如何检查应用程序是否在前台但在特定时间内未访问 - iOS

I am developing an app in Swift 2.0 that uses http requests. And I am passing a checksum value to the requests. This checksum is generated by the web server once a user logs in. The checksum value is refreshed once it is not used for 3 hours. Because of the change in checksum value, I am facing an issue. The problem is, the session gets expired and the user needs to login again to use the app.I am developing an app in Swift 2.0 that uses h