didChangeAppLifecycleState method
Called when the system puts the app in the background or returns the app to the foreground.
An example of implementing this method is provided in the class-level documentation for the WidgetsBindingObserver class.
This method exposes notifications from SystemChannels.lifecycle.
Implementation
@override
void didChangeAppLifecycleState(AppLifecycleState state) {
_didChangeAppLifecycleState?.call(state);
}