在前台(顶部)活动(应用程序)发生变化时收到通知

我想为Android平台编写一个服务，当当前前台活动发生变化时通知.基本上，只有当顶级活动发生变化时，服务才应该执行一些任务.有没有办法订阅并在发生此类事件时收到通知?或者没有可能，服务应该不时轮询正在运行的活动列表并检查前台活动是什么?不是更好的解决方案...

I want to write a service for Android platform that is notified when the current foreground activity changes. Basically the service should do some tasks only when the top activity changes. Is there any way to subscribe and to be notified when this kind of event occurs ? Or there is no possibility and the service should poll from time to time the list of running activities and to check what is the foreground activity ?Not preferable solution...