应用程序类是不知道哪些活动正在运行的。

The application class is unaware of what activity is running.

这是可以做的唯一的事情就是让应用程序类，了解活动是由不推荐，因为它可能会导致内存问题setter方法​​。

The only thing that can be done is to let application class know about activity is by a setter method which is not recommended as it may lead to memory problems.