How to know whether app is terminated by user or iOS (after 10min background)

How to know whether app is terminated by user or by iOS when restart app>

'By user' means "by Double-clicking Home Button and pressing - button". killed by user

'By iOS' means "app become background running state, and iOS terminate app after 10 mins"

If your app is in suspended state the applicationWillTerminate will never get called regardless who killed the app iOS or user.

Your applicationWillTerminate will only call when your app is in background and it gets killed (either by iOS or user) the term background means that it is running in background not in suspended state.

Just read this reference

applicationWillTerminate:—Lets you know that your app is being terminated. This method is not called if your app is suspended.

Here is the table of various states

Background - The app is in the background and executing code. Most apps enter this state briefly on their way to being suspended. However, an app that requests extra execution time may remain in this state for a period of time. In addition, an app being launched directly into the background enters this state instead of the inactive state. For information about how to execute code while in the background, see Background Execution.

iOS might terminate your app if system resources are low - if this happens, you will see applicationWillTerminate.

It used to be that if a user killed the app (task manager, via the button double-click and then hits the red '-') it's a SIGKILL and applicationWillTerminate is not called. A report mid-2013 suggests this has changed and applicationWillTerminate now is called.

You could use NSUserDefaults to write some state bit in applicationWillTerminate to note that this function was called and presumably that's a system kill rather than a user kill.