将一直运行到任务完成，即使进入前台也会柱塞到该任务完成；

有哪位高人可以帮忙解释一下，谢谢~~