How to check is user has closed app

To summarrise my app, Users choose a server they are currently on and are taken to that specific server page. The user clicks a button to show they are beginning their time of the server. On the right hand side, all the current players on that server are listed, all of those who pressed the button. There is also a button to show you have quit the server and your name is removed from the list of online users and you are taken back to the home page. However if the user closes the program without clicking the button, they still appear online. Is there a way of identifying if a user is online or they have closed the program? Im cannot find a way around this problem. Would appreciate any help possible. Thanks

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