Android：如何检测Back按钮是否会退出应用程序（即这是堆栈中剩余的最后一个活动）？

I'd like to warn the user if the back press is going to finish the last activity on the stack, thereby exiting the app. I'd like to pop up a little toast and detect a 2nd back press within the next few seconds and only then call I'd like to warn the user if the back press is